Studies On Root-Produced Cytokinins Of Intersubspecific Hybrid Rice

Hybrid rice, especially the inter-subspecific hybrid rice has strongheterosis but low seed setting rate, so yield potential can't been fullydeveloped, this is the main obstacle in exploring hybrid rice heterosis.Studies on rice seed setting physiology mainly concentrate on supply ofgrain-filling substances above the ground in the past 30 years, whichformed three theories in exPlanations for filling barrier in hybrid rice:"unbalance of source-sink". "energy barrier of grain filling" and"inter-grain apical dominance". These explanations accelerated ourunderstanding physiological processes of rice grain filling. Recently,with the increased research findings and the accumulation of practicalexPerience, crop physiologists focus on hormone control of rice grainfilling and relationship between root system activity and grain filling.These researches will fulfill and develop our physiological knowledgeof rice grain filling, and provide us with useful clues to eIucidate thebiological basis of hybrid rice filling problem.Inter-subspecific hybrid rice IIyou 2070. IIyou 4l9 and theirrestorer lines 2070 and Zhong4l9 were used as materials. These data aredetected and analyzed by using HPLC and ELISA, including the kindand amount of root-produced cytokinins in root exudate after initialheading stage, and changes of cytokinins in rice fiag leaves and grains.The results are briefiy described as folIows.l. Inter-subspecific hybrid rice has physiological superiority of rootsystem. In comParison with the greatly decreased root growth inrestorer lines from panicle initiation stage, hybrid combinations keepactive root growth uniII miIk ripening. This root growth heterosis aremainly due to different growth caPacity of root system in 0-l0cm soillayer. Hybrid combinations have strong root exudate and superiorroot activity. However, the amount of root exudate raPidly decreasedafter milk ripening stage. These dynamic changes are consistent with thephotosynthesis rate. chlorophyll content of leaf and green leaf areasduring filling stage. @ Quality of root exudate, which means variety andamount of cytokinin, is a major index that reflects root activity Hybridcombinations have higher cytokinin content in root exudate than that oftheir restorer lines. Strong root activity in hybrid combinations is relatedto slower decrement in total amount of cytokinins.2. By measuring the comPonents of cytokinins in rice root exudate,we have gained fOur kinds of cytokinins: Z diHZ diHZR and IPAs. Thisfinding from rice root exudate has not been reported.3. Resulting from our experiments show that cytokinins, especiallyzeatin, is very importam to sustain root system activity and leafphysiology. It is zeatin that mainly determines root activity and sustainleaf physiological function. However, not every kind of cytokinins hasthe capacity to promote or keep physiological function. Some kinds ofcytokinins are related to senescence. We suppose that the changes ofcytokinin sorts are results of catabolic enZyme.4. By analyzing the cytokinin changes of vigorous and weakspikelets, it is suPPosed that the increment of total cytokinin at maturestage cause weak spikeletS unfilled.