Correlation of continuous ryegrass regrowth with cytokinin induced by root nitrate absorption

This study investigated the separate and combined effects of nitrate (NO₃ ^?) and cytokinin additions on continuous ryegrass regrowth after defoliation and the underlying mechanisms. Our results showed that frequent defoliation reduced the biomass of newly grown leaves and roots, the root soluble carbohydrate contents, the root vitality (an indicator of root absorption capacity), and the leaf contents of NO₃ ^?, zeatin and zeatin riboside (Z + ZR), and isopentenyl adenine and isopentenyl adenosine (IP + IPA). NO₃ ^?addition to the roots or leaves increased the biomass of newly grown leaves as well as the leaf contents of NO₃ ^?, Z + ZR, and IP + IPA without increasing the root-to-shoot delivery of endogenous cytokinin. Interestingly, cytokinin directly added to the leaves also increased the biomass of newly grown leaves and their Z + ZR and IP + IPA contents, suggesting that nitrate-induced leaf cytokinin production mediates the growth-promoting effects of nitrate. We also found that cytokinin had a direct whereas NO₃ ^? had an indirect effect on the biomass of newly grown leaves. Taken together, our results indicate that leaf cytokinin production induced by NO₃ ^? absorbed through the roots plays a key role in continuous ryegrass regrowth after defoliation.