Influence of radiation and humidity on transpiration: Implications for calcium levels in tomato leaves

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Abstract
The transpiration of a leaf can be described by the Penman-Monteith equation as the sum of two terms, one giving the transpiration driven by the absorption of radiation and the other by diffusion to the unsaturated ambient air. Both terms for this relation may readily be calculated by a computer program. A model is developed to predict the energy absorbed from the incident radiation flux by a canopy and by young developing leaves for tomato. Solar radiation has a greater effect on the upper parts of the plants because they are exposed to the full incident light plus that reflected from below whereas lower parts of the canopy are progressively shaded. Humidity tends to act over the complete canopy. Hence a greater change in humidity would be required to maintain transpiration from the upper leaves at reduced irradiance than from the lower ones. The accumulation rate of calcium would be closely linked to the transpiration. Since calcium will not, in general, be redistributed subsequently, the model can explain the deficiency in calcium levels in leaves that develop during periods of low solar intensity and high humidity. It provides a basis for compensating by lowering the humidity of the environment by use of an evaporative equivalence factor relating vapour pressure deficit to radiation incident on the crop. It is estimated that the rate of transpiration should be at least 2.2 times the rate of leaf expansion to prevent any appreciable yield loss from calcium deficiency and that a total of 8.7 MJ m-2 d_l of radiation and of radiation equivalent would be required, 0.04 kPa vapour pressure deficit compensation per MJ m_2 d_1 shortfall.

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