Plant Physiology Laboratory Manual: Photosynthesis, Respiration & Water Relations
Rigorous experimental protocols in isolated thylakoid Hill reaction electron transport and DCPIP photoreduction, spectrophotometric chlorophyll $a/b$ and carotenoid quantitation via Arnon’s simultaneous equations, leaf tissue osmotic water potential ($Psi_w$) via the Chardakov density gradient falling-drop method, substrate-dependent respiratory quotient (RQ) manometry via Ganong’s respirometer, foliar in vivo nitrate reductase (NR) Griess diazotization assay, and stomatal turgor dynamics with cobalt chloride transpiration porometry aligned with CSIR-NET Unit 6 and GATE XL syllabi.
Volatile Organic Solvents, Photosystem Inhibitor Precautions & Dark Infiltration SOPs
Plant physiological investigations utilize organic extractants, photochemical dyes, and metabolic inhibitors requiring standard precautions:
- Flammable Solvent Extraction (80% Acetone / 1-Propanol): Acetone used for foliar pigment extraction is highly volatile and flammable ($Flash Point: -20^circtext{C}$). Never heat acetone over open flames or unprotected hotplates; conduct all blending and homogenate vacuum filtrations under a functional chemical fume hood. Dispose of organic waste in designated halogen-free solvent containers.
- Herbicide & Photosystem Inhibitor Handling (DCMU / Diuron): 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) is a potent specific photosystem II electron transport inhibitor and aquatic toxin. Prepare $10text{ mM}$ stock solutions in absolute ethanol using nitrile gloves; avoid skin contact and never mouth-pipette solutions.
- Carcinogen & Heavy Metal Precautions: Cobalt chloride ($text{CoCl}_2$) used in hygrometric paper is a toxic heavy metal salt and suspected carcinogen. Impregnate filter papers inside fume hoods, handle dry papers with forceps, and wash hands thoroughly after porometric timing experiments.
- Dark Infiltration & Anaerobiosis in Enzyme Assays: In vivo nitrate reductase assays require complete dark conditions and vacuum-infiltration to displace intercellular air from mesophyll lacunae. Use foil-wrapped incubation vials and maintain steady $30^circtext{C}$ water baths without exposure to stray laboratory light, which activates nitrite reductase (NiR) and consumes product nitrite.
Hill Reaction & Photosynthetic Electron Transport: Photoreduction of 2,6-Dichlorophenolindophenol (DCPIP)
In 1937, Robin Hill demonstrated that isolated chloroplasts illuminated in the presence of an artificial electron acceptor evolve molecular oxygen ($text{O}_2$) in the complete absence of carbon dioxide ($text{CO}_2$). This seminal finding proved that:
1. The light-dependent water-splitting reactions of photosynthesis are physically and chemically distinct from the light-independent enzymatic fixation of $text{CO}_2$ (Calvin-Benson cycle).
2. Water ($text{H}_2text{O}$), not $text{CO}_2$, is the ultimate source of evolved atmospheric oxygen.
In the classical Hill reaction, 2,6-dichlorophenolindophenol (DCPIP) is employed as an artificial, membrane-permeant electron acceptor. In its oxidized state, DCPIP is intensely blue with a prominent absorption maximum at $lambda = 600text{ nm}$ ($epsilon_{600} = 21,000text{ M}^{-1}text{cm}^{-1}$ at $text{pH } 6.5$). Upon photochemical reduction by electrons emerging from Photosystem II and the plastoquinone pool, DCPIP accepts two electrons and two protons to form colorless reducedindophenol ($text{DCPIPH}_2$):
$$text{DCPIP (Blue, Oxidized)} + 2 e^- + 2 text{H}^+ xrightarrow{text{Light, Thylakoids}} text{DCPIPH}_2 text{ (Colorless, Reduced)}$$
$$text{Overall Reaction: } 2 text{ H}_2text{O} + 2 text{ DCPIP} xrightarrow{hnu, text{Chloroplasts}} text{O}_2 uparrow + 2 text{ DCPIPH}_2$$
Mechanism of DCMU Inhibition: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU or Diuron) is a selective herbicide that competes directly with plastoquinone for the $Q_B$ binding site on the D1 reaction center protein of Photosystem II. By blocking electron transfer from $Q_A^-$ to the plastoquinone pool, DCMU completely halts DCPIP photoreduction, confirming that electron flow originates from the photochemical oxygen-evolving complex of PSII.
- Plant Material: Fresh, dark-adapted (1 h) market spinach leaves (*Spinacia oleracea*), destemmed and washed.
- Chloroplast Isolation Buffer (CIB, Ice-Cold): $0.4text{ M Sorbitol}$, $50text{ mM Tricine-KOH}$ ($text{pH } 7.8$), $10text{ mM NaCl}$, $1text{ mM MgCl}_2$, $0.1% (w/v)text{ BSA}$.
- Hypotonic Assay Buffer: $50text{ mM HEPES-KOH}$ ($text{pH } 6.5$), $10text{ mM NaCl}$, $5text{ mM MgCl}_2$ (osmotic shock lyses outer envelope to release naked thylakoids).
- Artificial Acceptor: $0.2text{ mM DCPIP}$ sodium salt in assay buffer (shielded from light).
- Inhibitor: $10text{ mM DCMU}$ stock solution dissolved in $95%$ ethanol (working concentration $10 mutext{M}$).
- Instrumentation: High-speed refrigerated centrifuge ($4^circtext{C}$), mechanical kitchen blender with chilled blades, UV-Vis spectrophotometer ($600text{ nm}$), $100text{ W}$ incandescent or cold-white LED illumination lamp, and water heat filter (to prevent heat transfer).
Homogenize $20text{ g}$ of destemmed spinach leaves in $60text{ mL}$ of ice-cold Isolation Buffer using 3 bursts of 5 seconds in a chilled blender. Filter homogenate through 4 layers of cheesecloth and 1 layer of Miracloth into a chilled beaker. Centrifuge filtrate at $2,000 times g$ for 5 minutes at $4^circtext{C}$. Discard green supernatant, gently resuspend green thylakoid pellet in $10text{ mL}$ of hypotonic assay buffer, and store on crushed ice in darkness.
Extract $50 mutext{L}$ of thylakoid suspension in $4.95text{ mL}$ of $80%$ acetone. Clarify at $3,000 times g$ for 3 minutes and measure $A_{652}$. Calculate total chlorophyll concentration using Arnon’s formula: $text{Total Chl (mg/mL)} = A_{652} times 100 / 34.5$. Adjust thylakoid stock to $0.10text{ mg Chl/mL}$.
Prepare 4 optical glass cuvettes:
• Tube 1 (Light Reaction): $2.7text{ mL}$ buffer + $0.2text{ mL}$ DCPIP + $0.1text{ mL}$ thylakoids (illuminated).
• Tube 2 (Dark Control): Same as Tube 1, wrapped completely in aluminum foil.
• Tube 3 (DCMU Inhibition): Same as Tube 1 + $3 mutext{L}$ of $10text{ mM DCMU}$ ($10 mutext{M}$ final).
• Tube 4 (Heat-Inactivated Control): Same as Tube 1, but using boiled thylakoids ($100^circtext{C}$ for 5 min).
Measure initial absorbance ($t = 0text{ min}$) at $600text{ nm}$ against a water blank. Place illuminated cuvettes $20text{ cm}$ from a $100text{ W}$ light source behind a $5text{ cm}$ water heat bath. Record $A_{600}$ at 2-minute intervals up to 10 minutes. Mix gently by inversion before each spectrophotometric reading.
| Experimental Condition | A₆₀₀ (0 min) | A₆₀₀ (2 min) | A₆₀₀ (6 min) | A₆₀₀ (10 min) | ΔA₆₀₀ / min | Rate (μmol DCPIP · mg Chl−¹ · h−¹) |
|---|---|---|---|---|---|---|
| 1. Light + Active Chloroplasts | 0.785 | 0.642 | 0.365 | 0.118 | 0.0667 | 114.3 μmol · mg−¹ · h−¹ |
| 2. Dark Control | 0.784 | 0.782 | 0.779 | 0.775 | 0.0009 | 1.5 μmol · mg−¹ · h−¹ (Baseline drift) |
| 3. Light + 10 μM DCMU | 0.786 | 0.783 | 0.778 | 0.772 | 0.0014 | 2.4 μmol · mg−¹ · h−¹ (97.9% Inhibition) |
| 4. Boiled Chloroplasts | 0.780 | 0.779 | 0.776 | 0.774 | 0.0006 | 1.0 μmol · mg−¹ · h−¹ (Inactivated) |
Kinetic Hill Reaction Calculation:
Total reaction volume $V = 3.0text{ mL} = 0.003text{ L}$.
Amount of chlorophyll present: $0.1text{ mL} times 0.10text{ mg/mL} = 0.010text{ mg Chl}$.
$$text{Rate} = frac{(Delta A_{600}/text{min}) times 60text{ min/h} times Vtext{ (L)} times 10^6 mutext{mol/mol}}{epsilon_{600} times l times text{mg Chl}}$$
$$text{Rate} = frac{0.0667 times 60 times 0.003 times 10^6}{21,000 times 1.0 times 0.010} = frac{12,006}{105} = mathbf{114.3 mutext{mol DCPIP reduced}cdottext{mg Chl}^{-1}cdottext{h}^{-1}}$$
Photochemical Conclusion: The profound drop in absorbance occurs exclusively when thylakoids receive light energy. The near-complete inhibition ($97.9%$) by $10 mutext{M DCMU}$ confirms that electron flow requires the functional Photosystem II $Q_B$ plastoquinone exchange mechanism.
Spectrophotometric Quantitation of Chlorophyll $a$, Chlorophyll $b$ & Carotenoids via Arnon’s Equations
Chlorophylls are the principal light-harvesting pigments of oxygenic photosynthesis. Both Chlorophyll $a$ ($text{Chl } a$) and Chlorophyll $b$ ($text{Chl } b$) share a cyclic tetrapyrrole porphyrin (chlorin) ring chelating a central $text{Mg}^{2+}$ ion, esterified to a hydrophobic $C_{20}$ phytol tail. They differ structurally by only a single substituent at carbon-3 of ring II:
• $text{Chl } a$ possesses a methyl group ($-text{CH}_3$).
• $text{Chl } b$ possesses an electron-withdrawing formyl group ($-text{CHO}$).
This minor electronegative modification alters electronic resonance within the conjugated macrocycle, shifting the red absorption maximum of $text{Chl } b$ to shorter wavelengths ($645text{ nm}$) relative to $text{Chl } a$ ($663text{ nm}$ in $80%$ acetone). Because both pigments absorb at each other’s wavelength maxima, their individual concentrations cannot be read directly. Instead, Daniel I. Arnon (1949) formulated a set of simultaneous linear equations by determining the specific extinction coefficients ($alpha$) of purified pigments and inverting the absorption matrix:
$$A_{663} = alpha_{a,663} cdot C_a + alpha_{b,663} cdot C_b = 82.04 cdot C_a + 9.27 cdot C_b$$
$$A_{645} = alpha_{a,645} cdot C_a + alpha_{b,645} cdot C_b = 16.75 cdot C_a + 45.60 cdot C_b$$
Solving this system of linear equations yields Arnon’s classical equations for pigment concentrations in $text{mg/L}$ ($mutext{g/mL}$):
$$text{Chl } a (text{mg/L}) = 12.7 cdot A_{663} – 2.69 cdot A_{645}$$
$$text{Chl } b (text{mg/L}) = 22.9 cdot A_{645} – 4.68 cdot A_{663}$$
$$text{Total Chl} (text{mg/L}) = 20.2 cdot A_{645} + 8.02 cdot A_{663}$$
$$text{Total Carotenoids} (text{mg/L}) = frac{1000 cdot A_{470} – 1.82 cdot text{Chl } a – 85.02 cdot text{Chl } b}{198}$$
Ecophysiological Significance of the $text{Chl } a/text{Chl } b$ Ratio: $text{Chl } a$ is present in both core reaction center complexes (PSII P680 and PSI P700) and peripheral light-harvesting complexes (LHC), whereas $text{Chl } b$ is located exclusively in peripheral antenna trimers (LHCII and LHCI). Sun-adapted leaves synthesize less antenna complex per reaction center, exhibiting high $text{Chl } a/text{Chl } b$ ratios ($3.0–3.8$). Conversely, shade-adapted leaves expand their peripheral antenna to maximize photon capture under canopy shade, lowering the ratio to $2.0–2.4$.
- Plant Material: Healthy foliage from sun-grown spinach (*Spinacia oleracea*) and shade-grown money plant (*Epipremnum aureum*).
- Extraction Solvent: $80% (v/v)$ Acetone in deionized water, buffered with $2.5text{ mM Sodium Phosphate}$ ($text{pH } 7.8$) to prevent pheophytinization (acid-catalyzed loss of central $text{Mg}^{2+}$).
- Porcelain Mortar & Pestle: Pre-chilled at $-20^circtext{C}$ with acid-washed quartz sand (grinding aid) and a pinch of calcium carbonate ($text{CaCO}_3$) to neutralize vacuolar organic acids.
- Instrumentation: UV-Vis spectrophotometer with $1.0text{ cm}$ quartz/glass cuvettes, benchtop clinical centrifuge, analytical balance ($pm 0.1text{ mg}$).
Accurately weigh $0.200text{ g}$ of fresh leaf tissue, excluding midribs and major veins. Transfer to chilled mortar, add $10text{ mg } text{CaCO}_3$, a pinch of quartz sand, and $3.0text{ mL}$ of ice-cold $80%$ acetone. Grind thoroughly under dim green/subdued light until the tissue pulp is completely bleached white and homogenous.
Quantitatively transfer the green extract through Whatman No. 1 filter paper into a $25.0text{ mL}$ volumetric flask. Wash mortar and residue with small aliquots ($2text{ mL}$) of $80%$ acetone until the washing liquid is completely colorless. Bring to final mark with $80%$ acetone and invert to mix.
Transfer extract to optical glass or quartz cuvette. Zero spectrophotometer against $80%$ acetone blank. Measure and record optical densities at exactly $663text{ nm}$, $645text{ nm}$, $470text{ nm}$, and $710text{ nm}$ (residual scattering/turbidity check). Ensure $A_{710} < 0.010$; subtract $A_{710}$ from all readings.
Calculate concentrations in $text{mg/L}$. Convert to tissue yield ($text{mg pigment / g fresh weight}$) using formula: $text{Pigment (mg/g FW)} = frac{Ctext{ (mg/L)} times Vtext{ (L)}}{text{Fresh Weight (g)}} = frac{C times 0.025text{ L}}{0.200text{ g}} = C times 0.125$.
| Leaf Sample | A₆₆₃ (Chl a) | A₆₄₅ (Chl b) | A₄₇₀ (Carotenoid) | Chl a (mg/g FW) | Chl b (mg/g FW) | Total Chl | Chl a/b Ratio |
|---|---|---|---|---|---|---|---|
| Sun Leaf (Spinach) | 0.924 | 0.342 | 0.582 | 1.352 mg/g | 0.439 mg/g | 1.791 mg/g | 3.08 (High) |
| Shade Leaf (Money Plant) | 0.648 | 0.395 | 0.412 | 0.896 mg/g | 0.502 mg/g | 1.398 mg/g | 1.78 (Low) |
Sample Calculations for Sun Leaf (Spinach):
$$text{Chl } a = 12.7(0.924) – 2.69(0.342) = 11.735 – 0.920 = mathbf{10.815text{ mg/L}}$$
$$text{Chl } b = 22.9(0.342) – 4.68(0.924) = 7.832 – 4.324 = mathbf{3.508text{ mg/L}}$$
$$text{Total Chl} = 10.815 + 3.508 = mathbf{14.323text{ mg/L}}$$
Per gram fresh tissue ($25text{ mL}$ extract from $0.200text{ g}$ tissue):
$$text{Chl } a = 10.815 times 0.125 = mathbf{1.352text{ mg/g FW}}$$
$$text{Chl } b = 3.508 times 0.125 = mathbf{0.439text{ mg/g FW}}$$
$$text{Chl } a / text{Chl } btext{ Ratio} = frac{10.815}{3.508} = mathbf{3.08}$$
Total Carotenoid content:
$$text{Carotenoids} = frac{1000(0.582) – 1.82(10.815) – 85.02(3.508)}{198} = frac{582 – 19.68 – 298.25}{198} = frac{264.07}{198} = mathbf{1.334text{ mg/L}} implies mathbf{0.167text{ mg/g FW}}$$
Ecological Deduction: The $text{Chl } a/text{Chl } b$ ratio of $3.08$ in spinach reflects sun acclimation with a smaller antenna trimer pool, while the ratio of $1.78$ in money plant reflects shade adaptation with abundant LHCII complexes to harvest scattered blue-green wavelengths.
Determination of Leaf Water Potential ($Psi_w$) via the Chardakov Density-Gradient Falling-Drop Method
Water potential ($Psi_w$) represents the free energy or chemical potential of water per unit volume relative to pure free water at standard atmospheric pressure and temperature ($Psi_w^circ = 0text{ MPa}$). In plant cells, total water potential is governed by the thermodynamic relationship:
$$Psi_w = Psi_s + Psi_p + Psi_m$$
where $Psi_s$ is osmotic (solute) potential (always negative or zero), $Psi_p$ is turgor (pressure) potential (positive in turgid cells, zero at incipient plasmolysis), and $Psi_m$ is matric potential (negligible in fully hydrated vegetative cells).
Chardakov’s Principle of Density Gradients: When plant tissue is incubated in an external sucrose solution:
1. Hypotonic Solution ($Psi_{text{sol}} > Psi_{text{tissue}}$): Water moves down its free energy gradient by osmosis from the solution into the tissue. As water is removed, the external sucrose solution becomes more concentrated and its physical density ($rho$) increases. When a drop of the unincubated control solution (tinted with methylene blue) is released into this test tube, the control drop is lighter and floats upward ($uparrow$).
2. Hypertonic Solution ($Psi_{text{sol}} < Psi_{text{tissue}}$): Water moves out of the tissue into the solution. The solution becomes diluted and its density decreases. The tinted control drop is denser and sinks downward ($downarrow$).
3. Isotonic Solution ($Psi_{text{sol}} = Psi_{text{tissue}}$): There is no net movement of water ($Delta text{Volume} = 0$). The density of the test solution remains identical to the control solution. The tinted drop neither rises nor sinks; it diffuses symmetrically / hovers stationary in place.
At this isotonic equilibrium point, the water potential of the tissue is equal to the osmotic potential of the sucrose solution, calculated via the van ‘t Hoff equation:
$$Psi_w = Psi_s = -i cdot M cdot R cdot T$$
where $i$ is the ionization constant ($1.0$ for non-ionizing sucrose), $M$ is sucrose molarity ($text{mol/L}$), $R$ is the universal gas constant ($0.008314text{ MPa}cdottext{L}cdottext{mol}^{-1}cdottext{K}^{-1}$), and $T$ is absolute temperature in Kelvin ($T = 273.15 + t^circtext{C}$).
- Plant Tissue: Uniform tissue cylinders cut with a $7.0text{ mm}$ cork borer from fresh potato tubers (*Solanum tuberosum*), sliced into uniform $2.0text{ mm}$ discs.
- Graded Sucrose Series: $1.0text{ M}$ primary analytical sucrose stock diluted into 8 graded test solutions: $0.05$, $0.10$, $0.15$, $0.20$, $0.25$, $0.30$, $0.35$, and $0.40text{ M}$.
- Color Indicator: Crystalline Methylene Blue (or $1% w/v$ aqueous solution) to dye the reference series without altering solution density ($< 0.01% w/v$).
- Glassware: Two matched sets of 8 test tubes (Set A: Test tubes with tissue; Set B: Control reference tubes without tissue), test tube racks, and fine-tip Pasteur pipettes.
Label two matched sets of 8 test tubes ($0.05text{ M}$ through $0.40text{ M}$). Dispense precisely $10.0text{ mL}$ of each sucrose solution into the corresponding tubes of both Set A (Experimental) and Set B (Control). Add a trace amount of methylene blue powder (approx. 1 mg) to each tube in Control Set B to impart a clear blue tint.
Cut 80 uniform potato discs ($2text{ mm}$ thickness, $7text{ mm}$ diameter). Rinse briefly in distilled water to wash off broken cell starch and blot dry on filter paper. Place exactly 10 discs into each tube of Experimental Set A. Stopper all tubes and incubate at room temperature ($22^circtext{C}$) for 90 minutes with gentle swirling every 15 minutes.
Remove potato discs from Set A. Draw a small volume of tinted control solution from Tube B (e.g., $0.05text{ M}$) into a fine-tip Pasteur pipette. Immerse the pipette tip halfway into the corresponding Experimental Tube A ($0.05text{ M}$) and carefully release a single tiny blue droplet (~$10 mutext{L}$) into the center of the column. Observe droplet movement against a white background: record whether the drop rises (↑), sinks (↓), or hovers stationary (•).
Identify the exact molarity where the drop remains stationary. If stationary behavior lies between two tested concentrations, record the midpoint. Apply the van ‘t Hoff equation to calculate total leaf/tuber water potential in MegaPascals (MPa).
| Sucrose Molarity (M) | Theoretical Ψ_sol at 22°C (MPa) | Observed Drop Behavior | Direction of Water Flow | Inference |
|---|---|---|---|---|
| 0.05 M | −0.123 MPa | Rapidly sinks ↓ (Fast) | Solution → Tissue (Swelling) | Hypotonic (Ψ_sol > Ψ_tissue) |
| 0.10 M | −0.245 MPa | Sinks ↓ (Moderate) | Solution → Tissue | Hypotonic |
| 0.15 M | −0.368 MPa | Slowly sinks ↓ | Solution → Tissue | Hypotonic |
| 0.20 M | −0.491 MPa | Barely drifts down ↓ | Slight influx | Near equilibrium |
| 0.25 M | −0.613 MPa | STATIONARY • Hovers in place | No Net Movement (ΔV = 0) | ISOTONIC POINT (Ψ_w = Ψ_sol) |
| 0.30 M | −0.736 MPa | Slowly rises ↑ | Tissue → Solution (Efflux) | Hypertonic (Ψ_sol < Ψ_tissue) |
| 0.35 M | −0.859 MPa | Rises ↑ (Moderate) | Tissue → Solution | Hypertonic |
| 0.40 M | −0.982 MPa | Rapidly floats ↑ (Fast) | Tissue → Solution | Hypertonic (Severe Plasmolysis) |
Calculation of Tuber Water Potential:
Isotonic sucrose concentration: $M = 0.25text{ M} = 0.25text{ mol/L}$.
Temperature: $t = 22^circtext{C} implies T = 273.15 + 22 = 295.15text{ K}$.
$$Psi_w = -i cdot M cdot R cdot T$$
$$Psi_w = -(1.0) times (0.25text{ mol/L}) times (0.008314text{ MPa}cdottext{L}cdottext{mol}^{-1}cdottext{K}^{-1}) times (295.15text{ K})$$
$$Psi_w = mathbf{-0.613text{ MPa}} quad (text{or } mathbf{-6.13text{ bar}})$$
Physiological Evaluation: A water potential of $-0.613text{ MPa}$ is typical for well-hydrated storage parenchyma tissue in equilibrium with soil moisture under ambient atmospheric conditions.
Determination of Respiratory Quotient (RQ) Across Substrates Using Ganong’s Respirometer
The Respiratory Quotient ($text{RQ}$) is defined as the volumetric ratio of carbon dioxide ($text{CO}_2$) produced to oxygen ($text{O}_2$) consumed during cellular respiration:
$$text{RQ} = frac{text{Volume of }text{CO}_2text{ evolved}}{text{Volume of }text{O}_2text{ absorbed}} = frac{text{Moles of }text{CO}_2}{text{Moles of }text{O}_2}$$
The numerical value of $text{RQ}$ depends strictly upon the chemical oxidation state of the respiratory substrate:
1. Carbohydrates (Glucose, Starch): Carbohydrates contain equal numbers of carbon and oxygen atoms ($text{C}_ntext{H}_{2n}text{O}_n$). Complete aerobic oxidation requires oxygen solely to burn hydrogen:
$$text{C}_6text{H}_{12}text{O}_6 + 6 text{O}_2 to 6 text{CO}_2 + 6 text{H}_2text{O} implies text{RQ} = frac{6 text{CO}_2}{6 text{O}_2} = mathbf{1.00}$$
Because $text{O}_2$ consumed equals $text{CO}_2$ evolved, there is no net change in internal gas volume in the respirometer manometer.
2. Fats & Lipids (Triacylglycerols, Fatty Acids): Lipids are oxygen-poor and hydrogen-rich macromolecules. Substantial molecular $text{O}_2$ must be imported to oxidize both carbon and hydrogen:
$$2 text{C}_{51}text{H}_{98}text{O}_6 (text{Tripalmitin}) + 145 text{O}_2 to 102 text{CO}_2 + 98 text{H}_2text{O} implies text{RQ} = frac{102 text{CO}_2}{145 text{O}_2} = mathbf{0.703}$$
Because $text{O}_2$ uptake exceeds $text{CO}_2$ release, internal gas pressure drops and the manometer fluid rises.
3. Organic Acids (Malate, Succinate, Citrate): Organic acids are oxygen-rich relative to carbon:
$$2 text{C}_4text{H}_6text{O}_5 (text{Malic Acid}) + 3 text{O}_2 to 8 text{CO}_2 + 6 text{H}_2text{O} implies text{RQ} = frac{8 text{CO}_2}{6 text{O}_2} = mathbf{1.33}$$
Internal gas pressure increases and the manometer fluid falls.
Ganong’s Respirometer Operation: The apparatus consists of a respiration bulb containing germinating seeds connected to a graduated vertical manometer tube and an adjustable leveling tube filled with saturated sodium chloride ($text{NaCl}$) solution (which minimizes $text{CO}_2$ gas dissolution). After recording the net volume change ($Delta V$), a solid pellet or concentrated solution of potassium hydroxide ($text{KOH}$) is introduced to absorb all accumulated $text{CO}_2$ ($2text{KOH} + text{CO}_2 to text{K}_2text{CO}_3 + text{H}_2text{O}$), allowing absolute measurement of total $text{O}_2$ uptake.
- Biological Material: Germinating wheat seeds (*Triticum aestivum*, 24 h imbibed) and germinating castor bean seeds (*Ricinus communis*, 48 h imbibed, testa peeled).
- Ganong’s Respirometer: Standard borosilicate glass apparatus with ground-glass stopcock, $100text{ mL}$ bulb, and $10text{ mL}$ graduated manometer tube ($0.1text{ mL}$ subdivisions).
- Manometer Confining Liquid: Saturated Sodium Chloride ($text{NaCl}$) solution (minimizes $text{CO}_2$ dissolution, unlike pure water which absorbs $text{CO}_2$).
- Absorbing Reagent: $20% (w/v)text{ Potassium Hydroxide}$ ($text{KOH}$) or solid $text{KOH}$ pellets.
- Ancillary Items: Petroleum jelly (vaseline for airtight stopcock sealing) and rubber tubing.
Fill the respirometer manometer and leveling tube with saturated $text{NaCl}$ solution. Place $20$ germinating seeds into the respiration bulb. Apply a thin film of vaseline to the ground-glass stopper and seal the bulb. Turn the stopcock open to atmosphere and adjust the leveling tube until the saline meniscus in the graduated tube aligns precisely with the zero mark ($0.0text{ mL}$). Close the stopcock tightly to isolate the chamber.
Allow the seeds to respire for 2 hours at constant room temperature ($25^circtext{C}$). Level the open leveling tube with the manometer tube to equalize hydrostatic pressure. Record the final meniscus reading ($V_1$, in $text{mL}$).
• For wheat: $V_1 approx 0.0text{ mL}$ (no level change).
• For castor: $V_1 > 0$ (meniscus rises, indicating negative pressure, $V_1 = xtext{ mL}$).
Add $2.0text{ mL}$ of $20%text{ KOH}$ into the leveling tube (or introduce a $text{KOH}$ pellet into the bent bulb). Raise and lower the leveling tube gently several times to draw the saline-KOH solution up the manometer tube without entering the respiration bulb. As $text{KOH}$ absorbs all $text{CO}_2$ gas, the fluid level rises further. Re-level the tube and record the new level ($V_2$, in $text{mL}$).
Calculate:
• Volume of $text{CO}_2$ evolved: $text{Vol}(text{CO}_2) = V_2 – V_1$.
• Volume of $text{O}_2$ absorbed: $text{Vol}(text{O}_2) = V_2$ (when $V_1$ is 0 or subtracted appropriately).
• For lipids: Total $text{O}_2$ absorbed $= V_2$; $text{CO}_2$ evolved $= V_2 – V_1$.
• Compute $text{RQ} = text{Vol}(text{CO}_2) / text{Vol}(text{O}_2)$.
| Seed Specimen | Primary Storage Substrate | Initial Level V_1 (mL) | After KOH V_2 (mL) | Vol CO₂ Evolved (mL) | Vol O₂ Absorbed (mL) | Experimental RQ |
|---|---|---|---|---|---|---|
| Germinating Wheat (*Triticum*) | Starch / Hexose Carbohydrate | 0.00 mL (No shift) | 4.50 mL | 4.50 mL | 4.50 mL | 1.00 ± 0.01 |
| Germinating Castor (*Ricinus*) | Ricinoleic Acid / Triglyceride | 1.85 mL (Rises ↑) | 6.35 mL | 4.50 mL (6.35 − 1.85) | 6.35 mL | 0.709 ± 0.02 |
Mathematical Deduction for Castor Bean:
Total contraction during aerobic run: $V_1 = 1.85text{ mL}$ (meaning $1.85text{ mL}$ more $text{O}_2$ was absorbed than $text{CO}_2$ evolved).
Total contraction after $text{KOH}$ addition: $V_2 = 6.35text{ mL}$ (representing the absolute volume of $text{O}_2$ consumed).
$$text{Volume of }text{CO}_2text{ evolved} = V_2 – V_1 = 6.35 – 1.85 = mathbf{4.50text{ mL}}$$
$$text{Volume of }text{O}_2text{ absorbed} = V_2 = mathbf{6.35text{ mL}}$$
$$text{RQ} = frac{4.50text{ mL}}{6.35text{ mL}} = mathbf{0.709}$$
Bioenergetic Conclusion: The experimental $text{RQ}$ of $1.00$ for wheat confirms pure carbohydrate glycolysis and Krebs cycle catabolism, whereas an $text{RQ}$ of $0.71$ for castor seeds demonstrates $beta$-oxidation of storage triacylglycerols into sucrose via the glyoxylate cycle.
In Vivo Assay of Foliar Nitrate Reductase (NR) Activity & Substrate Inducibility
Nitrate Reductase (NR; EC 1.7.1.1) is the first and rate-limiting enzyme in the plant inorganic nitrogen assimilation pathway. Located in the cytosol, NR is a homodimeric enzyme containing three internal prosthetic electron carriers: FAD, heme ($b_{557}$), and a molybdenum-pterin (Moco) cofactor. It catalyzes the two-electron reduction of nitrate ($text{NO}_3^-$) to nitrite ($text{NO}_2^-$) utilizing cytosolic $text{NADH}$ as the electron donor:
$$text{NO}_3^- + text{NADH} + text{H}^+ xrightarrow{text{Nitrate Reductase (NR)}} text{NO}_2^- + text{NAD}^+ + text{H}_2text{O}$$
In living illuminated leaf tissue, nitrite is rapidly imported into chloroplasts and reduced to ammonium ($text{NH}_4^+$) by light-driven Nitrite Reductase (NiR) utilizing reduced ferredoxin ($Fd_{text{red}}$):
$$text{NO}_2^- + 6 Fd_{text{red}} + 8 text{H}^+ xrightarrow{text{NiR (Chloroplast)}} text{NH}_4^+ + 6 Fd_{text{ox}} + 2 text{H}_2text{O}$$
The In Vivo Assay Strategy: To measure NR activity quantitatively without purifying the labile enzyme:
1. Leaf discs are vacuum-infiltrated with an assay buffer containing $0.1text{ M KNO}_3$ (substrate) and $5% (v/v)text{ 1-propanol}$ (surfactant that transiently permeabilizes cellular membranes, allowing endogenous $text{NADH}$ and exogenous nitrate to reach cytosolic NR while facilitating leakage of produced $text{NO}_2^-$ into the surrounding medium).
2. The incubation is conducted in complete darkness and anaerobic conditions. Depriving chloroplasts of photosynthetic light completely shuts down nitrite reductase (NiR, which requires light-generated ferredoxin), preventing any consumption of nitrite. Consequently, all nitrite produced by NR accumulates quantitatively in the extracellular buffer.
Griess Diazotization Colorimetry: Nitrite ($text{NO}_2^-$) is quantified by the classical two-step Griess reaction:
1. Under acidic conditions ($1.0text{ M HCl}$), nitrite diazotizes sulfanilamide to form a reactive diazonium salt.
2. The diazonium intermediate couples with N-(1-naphthyl)ethylenediamine dihydrochloride (NEDD) to form an intensely pink water-soluble azo dye with a sharp absorption maximum at $lambda = 540text{ nm}$ ($epsilon_{540} = 40,000text{ M}^{-1}text{cm}^{-1}$).
- Plant Material: Fresh primary leaves from 14-day-old maize (*Zea mays*) seedlings, pre-treated with $+10text{ mM KNO}_3$ (Induced) vs $-N$ nutrient solution (Uninduced).
- In Vivo Incubation Buffer: $0.1text{ M Potassium Phosphate}$ ($text{pH } 7.5$), $0.1text{ M KNO}_3$, $5% (v/v)text{ 1-Propanol}$.
- Griess Reagent A: $1.0% (w/v)$ Sulfanilamide dissolved in $3.0text{ M HCl}$.
- Griess Reagent B: $0.02% (w/v)$ N-(1-naphthyl)ethylenediamine dihydrochloride (NEDD) in deionized water (stored in dark amber bottle at $4^circtext{C}$).
- Nitrite Standard Stock: $1.0text{ mM Sodium Nitrite}$ ($text{NaNO}_2$, $69.0text{ mg/L}$), diluted to working standards: $0$, $5$, $10$, $15$, $20$, $25 mutext{M}$.
- Equipment: Vacuum desiccator with vacuum pump, shaking water bath ($30^circtext{C}$), foil-wrapped culture tubes, spectrophotometer ($540text{ nm}$).
Punch 20 uniform leaf discs ($5.0text{ mm}$ diameter) from healthy maize blades using a paper punch. Weigh exactly $0.200text{ g}$ of leaf discs and place into a foil-wrapped tube containing $5.0text{ mL}$ of incubation buffer. Place tubes without caps in a vacuum desiccator and apply vacuum ($70text{ kPa}$) for 3 minutes to displace intercellular air. Release vacuum suddenly; discs should sink to the bottom, confirming complete infiltration.
Cap tubes tightly, wrap completely in two layers of aluminum foil to exclude all photons, and incubate in a dark shaking water bath at $30.0^circtext{C}$ ($60text{ rpm}$) for precisely 60 minutes. Prepare an identical uninduced control tube (leaves grown without nitrate) and a blank tube (incubation buffer without leaf tissue).
Terminate incubation after 60 min. Pipette $1.0text{ mL}$ of the incubation medium into an assay tube. Add $1.0text{ mL}$ of Griess Reagent A ($1%$ Sulfanilamide) and mix. Incubate at room temperature for 5 minutes. Add $1.0text{ mL}$ of Griess Reagent B ($0.02%$ NEDD) and mix thoroughly. A vibrant magenta/pink color develops immediately. Allow 20 minutes for color stabilization.
Measure absorbance at $540text{ nm}$ against the reagent blank. Simultaneously prepare a $text{NaNO}_2$ standard curve ($0–25 mutext{M}$ nitrite). Interpolate nitrite concentration ($mutext{M}$) and calculate specific foliar enzyme activity in $mutext{mol NO}_2^-cdottext{g}^{-1}text{ FW}cdottext{h}^{-1}$.
| Experimental Treatment | A₅₄₀ (OD) | Assay Medium [NO₂−] (μM) | Total NO₂− in 5 mL (μmol) | Foliar FW (g) | NR Activity (μmol/g FW/h) |
|---|---|---|---|---|---|
| Blank (Buffer Alone) | 0.000 | 0.00 μM | 0.000 μmol | — | 0.00 |
| Uninduced (-N Maize) | 0.065 | 2.50 μM | 0.0125 μmol | 0.200 g | 0.625 μmol · g−¹ · h−¹ |
| +KNO₃ Induced Maize | 0.505 | 19.42 μM | 0.0971 μmol | 0.200 g | 4.855 μmol · g−¹ · h−¹ |
Enzyme Activity Calculation:
Nitrite standard calibration equation: $A_{540} = 0.0260 cdot [text{NO}_2^- (mutext{M})] quad (R^2 = 0.9995)$.
For induced sample: $[text{NO}_2^-] = frac{0.505}{0.0260} = mathbf{19.423 mutext{M}} = 19.423 mutext{mol/L}$.
Total nitrite released in $5.0text{ mL}$ ($0.005text{ L}$) incubation buffer:
$$text{Total }text{NO}_2^- = 19.423 mutext{mol/L} times 0.005text{ L} = mathbf{0.0971 mutext{mol}}$$
Specific in vivo NR activity:
$$text{Activity} = frac{0.0971 mutext{mol}}{0.200text{ g FW} times 1.0text{ h}} = mathbf{4.855 mutext{mol NO}_2^-cdottext{g}^{-1}text{ FW}cdottext{h}^{-1}}$$
Nitrogen Induction Factor:
$$text{Induction Ratio} = frac{4.855}{0.625} = mathbf{7.77text{-fold Induction}}$$
Biological Conclusion: Nitrate reductase demonstrates classic substrate-inducible gene expression and post-translational activation in response to soil nitrate availability.
Measurement of Stomatal Transpiration & Conductance ($g_s$) via Cobalt Chloride Porometry
Transpiration is the evaporation of water from plant aerial organs, occurring predominantly ($>90%$) through microscopic stomatal pores on the leaf epidermis. Stomatal movement is driven by changes in guard cell turgor pressure ($Psi_p$), mediated by specialized cell wall architecture and coordinated ion transport:
1. Radial Micellation: Cellulose microfibrils in guard cell walls are oriented radially, fanning outward from the ventral pore wall. This prevents lateral expansion during water uptake, forcing guard cells to lengthen longitudinally.
2. Differential Wall Thickness: The inner (ventral) wall bordering the stomatal pore is thick, rigid, and inelastic, whereas the outer (dorsal) wall is thin and flexible. When guard cell turgor increases, the outer wall stretches and bows outward, pulling the inner wall with it and dilating the central stomatal pore.
3. Proton-Pump Driven Ion Flux: Blue light and low intercellular $text{CO}_2$ activate plasma membrane $text{H}^+$-ATPases, hyperpolarizing the membrane and driving massive $text{K}^+$ influx through inward-rectifying channels, balanced by counter-ion accumulation ($text{Cl}^-$ and synthesized $text{malate}^{2-}$). Osmotic water influx inflates guard cell turgor.
Cobalt Chloride Hygrometric Principle: Anhydrous cobaltous chloride ($text{CoCl}_2$) is deeply blue. Upon exposure to transpirational water vapor, it undergoes hexahydrate coordination, transitioning into pink cobalt chloride hexahydrate:
$$text{CoCl}_2 (text{Anhydrous, Deep Blue}) + 6 text{H}_2text{O} rightleftharpoons text{CoCl}_2 cdot 6 text{H}_2text{O} (text{Hexahydrate, Pale Pink})$$
The transit time ($Delta t$, seconds) required for a calibrated square of $text{CoCl}_2$ paper to reach a standard reference pink tint is inversely proportional to the transpirational water vapor flux ($E$, $text{mmol}cdottext{m}^{-2}cdottext{s}^{-1}$) and stomatal conductance ($g_s$).
- Plant Material: Well-watered, light-adapted potted plants of dorsiventral leaves (*Ficus elastica*, *Hibiscus rosa-sinensis*, or *Spinacia oleracea*).
- Cobalt Chloride Paper: Whatman No. 1 filter paper strips soaked in $3.0% (w/v)text{ CoCl}_2$ in distilled water, dried thoroughly in an oven at $105^circtext{C}$ until deep sky-blue, and preserved in an airtight desiccator over dry silica gel.
- Standard Pink Color Match: Sealed control paper fully exposed to $100%$ relative humidity (standard endpoint match).
- Apparatus: Clean dry microscope glass slides, miniature binder clamps, digital stopwatch ($pm 0.1text{ s}$), compound light microscope with ocular micrometer grid.
Using dry forceps, quickly remove two $1.5text{ cm} times 1.5text{ cm}$ squares of blue $text{CoCl}_2$ paper from the desiccator. Simultaneously place one square against the upper (adaxial) surface and one square against the lower (abaxial) surface of an attached, intact leaf. Cover both papers immediately with clean, dry glass slides and clamp with binder clips to create an airtight seal excluding ambient atmospheric humidity.
Start the stopwatch immediately upon clamping. Watch the color transition through the clear glass slides. Record the precise time in seconds ($Delta t$) required for the blue paper to match the reference pink control tint. Repeat across 5 representative mature leaves under identical ambient conditions ($24^circtext{C}$, $50% text{RH}$, $400 mutext{mol}cdottext{m}^{-2}cdottext{s}^{-1}$ PAR).
Peel the lower and upper epidermises from the same leaf blade. Mount in water on microscope slides. Under $400times$ magnification ($0.15text{ mm}^2$ field of view), count the total number of open stomata across 10 random microscopic fields for both surfaces. Calculate stomatal frequency ($text{stomata / mm}^2$).
Calculate the transpiration index and relative rate ratio between the lower and upper surfaces: $text{Rate Ratio (Lower : Upper)} = Delta t_{text{Upper}} / Delta t_{text{Lower}}$. Compare with the observed stomatal frequency ratio.
| Leaf Epidermal Surface | Transit Time Δt (sec) | Transit Time (min) | Mean Stomatal Density (mm−²) | Stomatal Conductance g_s | Relative Transpiration Rate |
|---|---|---|---|---|---|
| Lower (Abaxial) Surface | 150 ± 12 s | 2.50 min | 185 ± 8 stomata/mm² | 245 mmol · m−² · s−¹ | 4.48 × Higher (81.8%) |
| Upper (Adaxial) Surface | 672 ± 35 s | 11.20 min | 32 ± 4 stomata/mm² | 55 mmol · m−² · s−¹ | 1.00 × (Baseline, 18.2%) |
Comparative Transpiration Index:
$$text{Transpiration Ratio} = frac{Delta t_{text{Upper}}}{Delta t_{text{Lower}}} = frac{672text{ s}}{150text{ s}} = mathbf{4.48 : 1.00}$$
Microscopic stomatal density ratio:
$$text{Stomatal Density Ratio} = frac{185text{ stomata/mm}^2}{32text{ stomata/mm}^2} = mathbf{5.78 : 1.00}$$
Ecophysiological Conclusion: The lower (abaxial) surface transpires nearly $4.5$-fold faster than the upper surface, closely mirroring the higher stomatal frequency. The slight residual transpiration on the upper surface represents a combination of scarce adaxial stomata and minor cuticular transpiration ($E_{text{cut}}$) across the waxy cuticle layer.
Comprehensive Practical Viva-Voce Examination & Plant Physiology Assessment Bank
Essential examination questions testing core biochemical and biophysical understanding of thylakoid photochemistry, Arnon pigment calculations, van ‘t Hoff water potential, respiratory quotients, nitrate assimilation kinetics, and guard cell turgor mechanics:
Ans: Hill proved that the light-dependent evolution of O2 can occur in isolated chloroplasts in the complete absence of CO2 fixation if an artificial electron acceptor is provided, demonstrating that photolysis of water is chemically distinct from carbon assimilation.
Ans: 2,6-dichlorophenolindophenol (DCPIP) is an artificial electron acceptor that intercepts electrons downstream of Photosystem II and plastoquinone. In its oxidized state, it is blue (Amax = 600 nm); upon reduction by 2 electrons and 2 protons, it becomes completely colorless (DCPIPH2).
Ans: DCMU competitively binds to the Q_B plastoquinone-binding niche on the D1 reaction center protein of Photosystem II, preventing electron transfer from Q_A^- to the plastoquinone pool and halting linear electron transport.
Ans: Chl a (mg/L) = 12.7 · A663 – 2.69 · A645; Chl b (mg/L) = 22.9 · A645 – 4.68 · A663; Total Chl = 20.2 · A645 + 8.02 · A663.
Ans: Chlorophyll b possesses an electronegative formyl group (-CHO) at carbon-3 instead of the electron-donating methyl group (-CH3) found in Chlorophyll a, altering macrocyclic conjugation resonance and blue-shifting the red absorption maximum.
Ans: Sun-adaptation: plants grown in high light have fewer peripheral light-harvesting antenna trimers (LHCII, rich in Chl b) per reaction center (rich in Chl a), maximizing photosynthetic capacity while avoiding photodamage.
Ans: Water potential is the chemical potential of water per unit volume relative to pure water (0 MPa). Components: Psi_w = Psi_s + Psi_p + Psi_m, where Psi_s is solute/osmotic potential, Psi_p is turgor pressure potential, and Psi_m is matric potential.
Ans: When plant tissue is placed in sucrose, it exchanges water by osmosis depending on its water potential. If the solution becomes concentrated (tissue absorbed water), a dyed control drop sinks; if dilute, it floats. At isotonic equilibrium (Psi_sol = Psi_tissue), the drop remains stationary.
Ans: Psi_s = -i · M · R · T, where i is the ionization constant (1.0 for sucrose), M is molarity (mol/L), R is the gas constant (0.008314 MPa·L·mol^-1·K^-1), and T is absolute temperature (K).
Ans: RQ = (Volume of CO2 evolved) / (Volume of O2 absorbed) during respiration. It indicates the chemical nature of the metabolic substrate being oxidized.
Ans: Glucose (C6H12O6) contains equal C and O atoms, so O2 is needed only for hydrogen (6 CO2 / 6 O2 = 1.0). Lipids (e.g. tripalmitin C51H98O6) are oxygen-poor and require extra atmospheric O2 to burn both carbon and hydrogen (102 CO2 / 145 O2 = 0.70).
Ans: Lipid oxidation consumes more O2 than the CO2 it produces (RQ = 0.7). The net loss of gas volume creates negative pressure, drawing the manometer fluid upward.
Ans: NR is the cytosolic, rate-limiting enzyme that catalyzes the 2-electron reduction of nitrate to nitrite: NO3^- + NADH + H^+ -> NO2^- + NAD^+ + H2O, using FAD, heme, and a molybdenum-pterin (Moco) cofactor.
Ans: Darkness deprives chloroplasts of light, deactivating Nitrite Reductase (NiR, which requires light-reduced ferredoxin). This prevents any downstream consumption of nitrite, allowing all NO2^- produced by NR to accumulate for colorimetric measurement.
Ans: In acidic conditions, nitrite diazotizes sulfanilamide into a diazonium salt, which couples with N-(1-naphthyl)ethylenediamine (NEDD) to form an intensely pink azo dye absorbing maximally at 540 nm.
Ans: Blue light activates plasma membrane H^+-ATPases, pumping protons out to hyperpolarize the guard cell membrane. This opens voltage-gated inward K^+ channels, driving K^+, Cl^-, and malate^2- influx, lowering osmotic potential and causing water influx and turgor-driven opening.
Ans: Cellulose microfibrils wrap radially around the guard cell circumference. When turgor pressure increases, the cells cannot expand laterally and are forced to elongate; the thick inelastic inner pore wall bows outward, opening the stomatal aperture.
Ans: Anhydrous cobalt chloride (CoCl2) is sky-blue. As it absorbs transpirational water vapor, it hydrates into cobalt chloride hexahydrate (CoCl2·6H2O), which is pale pink.
Ans: Dorsiventral leaves are hypostomatous or predominantly amphistomatous, concentrating most stomata on the shaded lower (abaxial) epidermis to minimize direct solar heating while maximizing gas exchange.
Ans: Drought stress triggers ABA synthesis, which binds PYR/RCAR receptors to activate OST1 kinase. This activates S-type anion channels (SLAC1), depolarizing the membrane and opening outward K^+ channels (GORK), driving massive ion and water efflux that collapses guard cell turgor.
