VARIATION IN YIELD OF NARROW-LEAFED LUPIN CAUSED BY TERMINAL DROUGHT

Narrow-leafed lupin is a. rain-fed crop in southern Australia whose yi elds are limited by the amount and distribution of rainfall. Drought t erminates the growing season, and the timing and intensity of developm ent of this (terminal) drought is a likely cause of much yield variabi lity. We investigated this yield variability by manipulating terminal drought with trickle irrigation. Despite comparatively high crop dry m ass 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 drou ght and thus prolonging the period for crop ripening raised crop dry m ass 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 da ys after sowing (DAS) but appreciable pod filling did not start until 140 DAS, by which time, in the unirrigated treatment, only about 15% o f the plant-available soil water remained, leaf diffusive conductance had fallen by 75%, and leaf water potential was -1.7 MPa. Leaf senesce nce and abscission had already begun; in the control treatment only 25 % of the green area remained at 150 DAS, when net vegetative growth ce ased and appreciable seed filling began. During irrigation, plant-avai lable soil water was maintained at 35-70% of that at field capacity, w ith correspondingly better plant water relations. Pod and seed filling started at the same times as in the unirrigated control, and vegetati ve growth ceased at the same time. However, green area declined more s lowly and reproductive growth continued for longer and at a faster rat e. Yields were highly correlated with the number of productive pods (a nd seeds) at maturity, which in turn was associated with pod survival rather than pod set. Irrigation increased the number of surviving pads and seeds, both of which sometimes aborted at comparatively late stag es of filling. Average seed weights were stable across treatments, exc ept where there was a period of drought before irrigation, in which ca se seeds were heavier, compensating for prior loss of pods. It is argu ed that a better ideotype for the Mediterranean environment of souther n Australia would switch to reproductive growth earlier, before severe water deficit develops, and with less overlap between vegetative and reproductive growth.