Increasing plant density improves grain yield, protein quality and nitrogen agronomic efficiency of soft wheat cultivars with reduced nitrogen rate

Application of nitrogen (N) fertilizer usually increases grain yield (GY) and grain protein content (GPC) simultaneously. But low GPC is desirable in soft wheat (Triticum aestivum L.). To break the unexpected link, we investigated the combined effects of reduced N rate, increased plant density on GY, nitrogen agronomic efficiency (NAE) and GPC targeting biscuit-making of two soft wheat cultivars. The field experiments involved four N rates (240, 180, 120 and 0 kg N ha−1) and three plant densities (180, 240 and 300 × 104 plants ha−1). Reducing N rate improved grain quality for biscuit-making and NAE in soft wheat, but resulted in lower dry matter accumulation and caused a significant GY loss. With the N rate ranged from 240 to 0 kg N ha−1, GY loss of 16.28–18.54 kg ha−1 was observed for every 1 kg N ha−1 reduction. Increasing plant density significantly enhanced the population dry matter accumulation and partially compensated for GY loss caused by reduced N via increasing the number of effective ears, meanwhile further improved grain quality and NAE. When the plant density increased from 180 to 300 × 104 plants ha–1, increasing 1.0 × 104 plants ha–1 led to an increment in average GY of 4.79, 6.78 and 3.98 kg ha–1 under 180, 120 and 0 kg N ha−1, respectively. The reduction of 1 kg N ha–1 was replaced by increasing app. 2.40–4.09 × 104 plants ha–1 to maintain same GY. The decrease in GPC by the combination of reduced N rate and increased plant density was mainly attributed to the variations in pre-anthesis accumulated N in vegetative organs as well as the whole plant. Moreover, reducing N rate showed a more remarkable effect on GPC than increasing plant density, and their interaction effect was significant. Our study therefore provided an agronomic approach to balance GY and GPC targeting biscuit-making in soft wheat production by the combination of low N rate and high plant density.