Photosynthesis, the process by which plants convert light energy into chemical energy, relies on three fundamental components: carbon dioxide, light, and water. Plants use light energy to convert carbon dioxide and water into glucose, a sugar used for fuel, and release oxygen as a byproduct. Water is a non-negotiable reactant in this biochemical pathway, and its availability is a primary limiting factor that dictates the overall productivity of a plant. When the amount of water is insufficient, the plant’s ability to manufacture its own food is impaired.
Water’s Essential Chemical Role in Photosynthesis
Water is a direct participant in the light-dependent reactions, occurring within the thylakoid membranes of the chloroplasts at Photosystem II (PSII). When light strikes the chlorophyll molecules in PSII, it excites electrons to a higher energy level, which are then passed along an electron transport chain.
This electron loss leaves the chlorophyll molecule oxidized and unstable. To restore stability, photolysis occurs, where an enzyme complex splits water molecules. The water molecule is cleaved into electrons, hydrogen ions (protons), and molecular oxygen. The electrons are immediately donated to PSII, allowing the light reactions to continue.
The hydrogen ions accumulate inside the thylakoid, creating a concentration gradient that powers the synthesis of adenosine triphosphate (ATP). The electrons and protons ultimately contribute to forming NADPH, another energy-carrying molecule. Without a continuous water supply, the flow of electrons ceases, the production of ATP and NADPH halts, and the entire photosynthetic process stops.
How Water Scarcity Triggers Stomatal Regulation
The primary impact of water scarcity on photosynthesis is the plant’s response to conserve moisture. Plants lose water through transpiration, the evaporation of water vapor through tiny pores on the leaf surface called stomata. Stomata must open to allow the intake of carbon dioxide (\(\text{CO}_2\)) from the atmosphere for the Calvin cycle, but this opening also allows water vapor to escape.
When water availability decreases, the roots sense the stress and produce the hormone abscisic acid (ABA), which signals the guard cells surrounding the stomata to close. This response prioritizes water retention over carbon fixation. The guard cells lose turgor, causing the stomatal pore to shrink or fully close, minimizing water loss.
While this conserves water, it reduces the amount of \(\text{CO}_2\) entering the leaf, leading to \(\text{CO}_2\) starvation within the plant tissue. With insufficient \(\text{CO}_2\), the Calvin cycle, which uses the ATP and NADPH generated by the light reactions, slows down. The photosynthetic rate declines rapidly. Stomatal closure is the primary factor limiting photosynthesis under mild to moderate drought conditions.
Long-Term Impact of Severe Water Stress
If water stress becomes severe and stomata remain closed for prolonged periods, the plant experiences non-stomatal limitations that damage the photosynthetic apparatus. With the Calvin cycle stalled due to \(\text{CO}_2\) deprivation, the plant still absorbs light energy, but that energy has nowhere to go. ATP and NADPH are not consumed, leading to a buildup of electrons in the electron transport chain.
This excess energy and electron buildup can damage the photosynthetic machinery, a phenomenon known as photoinhibition. Photosystem II is vulnerable, as the core D1 protein, responsible for water splitting, is prone to damage, and its repair mechanism is inhibited by stress. The over-reduction of the electron transport chain also leads to the production of reactive oxygen species (ROS).
ROS, such as superoxide and hydrogen peroxide, cause oxidative stress, damaging proteins, lipids, and DNA within the cell, including the chloroplasts. This cellular damage impairs the efficiency of the light-dependent reactions. The physical damage to the chloroplast structure is difficult to reverse, leading to a sustained reduction in the plant’s capacity for carbon fixation even after water is restored.
Plant Strategies for Water Conservation
In environments where water scarcity is common, plants have evolved specialized photosynthetic pathways to mitigate the negative effects of stomatal closure. These adaptations optimize \(\text{CO}_2\) capture while prioritizing water loss prevention.
C4 Photosynthesis
The C4 photosynthetic pathway, used by plants like corn and sugarcane, spatially separates the initial \(\text{CO}_2\) fixation from the Calvin cycle. C4 plants initially fix \(\text{CO}_2\) in outer mesophyll cells using an enzyme efficient even at low \(\text{CO}_2\) concentrations. The resulting four-carbon molecule is then transported to specialized bundle sheath cells, where \(\text{CO}_2\) is released and concentrated around the Calvin cycle enzyme. This spatial separation allows the plant to maintain high photosynthetic rates with partially closed stomata, reducing water loss while avoiding \(\text{CO}_2\) starvation.
Crassulacean Acid Metabolism (CAM)
Crassulacean Acid Metabolism (CAM) photosynthesis, found in cacti and succulents, employs a temporal separation. CAM plants only open their stomata at night when temperatures are cooler and humidity is higher, significantly reducing water loss. They fix atmospheric \(\text{CO}_2\) at night and store it as an organic acid, primarily malate. During the day, the stomata close, and the stored \(\text{CO}_2\) is released internally to fuel the Calvin cycle. This mechanism enables high water-use efficiency, allowing these plants to thrive in arid desert environments.