| Wheat(Triticum aestivum L.) grain yield is determined by spike number per square, grain number per spike and grain weight. These three components are interrelated and interact on each other. The structure of wheat population in the field consists of the main stem and tillers. Therefore, the occurrence and development of tillers play important roles in establishing structure of high yield and yield formation. Previous studies have suggested that plant hormones play major roles in the progress of tiller bud development. Hence, to enhance the yield potential of wheat, the better way is to combine nitrogen management with chemical control pathway. The purpose of this study was to test the hypothesis that whether and how cytokinin and nitrogen levels may be involved in mediating tiller bud development and yield formation of two different tillering ability wheat varieties. So JM22 and SN8355 were grown in the experimental plots with different seeding rates and nitrogen fertilizer levels. We conduct the experiments to study(1) the effects of seeding rates and nitrogen fertilizer levels on dynamic group development, the differences of grain filling between superior stems and inferior stems, and grain yield formation;(2) study the effects of cytokinin and nitrogen on tiller occurrence,(3) investigate the respondence of endogenous hormone levels in tiller buds to nitrogen levels and exogenous 6-BA, and(4) investigate the relationship between leaf photosynthesis characteristics, root activity and tiller growth and development. The main results were as follows: 1 The respondence of tillers to seeding rates and nitrogen fertilizer levelsThe grain number per spike decreased with the increase in tiller position. Compared with the main spike, grain number decreased by 5.8、5.1、9.3、12.4、15.5、18.9、17.4、22.2 kernels spike-1 in other tiller spikes of JM22 under N0D1 treatment, respectively. Similarly, grain number decreased by 3.3、10.9、18.1、22.9、22.2、30 spike-1 in other tiller spikes of SN8355 under N0D1 treatment, respectively. Grain weight expressed similar discipline with grain number. Compared with the main spike, grain weight decreased by 7.85%、40.80%、47.51% in other tiller spikes of JM22 under N1D2 treatment, respectively. However, grain weight decreased by 24.82%、36.06% of SN8355, respectively. The results indicated that the grain production decreased with the increase in tiller positions. One trend is that the widening individual production disparity between those at the top and bottom tillers. The grain number and grain weight decreased with the increase in seeding rates, but increased with the increase in application of nitrogen rates. Compared with N0D1 treatment, the grain number of the O, I, and II tiller of JM22 under N0D2 treatment was decreased by 7.0, 6.3, 16.8 kernels spike-1, respectively, and these of SN8355 was decreased by 15.2, 17.0, 11.4 kernels spike-1, respectively. Compared with N0D1 treatment, grain number of N1D1 and N2D1 treatment was averagely increased by 0.6 and 3.2 kernels spike-1, respectively, and that of SN8355 was averagely increased by 5.9 and 6.2 kernels spike-1, respectively.Cluster analysis showed that the tillers of JM22 and SN8355 were all divided into two groups: the superior stems and the inferior stems according to the grain number and grain weight of tillers in different positions. The grain-filling process of the two groups were changed as a s curve. And the grain filling rate was increased and then decreased with the increase of the days after anthesis(DAA). The grain filling rate of the superior grains was higher than that of the inferior grains. At 4 to 24 DAA, the grain filling rate was decreased with the increase in seeding rates, on the contrary, application of nitrogen increased grain filling rate and grain weight. The starch of superior and inferior grains consisted of the small starch granule(diameter < 10 μm). Volume of small starch granule in superior grains was increased by increasing seeding rates, however, the volume of medium granule was decreased. Application of nitrogen decreased the volume of large granule in superior grains and increased the volume of large granule in inferior grains. 2 The effects of seeding rates and nitrogen application rates on changes of wheat population and grain yieldTotal stem amount was increased until the jointing stage and the decreased. The population was increased with increase in seeding rates and nitrogen application rates. Compared with N0D1 treatment, total stem amount of N0D2 and N0D3 treatment was increased by 8.59% and10.16% at maturity, respectively. Compared with N0D2 treatment, total stem amount of N1D2 and N2D2 treatment was increased by 38.12% and 57.97%, respectively. The tillering number per plant was decreased with increase in seeding rates. Application of nitrogen increased the tillering number per plant. Seeding rate increased grain yield, however, there was no significant difference between N0D4 and N0D5 treatment. Application of nitrogen also significantly increased grain yield. Trend surface analysis method can be used to analyze the trend of the response of grain yield to nitrogen application rates and seeding rates. The fitted equation of quadratic trend surface was calculating as: z = 474.752 + 1.722 x + 0.844 y – 0.003x2 – 0.001y2 – 0.001 xy. Using the model obtained the highest ratio of fertilizer and seeding rate for wheat production:(x0, y0) =(235, 300), that is, when the amount of nitrogen fertilizer was 235 kg ha-1 and the seeding rate was 300 plant m-2 wheat production to obtain the maximum. 3 The relationship between tiller development and endogenous hormones in tiller budThe results showed that occurrence rate of I, II, and III tiller of HN treatment was significantly higher than those of LN treatment. The occurrence rate of I and II tiller of HN treatment was 36.43% and 187.83% higher than those of LN treatment, respectively. Exogenous 6-BA significantly increased tiller occurrence rate. Compared with HN treatment, HN+6-BA treatment was increased by 90.54%. The length of tiller bud and the weight of tiller bud were all increased by HN treatment, compared with LN treatment. Exogenous application of 6-BA significantly increased the length of tiller bud and the weight of tiller bud under the two nitrogen levels. Compared with LN treatment, endogenous Zt content in tiller buds was significantly increased by 6-BA and HN treatment. GA3, IAA and ABA content was reduced by application of 6-BA. On the contrary, lovastatin treatment increased endogenous ABA content under the higher nitrogen level. The expression of Ta CKX3 genne showed that LN treatment improved Ta CKX3 expression levels, but lovastatin decreased Ta CKX3 expression levels. Correlation analysis indicated that the length of tiller buds was significantly and positively correlated with the endogenous Zt content, whereas significantly and negatively correlated with the IAA and ABA content, and significantly and negatively correlated with the ratios of IAA to Zt, GA3 to Zt, and ABA to Zt. 4 The effects of cytokinin and nitrogen levels on photosynthetic characteristicsThe SPAD readings and Chl content of HN treatment were significantly higher than those of LN treatment. Application of 6-BA obviously increased SPAD values and Chl content. The Chl a and Chl b was increased by 35.83% and 48.57% at 12 days after treatment, respectively. The Chl a and Chl b of LN treatment was increased by 35.59% and 83.33%, respectively. The results suggest that cytokinin primarily improve the synthesis of chlorophyll b. Application of 6-BA increased stomatal conductance, transpiration rate, intercellular CO2 concentration, and the net photosynthetic rate. The analysis of chlorophyll fluorescence induction kinetic curve showed that the turning point K was appeared in the kinetic curve of LN treatment, compared with HN treatment. Application of 6-BA decreased the kinetic curve, whereas lovastatin increased the kinetic curve. Probability of transferring electrons to other electron acceptors downstream of QA in the electron transport chain(Ψo) and the performance indicator(PIabs) of 6-BA treated plants were significantly increased. The normalized relative variable fluorescence at the K step(Wk) and relative variable fluorescence at the J step(Vj) of 6-BA treated plants were significantly decreased. The results suggest that application of 6-BA protected oxygen evolving complex. Nitrogen fertilization and cytokinin significantly increased electron donor and acceptor performance in the reaction center of PS II. The photosynthetic performance of high nitrogen treatment and cytokinin treatment was superior to the LN treatment. Therefore, much more assimilates was produced by the leaf under HN and 6-BA treatment. Consequently, the car-bon source of the tiller buds growth and development was increased, marked by the increases in dry weight of tiller bud. 5 The effects of cytokinin and nitrogen levels on root developmentThe nitrogen content of HN treatment was higher than that of Ln treatment. Although application of 6-BA decreased the nitrogen content, the nitrogen accumulation in tiller bud was increased. Lovastatin played the adverse roles. Correlation analysis showed that nitrogen content in the tiller buds was negatively correlated with tiller bud growth, especially under the HN treatment, nitrogen content was significantly and negatively correlated with tiller bud growth. Whereas the nitrogen accumulation was positively and significantly correlated with tiller bud growth. The nitrogen for tiller bud growth ultimately comes from root uptake. The results indicated that HN and exogenous 6-BA decreased the length of root, however, HN and exogenous 6-BA treatment increased root activity. Consequently, the above biomass and nitrogen content of leaf under HN and 6-BA treatment was increased, compared with LN treatment.6 The effects of exogenous ABA on endosperm cell divison and grain fillingExogenous ABA significantly increased endosperm cell number and endosperm division rate, finally increased grain weight. Although ABA treatment decreased endogenous zeatin riboside(ZR) content in flag leaves from 7 to 28 days after anthesis(DAA), indole acetic acid(IAA) levels were significantly increased by spraying with ABA. Correlation analysis showed that endogenous contents of ZR, ABA and IAA in grains were positively and significantly correlated with grain-filling rate. IAA content in leaves was positively and significantly correlated with grain-filling rate. |
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