Effects Of Shading Between Jointing And Maturity On Yield, Quality Formation And Their Mechanism In Winter Wheat

The Huang-Huai-Hai (3-H) plain and the downstream plain of the Yangtze River are main wheat production areas in China. In these areas, low radiation affects wheat grain yield and quality formation in tree-crop intercropping systems. To elucidating the physiological mechanism and regulation principles for grain yield and quality formation in wheat is of important significance for understanding grain yield and quality physiology and guiding cultural management in wheat under low radiation. To evaluate the low radiation impact on crop yield and quality formation, two winter wheat (Triticum aestivum L) cultivars differing in shading (low radiation) tolerance, Yangmai 158 and Yangmai 11 were planted in a field experiment. Three treatments were designed from jointing to maturity, as non-shading (SO),22% less intercept radiation (S1) and 33% less intercept radiation (S2). The effects of shading between jointing and maturity on wheat canopy structure and light condition, accumulation and redistribution of water soluble carbohydrate and nitrogen, quality formation were elucidated. Here are the main results:1. Effect of shading during jointing and maturity on canopy photosynthetic properties of winter wheat.Shading significantly reduced canopy apparent photosynthetic rate (CAP) of both cultivars, and the reduction at 20 DAA (Days after anthesis) was higher than that at 10 DAA, when the CAP was active mostly. At 10 DAA, the decline in CAP in S1 (14.1-15.3%) was more less than the reduction in radiation (22%), and was consistent with the radiation reduction (33%) in S2 (29.5-37.4%). During the acclimation to long-term shading, the fraction of the top and bottom leaf layers increased, more 14.5-24.3% solar radiation was intercepted by the top fraction leaf layers (0.4-1.0 m) in the canopy, which partially compensated for the decreased LAI under shading. In addition, the photosynthetic rate (Pn) of the third leaf from the top increased to compensate the decreased Pn of the flag leaf: 88-97% of reduction of the flag leaf Pn was compensated by the third leaf in S1 and 70-80% was compensated in S2. These compensations were beneficial to allay the reduction of grain yield, ultimately, the grain yield of Yangmai 158 and Yangmai 11 were declined by 4.1-9.9% in S1 and 15.3-25.8% in S2, less than the reduction of solar radiation.2. Effect of shading during jointing and maturity on photosynthetic properties of winter wheat flag leaf.Shading during jointing and maturity declined the Pn of the flag leaf during grain filling and prolonged the photosynthetic active duration (PAD) in wheat flag leaf. Shading showed little impact on chlorophyll content in flag leaf of shading-resistant variety Yangmai 158 while decreased in shading-sensitive variety Yangmai 11 during the early grain filling stage. However, shading significantly increased flag leaf chlorophyll content at late grain filling stage for both varieties and prolonged the relative steady phase (RSP) of the chlorophyll content. Shading reduced soluble protein content, SOD activity and MDA content of wheat flag leaf. That indicated long-term low radiation postponed the senescence of wheat flag leaf. Shading reduced Fo/Fm, the PSII quantum yield (OPSII), electron transport rate (ETR), while increased Fv/Fo and non-photochemical quenching (NPQ). That indicated more excited energy was dispersed via non-photochemical reactions under long-term low radiation, thenΦPSⅡand ETR decreased, which lead to the reduction of Pn in wheat flag leaf.3. Effect of shading during jointing and maturity on redistribution of carbohydrate stored in vegetative organs pro-anthesis in winter wheat.Shading during jointing and maturity reduced the quantity of carbohydrate stored in vegetative organs pro-anthesis, but increased the PSGS (percentage of total soluble sugar remobilization quantity to grain starch yield). That indicated with current assimilation was reduced by low radiation, grain starch more depend relatively on remobilized carbohydrate. Activity of SST (Sucrose:sucrose fructosyltransferase, EC 2.4.1.99) and FFT (Fructan: fructan fruc-tosyltransferase, EC 2.4.1.100) in wheat stem were reduced under shading conditions, led to less fructan accumulated and then less water soluble carbohydrate (WSC) stored in stem. At late grain filling, FEH (fructan exohydrolase) activity decreased by shading, and ability of hydrolyzed fructans and their transfer to grain were weakened. The higher reduction in carbon assimilation post-anthesis than that in RSQ (total soluble sugar redistribution quantity) led to a relative increase in PSGS. But the quantity of carbohydrate stored in vegetative organs pro-anthesis was less, and the remobilization quantity to grain was still low. That indicated shading during jointing and maturity reduced the amount of fructan and sucrose stored in wheat stem, and the WSC remobilization quantity in stem declined subsequently, then led to a lower contributive ability to grain and starch, and resulted in lower grain yield and grain starch yield.4. Effect of shading during jointing and maturity on nitrogen metabolism in winter wheat.Shading during jointing and maturity reduced accumulation of grain protein:in S1 and S2, protein accumulation decreased by 2.91-6.48% and 11.97-13.52% than that in S0, and the more light was obstructed, the more grain nitrogen came from post-anthsis accumulation. Grain nitrogen mainly from remobilization of previous assimilated N. In control (SO), the contribution of RANP (amount of nitrogen accumulated pre-anthesis) to grain N (CRNP) was 79.48-82.88%, while decreased to 71.77-79.22% and 54.89-73.48% in S1 and S2, respectively. RANP of each vegetative organ decreased under low radiation and resulted the reduction of the total RANP. Leaf RENP (remobilization efficiency of nitrogen accumulated pre-anthesis) increased to compensate the decrease of RENP of other vegetative organs, and resulted no obvious changes occurred in total RENP. That indicated shading during jointing and maturity reduced the amount of nitrogen stored pre-anthesis and then the RANP, finally led to the reduction of the CRNP. Shading significantly increased NAP (nitrogen amount accumulated post-anthesis):S1 and S2 increased by 16.79-52.40% and 40.87-92.70% than that in S0, respectively, and CNP (contribution of NAP to grain nitrogen) then improved. Shading showed no significant effect on the activity of grain GS (glutamine synthetase, E.C.6.3.1.2) and GPT (glutamate pyruvate transaminase, E.C.2.6.1.2), that indicated low radiation did not affect on nitrogen metabolism post-anthesis, accordingly, the increase of NAP had few relationships with the nitrogen metabolism post-anthesis. At mid and anaphase of grain filling, grain protein content was significantly increased by low radiation, which reasoned for less reduction of grain protein accumulation than grain yield (reduced 4-10% and 15-26% in S1 and S2 than that in S0, respectively).5. Effect of shading during jointing and maturity on quality of wheat grain and flour.Long-term low radiation showed no effect on amylose content while significantly reduced amylopectin content, which resulted in the significant decrease in the ratio of amylopectin to amylase content in grain. Long-term low radiation also reduced the starch peak viscosity, reduced starch through viscosity and increased pasting temperature in shading intolerant cultivar of Yangmai 11, while showed no effect on starch through viscosity and pasting temperature in shading tolerant cultivar of Yangmai 158. Under long-term low radiation conditions, grain protein content increased with no change in albumin and globulin contents and increasing in the contents of gliadin, glutinin and GMP (glutenin macropolymer), which caused the increase of wet gluten content, SDS-sediment, development time and stability time, and the decline of softening degree. This study indicated that shading from jointing to maturity was disadvantaged to cooking qualities of wheat, but improved baking qualities.In conclusion, shading between jointing and maturity reduced wheat photosynthetic capacity and then carbon assimilation. The reductions in amount of photosynthetic assimilates after anthesis and amount of pre-anthesis stored photosynthetic assimilates and nitrogen in vegetative organs which transferred to grain after anthesis were involved in the depressed grain yield, and accumulations of protein and starch in wheat under low radiation. Further studies revealed that the carbon metabolism was depressed more than the nitrogen metabolism under shading between jointing and maturity. Thus, grain starch content decreased while protein content increased, content of starch and protein components altered, which resulted in significant changes in qualities of grain and flour under low radiation.