Factors Affecting Photosynthesis Environmental Factors

Although it is obvious that light affects photosynthesis, soil moisture and temperature also have a role. Water stress not only reduces photosynthesis per unit leaf area but also reduces leaf size, so overall capacity is reduced. In extreme situations, premature leaf abscission occurs. Stomates can close in response to low humidity. Wind can alter leaf water loss by affecting the boundary layer and by causing an increase in transpiration. Excess soil moisture affects trees by reducing oxygen to the roots and possibly by increasing the accumulation of soil carbon dioxide. Apple transpiration, photosynthesis, leaf growth, and root growth can be reduced by several days of flooding. The sensitivity to flooding appears to be related to periods of most active growth.

Lakso (1994) reports that photosynthesis of apple leaves does not have a strong response to temperatures over a fairly wide range— from 15 to 35°C, with the optimum generally near 30°C. Temperatures of 37°C and above are deleterious to photosynthesis. Frosts in the fall decrease photosynthesis and, if severe, will kill the leaves. Growing areas with long periods of frost-free conditions following apple harvest tend to have higher fruit yields and size because of increased storage reserves and bud development during this period.

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