Q.How does water stress decrease the rate of photosynthesis ?

Factors Affecting Photosynthesis

Think of photosynthesis as a factory inside the leaf. The raw materials are carbon dioxide and water, the energy comes from sunlight, and the factory floor is kept running by enzymes that work best at a certain temperature. If any one of these inputs runs short, the entire production line slows down — no matter how much of the other inputs you supply.

That single idea — that the factor in shortest supply controls the rate — is the heart of the whole topic.

The Four Major Factors

Light provides the energy. More light generally means a faster rate, but only up to a point. At very high intensities, the rate plateaus because the plant's machinery is already working at full capacity. Beyond that, excess light can actually damage the photosynthetic apparatus (photoinhibition), though that's a more advanced detail.

Carbon dioxide is the carbon source. Normal air has about 0.03–0.04% CO₂. If you increase it, the rate rises — again, until something else becomes limiting. In many plants, CO₂ is the most common limiting factor in bright sunlight.

Temperature affects the enzymes, especially RuBisCO, which fixes CO₂. The rate increases with temperature up to an optimum (typically 25–35°C for most crops), then drops sharply as enzymes denature. This is why photosynthesis nearly stops on a scorching summer afternoon.

Water is less direct. Water shortage causes stomata to close to prevent transpiration loss, which simultaneously cuts off CO₂ entry. So the real effect of water stress is often a CO₂ shortage inside the leaf.

Blackman's Law of Limiting Factors

This is the precise statement you need. In 1905, F.F. Blackman proposed that when a process depends on several factors, its rate is limited by the factor that is in the minimum supply relative to the plant's needs.

This is not a vague idea — it is a testable, quantitative principle. If you plot rate against one factor (say, light intensity) while keeping all others constant, you get a curve that rises linearly then flattens. The flat part means some other factor (usually CO₂ or temperature) has become limiting. …