The Involvement of Cytokinin and Abscisic Acid Levels in Roots in the Regulation of Photosynthesis Function in Flag Leaves during Grain Filling in Super High‐yielding Rice (Oryza sativa)

Abstract

AbstractA few rice cultivars with super high‐yielding potential have successfully been released. However, little is known about the physiological mechanisms underlying the formation of super high yield in rice. Here, we report changes of cytokinin (zeatin + zeatin riboside, ZR) and abscisic acid (ABA) contents in roots and leaves during grain filling and their relation to declining photosynthesis function of leaves in super high‐yielding rice cv. Xieyou 9308 and standard rice cv. Shanyou 63, as a control. ZR contents in roots and leaves decreased in a time‐dependent manner, while their ABA contents were relatively stable at earlier grain filling stage and rapidly increased at later grain filling stage in both cultivars. A rapid decrease in ZR content and a rapid increase in ABA content were observed earlier in roots than in leaves for both cultivars. Declining instantaneous photosynthesis rate (IAPS) of leaves in both cultivars coincided with decreasing ZR contents and increasing ABA contents in both roots and leaves. Leaves with 6‐benzyl aminopurine (6‐BA) treatment exhibited increased IAPS and extended active photosynthesis duration (APD), and the result was reversed in leaves with ABA treatment. A rapid decrease in ZR contents and a rapid increase in ABA contents of roots and leaves in Xieyou 9308 obviously lagged behind those in Shanyou 63. These results suggest that decreased ZR content and increased ABA content in roots cause the decline of photosynthetic function during grain filling stage. The highly efficient photosynthetic function of Xieyou 9308 is due to the delayed change of ZR and ABA contents in its roots.