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

Abstract

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/m², 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, <20% of the PLA was contributed from leaves originating from branches, compared with about 40% in phasey bean and about 75% in vigna. The slope of the linear component of the relationship between PLA and PI in sesbania (85 cm²) was slightly less than the maximum leaf area per node recorded. The comparable slope for phasey bean was 57 cm², being greater than the area of any individual leaf (40–50 cm²) because of the greater contribution of leaves originating from branches. In vigna, the very large contribution of leaves originating from branches to PLA, together with the slower DIPI for that species, resulted in the relationship between PI and PLA being much steeper than in either of the other species. 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 successfully predicted for these 3 tropical legumes at the set plant density. The successful prediction of PLA for legumes with a high degree of branching is of special significance, as it indicates that these methods may have application in predicting leaf growth in legumes which are being grazed and which have a number of active growing points. Nevertheless, the stability of the derived parameters needs to be assessed under a range of plant densities before they can be widely used to predict leaf area development in ley legumes.