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 o f managing N and water in these systems is increasingly being aided by the use of crop models which require quantitative estimates of the various comp onents of growth. A study to predict plant leaf area (PLA) of 3 tropical fo rage legumes as a function of temperature under irrigated conditions was co nducted 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 es tablishing a relationship between plastochron index (PI) and accumulated th ermal time, and then between PLA and PI. Daily rate of increase in PI (DIPI) was strongly related to accumulated the rmal time in all species, with DIPI higher for phasey bean (0.51 degree-day s) 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 la rgely dependent upon the proportion of PLA originating from branches rather than from the main stem. In sesbania, <20% of the PLA was contributed from leaves originating from branches, compared with about 40% in phasey bean a nd about 75% in vigna. The slope of the linear component of the relationshi p between PLA and PI in sesbania (85 cm(2)) was slightly less than the maxi mum leaf area per node recorded. The comparable slope for phasey bean was 5 7 cm(2), being greater than the area of any individual leaf (40-50 cm(2)) b ecause of the greater contribution of leaves originating from branches. In vigna, the very large contribution of leaves originating from branches to P LA, together with the slower DIPI for that species, resulted in the relatio nship between PI and PLA being much steeper than in either of the other spe cies. The parameters developed for each species were successfully validated using independent data from other experiments in south-eastern Queensland. Increase in PLA in response to accumulated thermal time can be successfull y predicted for these 3 tropical legumes at the set plant density. The succ essful prediction of PLA for legumes with a high degree of branching is of special significance, as it indicates that these methods may have applicati on in predicting leaf growth in legumes which are being grazed and which ha ve a number of active growing points. Nevertheless, the stability of the de rived parameters needs to be assessed under a range of plant densities befo re they can be widely used to predict leaf area development in ley legumes.