Crop characteristics and the ratio between assimilation and transpiration.
A simple model is used to calculate transpiration and assimilation rates for leaves or canopies. This is then used to investigate interactions between the various crop and environmental parameters that determine the 'assimilation ratio' (the ratio between assimilation and transpiration). The output from the model is discussed in relation to breeding for drought tolerance. It is concluded that the plant breeders' ideotypes will depend on the particular climate in which a crop is grown. Selection for specific characters, including high photosynthetic activity, low respiration rates, and certain morphological attributes such as leaf size or disposition, could all lead to increased assimilation ratios. The greatest scope for maximizing the assimilation ratio, particularly when measured over the whole growth period of a crop, may be offered by the selection for stomatal behaviour appropriate to the expected environment. Figures are presented which provide estimates of the relative effects of various plant attributes on the assimilation ratio in particular situations.