Effects of potassium nutrition on carbohydrate and endogenous hormone contents of sweet potato under drought stress

Objectives An experiment was carried out to investigate the effects of different potassium application rates on carbohydrate and endogenous hormone contents of sweet potato at different growth stages in order to provide theoretical basis for high yield cultivation of sweet potato under drought condition.

Methods Sweet potato cultivar of Taizhong 6 was used as tested crop and potassium sulphate (K₂SO₄) as tested K fertilizer. The water treatments included normal water supply (W1, soil with water content of 60%−70% field capacity) and drought stress (W0, soil with water content of 30%−40% field capacity). The K fertilizer application treatments included K₂O rates of 0 (K0), 120 (K1), 240 (K2) and 360 kg/hm² (K3). The contents of dry matter, starch, soluble sugar and endogenous hormones in different growing stages were measured, and the storage root yields were weighed.

Results The dry matter contents, the storage root and starch yields under the drought stress were significantly lower than those under normal water condition. The highest dry matter contents in both plants and storage roots, the starch contents and accumulation in storage roots were all obtained in K2 treatment, with the increment of 31.7%, 43.6%, 10.6% and 50.6% in turn, compared with K0. At the same potassium rate, the single tuber weight was higher and the tuber number per plant was lower under W0 than under W1. Under W1, the soluble sugar contents in the leaves were reduced and the soluble sugar contents in the storage roots were increased after potassium application, while under W0, the soluble sugar contents in both the leaves and storage roots were increased in K1, with the highest increase of 31.7% and 31.4%, compared with K0. Under drought stress and compared with K0, the IAA, ABA, ZR and GA contents in the leaves and storage roots were significantly increased by potassium application, with the highest increase of 12.7%, 15.7%, 12.0% and 10.4% in leaf, and 21.4%, 15.6%, 65.7% and 13.0% in storage root. The increased hormone concentrations brought to higher carbohydrate and dry weight accumulation amount and rate in the plants and storage roots.

Conclusions Under the drought stress, potassium application promoted the dry matter allocating to storage roots, and increased the single tuber weight, which increased storage roots yield per unit area. Potassium application increased the endogenous hormone (ABA, IAA, ZR and GA) contents in leaves and. The increased endogenous hormone contents in roots promoted starch synthesis and accumulation in storage roots. The increased endogenous hormone contents promoted stem and leaf growing, dry matter of stem and leaf and soluble sugar content of leaves, and enhanced drought resistance of sweet potato.