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Leaf area dynamics are controlled by genotypic and environmental influences. In this series of papers, general models of leaf area dynamics of uniculm and tillering sorghum (Sorghum bicolor (L.) Moench) at the whole plant and individual leaf levels were developed to examine and quantify both genotypic and environmental controls. Green leaf area was modelled by examining leaf area production and senescence separately. Data from field experiments involving broad ranges of hybrids and environments were collated and analysed. Crops were grown with adequate water and nutrient supply.

In this paper, a general framework to model leaf area production at the whole plant level is presented. Accumulation of total plant leaf area (TPLA) (without losses due to senescence) was simulated using a logistic function of thermal time (TT) from emergence and it increased to its maximum value (TPLA(max)) shortly before flowering. To calculate TT, base, optimum and maximum temperatures of 11, 30 and 42-degrees-C respectively were derived by examining the effect of temperature on rate of appearance of fully expanded leaves. Hence, TT incorporated the major effect exerted on TPLA by temperature. Most remaining genotypic and environmental variation in TPLA was related to variation in TPLA(max). Values of TPLA(max) were predicted from total leaf number on the main culm (TLN) and fertile tiller number per plant (FTN) by allowing for a curvilinear increase in TPLA(max) with TLN and a sequential decrease in total leaf area produced by successive surviving tillers relative to that on the main culm. The potential genotypic and environmental controls on TPLA, introduced via factors affecting TLN and FTN, were considered. For seven hybrids grown in eight environments (locations and times of planting), this simple general model accounted for 93% of observed variation in TPLA with time, with a root mean square deviation of 664 cm2 for observed values of TPLA ranging from 161 to 11 302 CM2.