Variability of harvest index in four grain legume species

Grain legumes have the potential to be high yielding crops in Canterbury. However, like most grain legumes the problem of harvest index (HI) variability in Cicer arietinum, Lens culinaris, Lupinus angustifolius and Pisum sativum hinders the realisation of their full potential yield. In 1998/99, the four species were sown at four populations, viz. lentils cv. Rajah (15, 150, 300 and 600 plants m⁻²), desi chickpeas (5, 50, 100 and 200 plants m⁻²), lupins cv. Fest and field peas cv. Beacon (10, 100,200 and 400 plants m⁻²). In 1999/2000, all species were grown at the same three plant populations (10, 100 and 400 plants m⁻²) and at three sowing depths (2, 5 and 10 cm). Also a central plant was treated with N surrounded by other four plants to determine neighbouring plant effects, under ideal conditions. Agronomic and physiological parameters were measured to identify the causes of HI variability in grain legumes in relation to season, different populations, sowing depths and N application. Seed yield depended on the interaction between species and plant population, and species and sowing depth at harvest maturity. The highest potential seed yield of > 6.5 t ha⁻¹ in chickpeas and lupins, and about 4.5 t ha⁻¹ in lentils and peas was recorded at the higher plant populations. Seed yield was doubled at the higher populations compared to the lowest population. Trends were similar for total dry matter (TDM) production. The increase in seed yield due to population was a function of greater TDM production, crop HI (CHI), pods m⁻² and mean seed weight. The CHI values were variable and ranged from 0.31 to 0.66 over species and populations across seasons. Averaged over both seasons, the CHI was 0.59 in lentils and 0.49 in lupins, while chickpeas and peas were intermediate with 0.54 and 0.56, respectively. Sowing depth contributed less to variation in yield and yield components. Seed yield and CHI were strongly correlated both seasons. The number of pods m⁻² was the major yield component and correlated nearly perfectly with seed yield and CHI in both trials. Green area index (GAI), intercepted radiation (Fᵢ) radiation use efficiency (RUE) and total seasonal intercepted PAR significantly (P < 0.001) increased with increased plant population. Lupins produced the greatest maximum biomass and intercepted more radiation than the other three legumes. In all species in both trials the highest populations reached their peak GAI about 7 - 10 days earlier than the low populations. The cumulative intercepted PAR had significant and positive association with seed yield and CHI. The RUE varied widely within and among species and populations. The RUE increased as plant population increased and was highest in the greatest yielding legume species. However, RUE was inversely related to the extinction coefficient (k) and was strongly associated with CHI. The relationships among individual plants were studied to quantify the influence of neighbouring plants using the principal axis model (PAM). The PAM was based on a principal axis, which represented the linear relationship (r²> 0.90) between SWT and PWT. A negative SWT-axis intercept was confirmed for each species in this study. Both mean SWT and PWT were inversely related to plant population. Yield, yield components and plant HI (PHI) were decreased significantly in the southerly plant compared with the N treated and with control plants. The PAM took into account the performance of individual plants and identified the variability and lowest or highest yielding plants. The relationship between PHI and PWT was asymptotic. The minimum plant weight (MPW) was species dependent and was highest for plants grown at the lowest population and decreased with increased population. In all four species N concentration was highest in seed followed by senescent leaves, while the lowest level was in straw. Lupins produced the greatest amount of seed N (16.82 and 19.29 g m⁻²) followed by chickpeas (10.26 and 13.10 g m⁻², during 1998/99 and 1999/2000, respectively). Lentils had the lowest N concentrations and yield in this study. The distribution of N to different parts of legumes increased with increased population. Averaged over all populations, the NHI was stable in each species across the two seasons. However, it changed with changed population and sowing depth. The N accumulation efficiency (EN) was highest in lentils (0.024 and 0.027 g N g⁻¹ DM) and lowest in chickpeas (0.018 and 0.021 g N g⁻¹ DM) in 1998/99 and 1999/2000, respectively. The NHI and the CHI were correlated and both were strongly associated with seed yield.