Light is the primary environmental factor driving the movement of water through a plant, a process known as transpiration. Transpiration is the loss of water vapor, mainly from the leaves, and is an unavoidable consequence of the gas exchange required for photosynthesis. Light influences this water movement in two ways: by providing a signal that triggers physiological changes in the leaf, and by delivering energy that drives evaporation. Understanding these roles is necessary to comprehend how light dictates the overall rate of water loss from a plant.
The Mechanism of Water Movement in Plants
Water movement in plants begins with uptake from the soil by the roots, then transported upwards through specialized vascular tissue called the xylem. This upward flow is driven by the cohesion-tension theory, where evaporation from the leaves creates a negative pressure, or tension, that pulls the continuous column of water up from the roots. Cohesive forces between water molecules, and their adhesion to the xylem walls, prevent this column from breaking under tension.
The ultimate exit point for water vapor is through microscopic pores on the leaf surface called stomata, which are bordered by a pair of specialized cells known as guard cells. These guard cells function as hydraulic valves, controlling the size of the stomatal pore. Stomata are necessary for allowing carbon dioxide to diffuse into the leaf for photosynthesis, but the water vapor loss is substantial, with over 97% of the water absorbed by the roots being lost through this process.
Light’s Direct Control Over Stomata
The most immediate effect of light on transpiration is the direct physiological signal it sends to the guard cells to open the stomatal pore. This process involves photoreceptors that detect certain wavelengths of light. Blue light is perceived by phototropins, which are protein kinases located within the guard cell membranes.
The activation of phototropins triggers a cascade of biochemical events, including the activation of a plasma membrane proton pump (H+-ATPase). This pump actively moves hydrogen ions out of the guard cells, creating an electrochemical gradient that drives the rapid influx of potassium ions and other solutes into the cells. The increase in solute concentration lowers the water potential inside the guard cells, causing water to rush in via osmosis. This influx of water increases the internal fluid pressure, or turgor, which swells the guard cells and pulls the stomatal pore open for carbon dioxide uptake.
How Light Intensity Governs the Rate
Beyond the initial signal to open, the quantity of light, or its intensity, exerts strong quantitative control over the rate of transpiration. As light intensity increases, the stomata open wider to maximize carbon dioxide uptake, leading to a faster rate of water loss. This direct relationship is observable across a wide range of light levels, as a brighter environment supports a higher potential for photosynthesis.
However, this increase in the transpiration rate is not indefinite, and the effect plateaus once the light saturation point is reached. At this point, the stomata are fully open, or other factors, such as water availability or internal carbon dioxide concentration, become the limiting constraints on the process. Further increases in light intensity beyond the saturation point do not lead to a higher rate of water loss. The duration of light exposure, known as the photoperiod, also dictates the total time available for transpiration to occur during a given day.
The Indirect Heating Effect of Solar Energy
Light also affects the transpiration rate through a physical, indirect mechanism: the conversion of solar energy into heat. Solar radiation that strikes the leaf surface is absorbed and converted to thermal energy, which raises the leaf’s temperature. This increase in temperature affects the rate at which water evaporates from the moist cell walls inside the leaf’s air spaces.
The warmer the leaf becomes, the higher the concentration of water vapor inside the leaf’s intercellular spaces. This creates a steeper gradient between the high vapor concentration inside the leaf and the lower concentration in the surrounding atmosphere, a difference known as the vapor pressure deficit. This steep gradient accelerates the diffusion of water vapor out through the open stomata. Even with the stomata already open due to the direct light signal, the sun’s thermal energy drives a faster rate of water loss by increasing the potential for evaporation.