Q29 - Water Stress, Photosynthesis and Turgor

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Water Transport & Transpiration Plant Stress Physiology & Defense

Water deficit (insufficient water availability) occurs in most natural and agricultural habitats and is caused mainly by periods without precipitation. When plant cells experience water deficit, cell dehydration occurs. Cell dehydration leads to plant water stress, which adversely affects many basic physiological processes.

Figure 1. Plot with leaf water potential (MPa) on the x-axis (becoming more negative to the right). Curve A (leaf expansion, blue) drops sharply at mild water stress near -0.4 MPa, while Curve B (photosynthesis, red) declines more gradually at more negative water potentials. Y-axis (left) = leaf expansion rate (% increase in leaf area per 24h), Z-axis (right) = photosynthesis rate. Figure 1. Effects of water stress on photosynthesis and leaf expansion of sunflower. X = Leaf water potential (MPa), A = Leaf expansion, B = Photosynthesis.

Sunflower plants were grown either with ample water, or with limited soil water to produce mild water stress. After re-watering, plants of both treatment groups were stressed by the withholding of water, and leaf growth rates (GR) and turgor (P) were periodically measured.

Figure 2. Plot with turgor (MPa) on the x-axis (B) and leaf growth rate on the y-axis (A). Two lines are shown: Plant C (black triangles) has a steeper slope and higher x-intercept (Y). Plant D (red dots) has a shallower slope and lower x-intercept (Y). Y marks the turgor threshold below which growth ceases. Figure 2. Leaf growth rate (A) vs turgor (B) for Plant C and Plant D. Y = x-intercept (yield threshold).

Q29.1. Photosynthetic rate is not as responsive to mild water stress as leaf expansion, because photosynthesis is much less sensitive to turgor than is leaf expansion.

Q29.2. As stomata close during early stages of water stress, water-use efficiency may increase because stomatal closure inhibits transpiration more than it decreases intercellular CO2 concentrations.

Q29.3. Growth slows down when stress reduces turgor. Turgor needs to decrease to zero to eliminate leaf expansion.

Q29.4. Plant 1 was pre-stressed by growing it with limited soil water to produce mild water stress. Plant 2 was grown with ample water.

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF