Environmental Conditions Can Affect Foliar Absorption

Plants undergo a variety of changes in response to changing environmental conditions. On an overcast day, the amount of light energy available for photosynthesis can be reduced by as much as 50%. Plants will increase leaf area and produce more chlorophyll in order to maximize photosynthesis under low light conditions. Leaves will orient parallel to the ground to capture any available light. If other growing conditions are favorable (temperature and moisture), all metabolic pathways will be active and sugars produced during photosynthesis will translocate to areas of active growth (young leaves, roots and shoot meristems). During periods of moisture stress, plants respond in very different ways. Moisture-stressed plants tend to minimize their leaf surface area and orient leaves perpendicular to the ground to reduce light interception and temperature. Plants also conserve moisture by closing their stomata, while this reduces water loss it also reduces photosynthesis because carbon dioxide is not absorbed. When photosynthesis is reduced many other metabolic pathways slow or stop completely. The demand for chlorophyll is greatly reduced and translocation of sugars will be limited because of reduced photosynthesis in source leaves resulting in reduced growth of young leaves and root and shoot meristems.

Changes in composition of leaf cuticle have also been observed when plants undergo moisture stress, with more wax being produced under water stress conditions. The cuticle can swell or shrink depending on relative humidity and the moisture status of the plant. When the cuticle shrinks due to moisture stress wax units in the cuticle are pulled closer together, decreasing permeability of polar herbicides (Kogan and Bayer 1996).