Hormonal Changes in the Grains of Rice Subjected to Water Stress during Grain Filling

Abstract

Lodging-resistant rice (Oryza sativa) cultivars usually show slow grain filling when nitrogen is applied in large amounts. This study investigated the possibility that a hormonal change may mediate the effect of water deficit that enhances whole plant senescence and speeds up grain filling. Two rice cultivars showing high lodging resistance and slow grain filling were field grown and applied with either normal or high amount nitrogen (HN) at heading. Well-watered and water-stressed (WS) treatments were imposed 9 days post anthesis to maturity. Results showed that WS increased partitioning of fixed 14 CO 2 into grains, accelerated the grain filling rate but shortened the grain filling period, whereas the HN did the opposite way. Cytokinin (zeatin ϩ zeatin riboside) and indole-3-acetic acid contents in the grains transiently increased at early filling stage and WS treatments hastened their declines at the late grain filling stage. Gibberellins (GAs; GA 1 ϩ GA 4 ) in the grains were also high at early grain filling but HN enhanced, whereas WS substantially reduced, its accumulation. Opposite to GAs, abscisic acid (ABA) in the grains was low at early grain filling but WS remarkably enhanced its accumulation. The peak values of ABA were significantly correlated with the maximum grain filling rates (r ϭ 0.92**, P Ͻ 0.01) and the partitioning of fixed 14 C into grains (r ϭ 0.95**, P Ͻ 0.01). Exogenously applied ABA on pot-grown HN rice showed similar results as those by WS. Results suggest that an altered hormonal balance in rice grains by water stress during grain filling, especially a decrease in GAs and an increase in ABA, enhances the remobilization of prestored carbon to the grains and accelerates the grain filling rate.

Cytokinins are central regulators of cambial activity

“…They decrease by drought in rice (12) and increase quickly and greatly by nitrate nutrients in sunflower (9), barley (11), and Arabidopsis (10). Cytokinin levels also respond to phosphate, although not as much as to nitrate (9).…”

“…Cytokinin levels dramatically increase in response to nitrogen and phosphate nutrients (8-11) and decrease in harsh conditions such as nutrient deficiency or drought (12). These environmental conditions are correlated with changes in cambial activity, suggesting that cytokinins may function directly as regulators of cambium development.…”

Alterations of Endogenous Hormonal Levels in Plants under Drought and Salinity

“…Thus, several studies showed that under drought the endogenous GAs levels markedly decreased because this stress appeared to inhibit the GA biosynthesis and/or increase its degradation. Early reports demonstrated that drought reduced the GAs accumulation in lettuce [53], cottonwood saplings [54] and in rice grains during early grain filling [55]. Similarly, in leaves of unstressed and water stressed maize seedlings showed that GA 3 decreased considerably during drought [56].…”

ABA mediates PEG-mediated premature differentiation of root apical meristem in plants

“…Measurements of ABA content ABA extraction and ABA contents in root tips were quantified by enzyme-linked immunosorbent assay (ELISA) as described in Yang et al 4 …”