Differences In Physiological Basis Of Floret Development And Response To Cultivation Regulation In Wheat

Wheat (Triticum aestivum L.) production is considered vital to ensure global food security.It has been recently agreed that the grain number per spike is still the main factor limitingwheat super high yield potential. Physiological basis of floret development and grain numberper spike and its regulation in wheat play a major role in the remarkable grain yield in wheat.In this study, wheat cultivars differing in spike-types were grown at Tai’an ExperimentalStation of Shandong Agricultural University during the2010-2011and2011-2012growingseasons. The studies were focused on the changes endogenous hormones in spike, floret andleaf. The result elucidated the relationship between endogenous hormones and floretdevelopment and grain set, discussed the regulatory mechanism of endogenous hormones onfloret development, and physiological and chemical mechanism of plant densities, nitrogenapplication rates and exogenous hormone effect on floret development in wheat. Informationobtained will help to provide a theoretical basis for obtaining higher floret setting rate andimproving floret setting uniformity. The main results were as follows.1Physiological Basis of Floret Development in different spike type wheatLarge-spike cultivars had the faster floret differentiation rate, more floret number, fertilefloret number and grain number per spike than medium-spike cultivars. There were littledifferences in superior floret setting rate between large-spike cultivars and medium-spikecultivars, but a large difference in inferior floret setting rate between them. Analyzing fromthe photosynthate, at the later floret development stage, the rates of photosynthate ofspike/culm in large-spike cultivars were significantly larger than that in medium-spikecultivars. Correlation analysis showed that there was a significantly positive relationshipbetween spike dry weight at anthesis and grain number per spike. It was showed that more photosynthate to spike was favored for more grain number.Sucrose content in leaves, total soluble sugar content, sucrose content and soluble proteincontent in spike were more in large-spike cultivars than in medium-spike cultivars. After theanther formation, the C/N ratio in spike in large-spike cultivars was larger than that inmedium-spike cultivars. It was showed that N metabolism in spike was advantageous to thefloret primordium differentiating, and C metabolism was in favour of floret development, andthe higher C/N ratio was beneficial to grain number per spike.During floret development stage, the changes of GA3content in spike showed the ‘W’model with the two lowest contents at stamen and pistil initiation and floret degeneration. AndIAA content in spike showed the ‘S’ model during floret development stage. Compared withthe medium-spike cultivars, the GA3and IAA peak levels of spike were delayed in thelarge-spike cultivars. Furthermore, the GA3and IAA content in spike of large-spike cultivarsat anther formation were much higher than that in the medium-spike cultivars. However, ZRcontent in spike of large-spike cultivars showed the ‘W’ model, but the ‘V’ model ofmedium-spike cultivars. And at anther formation, the ZR content in spike of the large-spikecultivars was much higher than that in the medium-spike cultivars. It was suggested thathigher GA3, IAA and ZR contents at anther formation were useful for the formation of lagerspike. At anther formation, ABA content in spike was dropped rapidly in large-spike cultivars,but slowly in medium-spike cultivars. It indicated that ABA content from anther formation tomeiosis dropped rapidly was beneficial for the formation of lager spike. The grain number perspike was highly positively correlated with GA3in spike and in spike/leaf at anther formation,and the IAA content in spike anther formation (P<0.01). There was a negative relationshipbetween grain number per spike and ABA content at anther formation and meiosis (P<0.01).During the floret development and degeneration stage, GA3and ABA in fertile floretsshowed ‘V’ model, but IAA content in fertile florets remained a relative stable and low level.But there was a much higher IAA content in stertile florets at the beginning of the floretdevelopment and degeneration stage. ABA content in stertile florets was significantlyincreased during the floret development and degeneration stage. However, GA3/ABA,IAA/ABA and ZR/ABA in stertile florets was much higher than that in fertile florets, but thendecimated. It was showed that the higher IAA content and the decimated of GA3/ABA,IAA/ABA, ZR/ABA in stertile florets may be one of the cases of floret degeneration.2The study of regulation of cultivation measures on floret development and mechanism2.1The study of floret development and mechanism under different plant densities During floret development stage, the sucrose content in leaves, the total soluble sugarcontent and the sucrose content in spike was decreased as increased plant densities. Butsoluble protein content in spike was increased with the increasing plant densities. The resultsuggested that low plant density was in favor of the accumulation of total soluble sugar inspike, or low plant density was beneficial to sucrose to spike within the leaf. The C/N ratio inspike was decreased as increased plant densities, and it was showed that higher plant densitywas not conducive to carbon metabolism, then to large spike formation.After anther formation, the GA3, IAA and ZR contents in spike were decreased with theincreasing plant densities, but ABA content in spike was increased. The result suggested thatlow plant density was in favor of the accumulation of GA3, IAA and ZR in spike at later floretdevelopment stage, but high plant density was in favor of the accumulation of ABA.The results showed that the grain number, spikelet weight, and grain weight with thespikelet positions from the bottom to the top increased first and then decreased, showedparabolic change. Meanwhile, grain setting traits of main spike were superior to that oftillering spike. Under low plant density, the grain setting characteristics and grain weight inlarge-spike cultivars were more superior to those in medium-spike cultivars, and those ofmain stem spike were more superior to those of tillering spike. In fact, high plant density isnot conducive to high yield.2.2Effect of nitrogen application rate on floret development and mechanismAt the early floret development stage, total floret number was decreased with increasingnitrogen (N) levels. The result suggested that low N level promoted the floret development,but the floret development was delayed under high N level. With the increaseing N levels, thegrain number per spike, total floret number, fertile floret, differentiation rate and spike dryweight was increased first, then decreased, and showed the largest increased at N level of240kg N ha-1. It was indicated that N level of240kg N ha-1was in favor of grain number.Nitrogen increased the sucrose content and the total soluble sugar content in spike of wheat.Sucrose content in wheat spike under240kg N ha-1treatments were significantly higher thanthat under N0treatment. However, the excess360kg N ha-1treatment led to a significantdecline in sucrose content in spike of the cultivars. These indicated that appropriate nitrogencould increase the sucrose, and excess nitrogen could decrease the transfer of the sucrose fromleaves to spike. The soluble protein content in spike was increased with the increasing Napplications, and it was suggested that N fertilizer was in favor of the accumulation of solubleprotein. N fertilizer generally increased the C/N ratio in spike as N rate was increased from0 to240kg N ha-1, and then decreased the C/N ratio in spike as N rate was further increased to360kg N ha-1. This indicated that N generally favored C/N in spike up to the N level of240kg N ha-1, while excessive N (360kg N ha-1) decreased the C/N ratio in spike.After anther formation, the GA3, IAA and ZR contents in spike were increased with theincreasing nitrogen applications, but the ABA content in spike was increased. The resultsuggested that low plant density was in favor of the accumulation of GA3, IAA and ZR inspike at later floret development stage, but high plant density was in favor of theaccumulation of ABA. GA3/ABA, IAA/ABA and ZR/ABA in spike were increased withincreasing0-240kg N ha-1levels, but decreased at360kg N ha-1level. It was showed thatnitrogen application was in favor in increasing GA3/ABA, IAA/ABA and ZR/ABA. Nitrogenapplication increased spike/leaf GA3, IAA and ABA during the later floret development stage.Cytokinin oxidase activity in spike was decreased with the increasing nitrogen treatment, andthe least level was under the240kg N ha-1treatment. However, there was no significantdifference of Cytokinin oxidase activity between the240kg N ha-1treatment and the360kg Nha-1treatment. Transcript levels for TaCKX3in spike under nitrogen treatment (240kg N ha-1)were significantly higher than that under nitrogen treatment (0kg N ha-1) from meiosis toanthesis, which indicating that nitrogen could increase the expression of TaCKX3genes inspike, and it is possible that nitrogen could regulate Cytokinin oxidase expression in transcriptlevels.2.3Effect of exogenous ZT or ABA on the floret development and the grain in wheatFloret development was obviously effected by exogenous hormone ZT and ABA.Exogenous hormone ZT increased the grain number by increasing the setting rate of inferiorfloret, especially for ZT application at anther formation. Exogenous ZT significantlyincreased sucrose content and C/N in spike, that provide much more photosynthate forlarge–spike cultivars. But Exogenous ZT increased the GA3, IAA and ZR content in spike,especially for ZT application at anther formation. Exogenous hormone ABA inhibited floretdevelopment, and decreased the fertile florets and grain set almost at all floret developmentstage. But at anther formation, the spike was more tolerant to ABA without significant lose offertile florets. The inhibition effect of ABA was mainly on inferior floret on a spikelet.