Bc. Pengelly et al., Predicting leaf area development in response to temperature in three tropical annual forage legumes, AUST J AGR, 50(2), 1999, pp. 253-259
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.