Effects of exogenous abscisic acid and gibberellic acid on filling process and nitrogen metabolism characteristics in wheat grains

Hormones play important roles in regulating the grain filling process and protein accumulation. In this study, two wheat cultivars differing in grain protein concentration were used to investigate the changes in endogenous hormone contents and the mechanism of hormonal regulation of grain filling and protein accumulation under exogenous application of abscisic acid (ABA) or gibberellic acid (GA3). Results showed that the mean grain weight of the ABA treatment was 11.8% and 5% higher than that of control in Shannong1391 (SN1391) and Gaocheng8901 (GC8901), respectively. The equivalent values for SN1391 and GC8901 were 16.9% and 6.9% after GA3 was applied. The protein concentration was significantly increased by 17.6% and 15.3% in response to application of ABA in SN1391 and GC8901, respectively, and by 21.2% and 12.0% responding to GA3. GA3 showed much higher effects than ABA on glutamine synthetase (GS) activity and free amino acid contents. The elevated contents of endogenous ABA and gibberellins (GAs) were closely associated with the exogenous ABA and GA3, respectively. The indole-3-acetic acid (IAA) contents reduced at the early grain filling period under the application of ABA, but it increased responding to exogenous ABA and GA3 at the mid and later grain filling period. Correlation analysis demonstrated that grain filling rate positively and significantly correlated with the contents of GAs and IAA and ABA, free amino acid contents and GS activity significantly correlated both with the contents of ABA and GAs, and soluble protein contents significantly correlated with the contents of ABA and IAA. It can be concluded that the effects of exogenous hormones were predominantly mediated by changes in endogenous hormone contents, which affected grain filling process, nitrogen metabolism characteristics, and, therefore, changed protein accumulation amounts.