Both diffusional and non-diffusional limitation factors and their contribution to the phenomenon known as midday depression were studied; a decrease in photosynthesis due to environmental stress. Measurements of leaf gas exchange (transpiration rate, stomatal conductance, photosynthetic rate) and related parameters (intercellular CO2 concentration, maximum carboxylation, diffusional limitation index, leaf water potential) were determined over time from cucumber plants under constant environmental conditions. The experiment may be divided into three periods. Period 1 occurred when both photosynthetic rate and stomatal conductance increased, period 2 occurred when photosynthetic rate decreased but stomatal conductance remained constant, period 3 occurred with the decrease in both photosynthetic rate and stomatal conductance. Photosynthetic rate increased immediately, post-illumination, and reached its maximum value during period 1, then it decreased to half this value for the remainder of the experiment, indicating that a midday depression had occurred. During period 2, diffusional limitation index was around 50% and then increased while maximum carboxylation sharply decreased, suggesting the existence of both non-diffusional and diffusional limitations. In period 3, stomatal conductance decreased due to decreasing leaf water potential, and diffusional limitation index increased, suggesting that diffusional limitation was dominant at this stage. Thus, it was found that there are multiple dominant factors in midday depression, and that these factors are dynamic even under constant environmental conditions.
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