Effects of Potassium and Calcium on Root Exudates and Grain Quality During Grain Filling

The purposes of this study were to understand the root activity and root exudates of rice (Oryza sativa L.) in response to different levels of potassium (K) and calcium (Ca) nutrients and the effect of K and Ca on rice grain quality. Cultivars Yangdao 6 (indica) and Yangjing 9538 (japonica) were cultured in hydroponical solutions and treated with different Espino nutrition solutions from heading to maturity, which were supplemented with complete K and Ca (standard Espino nutrition solution, control), half K (1/2K), zero K (0K), half Ca (1/2Ca), and zero Ca (0Ca) nutrition. Compared to the control, the 0K treatment significantly decreased root activity, ATPase activity, and concentrations of citric acid, Ca²⁺, K⁺, and NH₄⁺ in the root exudates, but accelerated root senescence; and the 0Ca treatment significantly decreased concentrations of oxalic acid, Ca²⁺, K⁺, and NH₄⁺ in the root exudates. The oxalic acid concentration in root exudates was positively correlated (P < 0.01) with chalky grain percentage, chalkiness, and gel consistency of rice grain; whereas the citric acid concentration in exudates was negatively correlated (P < 0.01) with chalky grain percentage, chalkiness, and breakdown value, but positively correlated (P < 0.05) with setback value. At early (10 d after heading) and mid grain-filling (20 d after heading) stages, the concentrations of K⁺ and Ca²⁺ in root exudates were negatively correlated (P < 0.05) with chalky kernel percentage, chalkiness, and amylose contents, and the NH₄⁺ concentration was also negatively correlated (P < 0.05) with amylose content of grain. This result indicates that root exudates are closely associated with grain quality. K and Ca nutrition regulate root exudates and consequently affect grain quality of rice.