Narrow-leafed lupin is a rain-fed crop in southern Australia whose yields are
limited by the amount and distribution of rainfall. Drought terminates the
growing season, and the timing and intensity of development of this
(terminal) drought is a likely cause of much yield
variability. We investigated this yield variability by manipulating terminal
drought with trickle irrigation. Despite comparatively high crop dry mass of
nearly 9 t/ha in the absence of irrigation, seed yield was only 1·2
t/ha with a harvest index of only 14%. Delaying the terminal
drought and thus prolonging the period for crop ripening raised crop dry mass
by 18-42%, with large increases in harvest index and seed yields, which
rose by 45-75% and 95-135%, respectively.
Pod set started 110 days after sowing (DAS) but appreciable pod filling did
not start until 140 DAS, by which time, in the unirrigated treatment, only
about 15% of the plant-available soil water remained, leaf diffusive
conductance had fallen by 75%, and leaf water potential was -1·7
MPa. Leaf senescence and abscission had already begun; in the control
treatment only 25% of the green area remained at 150 DAS, when net
vegetative growth ceased and appreciable seed filling began. During
irrigation, plant-available soil water was maintained at 35-70% of that
at field capacity, with correspondingly better plant water relations. Pod and
seed filling started at the same times as in the unirrigated control, and
vegetative growth ceased at the same time. However, green area declined more
slowly and reproductive growth continued for longer and at a faster rate.
Yields were highly correlated with the number of productive pods (and seeds)
at maturity, which in turn was associated with pod survival rather than pod
set. Irrigation increased the number of surviving pods and seeds, both of
which sometimes aborted at comparatively late stages of filling. Average seed
weights were stable across treatments, except where there was a period of
drought before irrigation, in which case seeds were heavier, compensating for
prior loss of pods.
It is argued that a better ideotype for the Mediterranean environment of
southern Australia would switch to reproductive growth earlier, before severe
water deficit develops, and with less overlap between vegetative and
reproductive growth.
Risk Management Potential of Precision Farming Technologies