Effects Of Cytokinin Induced By Root Nitrate Absorption On The Continuous Regrowth Of Defoliated Ryegrass

After being grazed by herbivores, the aboveground parts of forage grasses,especially the leaves often exerts regrowth. The ability of grass continuous regrowthafter defoliation is an important factor in resisting hurt by herbivores and maintainingthe continuous production of grassland. In the current study, annual ryegrass was usedas the subject materials, with method of pot and sand culture, means of exogenouscytokinin, gibberellin and nitrate spraying, root local nitrate supply as well as rootbreaking, we studied the effects of exogenous cytokinin, gibberellin and nitratespraying on biomass of newly grown leaves, effects of root local nitrate supply both onroot-leaf transport of cytokinin and biomass of newly grown leaves, and effects ofchangs of root absorption ablity caused by root breaking on biomass of newly grownleaves. This study ascertained the effects of nitrate on regrowth of defoliated ryegrassand its regulatory mechanism, and provided the theoretical basis for study of grazingtolerance. The main results are as follows.1. Frequent defoliation decreased root biomass, soluble root carbohydrate content,root bleeding sap quantity and biomass of newly grown leaves. Exogenous gibberellinspraying easily increased GA content of newly grown leaves, and cytokinin sprayingeasily increased ZR content of newly grown leaves. Exogenous Cytokinin sprayingincreased the regrowth of leaves under frequent defoliation, and gibberellin sprayingincreased the regrowth of leaves only under fewer clippings.2. Without exogenous NO₃^-spraying to leaves, frequent defoliation decreasedNO₃^-, ZR and iPA content in leaves as well as biomass of newly grown leaves. Thespraying of exogenous NO₃^-easily increased NO₃^-, ZR and iPA content as well asbiomass of newly grown leaves whenever NO₃^-was sprayed after the first or thirdclipping. Moreover, under certain extent （0-1400mg·L-1） the higher NO₃^-concentration it sprayed, the greater stimulation effects it would be. The high NO₃^-content in the newly grown leaves improved the continuous regrowth of ryegrass, which is caused by the stimulation effects of NO₃^-on accumulation of cytokinin inleaves.3. Frequent defoliation caused a significant decline in root bleeding sap quantity,NO₃^-and cytokinin content in leaves, as well as biomass of newly grown leaves whenno external nitrate source was available. The absorption of NO₃^-by roots easilyincreased root bleeding sap quantity, NO₃^-content in roots, accumulated NO₃^-quantityin bleeding sap, NO₃^-and cytokinin content in leaves as well as biomass of newlygrown leaves whenever the external nitrate source was supplied after the first or thirdclipping. The stimulation effect of leaf cytokinin on the continuous regrowth ofdefoliated ryegrass is not depending on the cytokinin transported from root to leaf, buton the synthesis of leaf cytokinin induced by the NO₃^-transported from roots to newlygrown leaves.4. Under frequent defoliation, the ability of ryegrass continuous regrowth wasdirectly depending on cytokinin content in newly grown leaves, but had no directrelation with NO₃^-content in newly grown leaves. The root absorption ability of NO₃^-had a direct influence on NO₃^-content in leaves, and the great effects of leaf NO₃^-oncontinuous regrowth of ryegrass was realized by the synthesis of leaf cytokininregulated by NO₃^-.