Effects of potassium fertilizer rate on growth, seed quality and potassium use efficiency in Brassica napus under drought stress

Objectives Under long-term drought stress, rate of potassium application could influence rape growth, seed quality, and potassium utilization. A pot experiment was conducted to clarify the mechanism of growth regulation and nutrient allocation of rape under drought stress in response to potassium fertilizer levels.

Methods Two rape cultivars Chuanyou 36 and Youyan 57 differ in drought resistance were used in a pot experiment. The pot was filled with 10 kg of air-dried soil, and three K₂O rates of 0, 80 and 160 mg/kg soil were applied, respectively, and the drought stress was conducted by controlling soil moisture at 45%–50% of field water capacity after flowering period, denoted as DK0, DK80, DK160, respectively. Applying K₂O 80 mg/kg soil and keeping soil moisture at 65% of field water capacity was used as control (CK). Rape growth, yield, seed quality and K utilization characteristics were investigated.

Results Rape growth was inhibited significantly by drought stress. Compared to the DK0, the root dry mass, total root length, root surface area, total plant dry mass and seed yield increased significantly under DK80 and DK160 treatments for both cultivars. Drought stress reduced seed quality however, potassium fertilizer relatively alleviated it. Rapeseed oleic acid content in DK160 and DK80 treatments were significantly higher than that in DK0 in ‘Youyan 57’, but the erucic acid content decreased with increasing rate of potassium fertilizer. Drought stress led to early senescence of leaves, the proportion of potassium in the early decayed leaves during flowering stage increased significantly as the rate of potassium fertilizer increased. Conversely, the proportion of potassium in the stem and pericarp decreased significantly and decreased with increasing rate of potassium application under drought stress, which led to the reduction of rape K-use efficiency. Drought-tolerant cultivar ‘Youyan 57’ maintained higher proportions of K in both stem and pericarp and conferred a superior K-use efficiency, improved yield and higher oleic acid content in seeds than those of ‘Chuanyou 36’ under potassium deficiency.

Conclusions Drought stress after flowering stage will inhibit root growth, cause leaf senescence, and decrease seed oil nutrition quality and yield. Applying potassium fertilizer could alleviate drought stress to some extent by increasing the allocation of K in leaves, and increase the seed yield and quality. However, the K in the fallen senescent leaves is unable to participate in the later growth, inducing the decrease of potassium use efficiency.