Effects of Salt Stress after Late Booting Stage on Yield and Antioxidant Capacity in Pigmented Rice Grains and Alleviation of the Salt-Induced Yield Reduction by Exogenous Spermidine

Abstract

Pigmented rice is receiving much attention due to the large amounts of bioactive compounds and various health benefits. However, little information is available on its agronomic and physiological aspects. This research aimed to explore the effects of salinity on yield and grain quality of pigmented rice and the modulation of salinity responses by exogenous application of spermidine (Spd). Four cultivars of rice were grown in pots until the early booting stage. Thereafter, the plants were sprayed with 1 mM Spd for 7 successive days before being irrigated with 25 mM NaCl instead of tap water until maturity. Grain yield, yield components and harvest index in all rice cultivars were negatively affected by this salinity stress. Pretreatment with Spd dramatically improved yield and yield components of salt-treated plants, particularly the salt-sensitive cultivar. The major yield components which were improved and contributed most to the dramatic increase in seed yield were the number of filled grains per panicle and panicle fertility. Moreover, Spd pretreatment resulted in an increase in K⁺/Na⁺ ratio in rice grains. Salt stress increased nutritional quality of mature grains, i.e., total phenolic content, anthocyanins, proanthocyanins and antioxidant activities (evaluated by FRAP, DPPH and ABTS assays) in all rice cultivars. Furthermore, all aspects of health-promoting nutritional characters were further enhanced by Spd pretreatment. Thus foliar spraying of Spd to rice plants prior to salt application improved grain yield as well as nutritional quality of colored rice grains in relation to total phenolics, flavonoid pigments and antioxidant capacities.