Abstract

In Australia, grain sorghum [Sorghum bicolor (L.) Moench] hybrids are often grown under conditions of low soil nitrogen (N) availability with suboptimal levels of N fertiliser supplied. However, little is known about the traits that contribute to sorghum hybrid performance in environments with low available N. We examined plant traits that may contribute to adaptation of sorghum to low soil N conditions, and the influence of genotype × N environment interactions on yield and grain N concentration. Two experiments were conducted using 3–6 hybrids with similar phenology. Three N fertiliser application rates (0, 60, and 240 kg/ha) were used in Expt 1, and 2 application rates (0 and 60 kg/ha) were used in Expt 2. Hybrid yield was associated with plant N content at maturity. The ability of a hybrid to take up N continuously during grain filling, under N limiting conditions, was identified as an important component contributing to high yield. In the non-fertilised treatment of Expt 2, where plants suffered the most severe N limitation before anthesis (e.g. total plant N content at anthesis <3 g/m²), hybrid yield was associated with biomass production and duration of effective grain filling. The dependence of the expression of the higher N uptake trait on N availability and other environmental factors resulted in genotype × environment interactions for yield. Differences among hybrids in leaf senescence and grain growth rate had little effect on yield.

Genotypic variation for grain N concentration was consistent across experiments for hybrids with and without the staygreen attribute. In Expt 2 the magnitude of leaf senescence and amount of N mobilised from leaf to grain were greater at 60 kg N/ha than in the non-fertilised treatment. In addition, the staygreen hybrid 72389–1-1–3/QL36 had a slower rate of leaf senescence, took up larger amounts of N after anthesis, and had higher grain N concentration (1·07%) than the senescent hybrids ATx623/RTx430 (0·95%) and QL41/69264–2-2–2 (0·90%).