Predicting leaf area development in response to temperature in three tropical annual forage legumes

Ley legumes are an important option in nitrogen (N) management in cropping–livestock systems in Australia’s northern cropping region. The complexity of managing N and water in these systems is increasingly being aided by the use of crop models which require quantitative estimates of the various components of growth. A study to predict plant leaf area (PLA) of 3 tropical forage legumes as a function of temperature under irrigated conditions was conducted at Gatton, south-eastern Queensland. The legumes were phasey bean [ Macroptilium lathyroides (L.) Urban.], vigna [ Vigna trilobata (L.) Verdc.], and sesbania ( Sesbania cannabina Retz.). The PLA was to be predicted by establishing a relationship between plastochron index (PI) and accumulated thermal time, and then between PLA and PI. Daily rate of increase in PI (DIPI) was strongly related to accumulated thermal time in all species, with DIPI higher for phasey bean (0.51 degree-days) and sesbania (0.54 degree-days) than for vigna (0.14 degree-days). At a plant density of 50 plants/m 2 , the relationship between PI and PLA was largely dependent upon the proportion of PLA originating from branches rather than from the main stem. In sesbania,