Variation in yield of narrow-leafed lupin caused by terminal drought

Abstract

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.