MODELING GENOTYPIC AND ENVIRONMENTAL-CONTROL OF LEAF-AREA DYNAMICS IN GRAIN-SORGHUM .3. SENESCENCE AND PREDICTION OF GREEN LEAF-AREA

This is the third paper in a series that examines the genotypic and environmental controls of leaf area dynamics in grain sorghum. In this paper, genotypic variation in leaf area senescence was firstly quantified both at the whole plant level and at the level of individual leaves. Green leaf area was then simulated at both levels, and predictions were compared with independent data on crop leaf area index. At the whole plant level, senesced plant leaf area (SPLA) was modelled as a logistic function of thermal time after emergence, where the asymptote of the relationship is determined by maximum leaf area produced per plant. At the level of individual leaves, where leaf size was predicted, SPLA was simulated by a constant rate of senescence of individual leaves. The relationships at the two levels of simulation were both affected by different leaf numbers per plant. While there were genotypic differences in the senescence of sorghum leaf area, the genotypic control of sorghum leaf area dynamics was generally less influential than the effect of the environment. Relationships to predict SPLA were added to those to predict total plant leaf area (TPLA), previously developed in this series, to enable prediction of green leaf area development of sorghum crops. There was little difference in the accuracy with which the models at the whole plant and individual leaf levels simulated independent data. The model at the whole plant level was simpler and required less initial information. The model at the level of individual leaves was more complex and needed greater information, but provided a more detailed treatment of leaf area dynamics. The choice between the two approaches will depend to a large degree on the demands of the study for which a sorghum leaf area model is to be applied.