| Rice,as a photophilous crop,often encounters shading stress during the growth stage,particularly in Sichuan,China.The Sichuan Basin receives fewer than 1200 hours of sunshine annually and only 3345-3763 MJ m-2 y-1 of annual solar radiation.This ecological condition seriously restricts to the further increase in rice yield and quality.Based on this,the field experiments were conducted to investigate the effect of shading stress on photosynthetic physiological characteristics and grain development of rice from the years of 2010-2013 and 2015,in Sichuan,China.Rice cv.IIyou 498,Gangyou 188,Dexiang 4103,Gangyou 527,Chuanxiang 9838,and Yixiangyou 2115 were selected and shaded during grain filling period,which resulted in 53%shading environment(achieved with white cloth above the rice canopy).We studied the rice physiological adaptability,yield and quality formation characteristics.Moreover,the response and adaptation mechanisms of rice subjected to shading stress and the reason of quality deterioration were investigated.The results are showed as follows:1.The field experiments demonstrated that no significant difference in spikelets per panicle or per square meter was observed between control and shading stress.Shading stress significantly(P<0.05)decreased grain yield mostly by reducing spikelet filling and grain weight.Under shading stress,the grain filling of bottom grains was seriously destroyed,which resulted in sharp reduction in spikelet filling and grain weight,while the top grains possessed the smallest reduction in grain yield.However,the influence of shading stress on grain yield was differed to rice varieties.Both IIyou 498 and Yixiangyou 2115 could decrease the damage of shading stress by maintaining a higher spikelet filling and grain weight,which just led to 30.6%and 28.3%reduction in grain yield,respectively.Nevertheless,the spikelet filling and/or grain weight of Gangyou 527 and Chuanxiang 9838 were severely decreased by shading stress,resulting in more than 45%reduction.2.The appearance,milling,and eating and cooking qualities were markedly deteriorated by shading stress.Under shading stress,the brown rice rate,milled rice rate,head rice rate,and breakdown were significantly reduced,while the percentage of chalky rice and chalk degree were increased.Shading stress also significantly influenced the RVA profile characteristics of rice.Shading stress enhanced the setback but decreased breakdown.In general,the damage of shading stress on rice quality of bottom grains was more serious than that of top and middle grains.Moreover,both IIyou 498 and Yixiangyou 2115 could be selected as shading tolerance varieties for keeping good rice quality.3.Under shading stress,the concentration of total chlorophyll,chlorophyll a,and chlorophyll b in leaf lamina were significantly(P<0.05)enhanced,while the ratio of chlorophyll a and chlorophyll b were differed to rice varieties.The effectiveness of shading stress on photosynthetic and chlorophyll fluorescence characteristics were varied to variety and shading progress.Shading stress significantly improved the original light energy conversion efficiency,but reduced the photosynthetic rate,which resulted in the decrease in electron transport rate and saturation light intensity of rice.IIyou 498,a rice variety with higher shading tolerance,obviously increased the light use efficiency by improving the thickness of leaf lamina,actual photochemical efficiency of PSII photochemistry reaction centers,and photochemistry reaction ability and reducing non-photochemical efficiency,as well as maintaining a relatively higher photosynthetic rate.Under shading stress,the stomatal conductance of leaf lamina was negatively related to the carbon isotope discrimination(△)of stem plus sheath at grain filling stage,while the photosynthetic rate was positively associated with the △ of stem plus sheath at maturity stage.Therefore,the △of organs was closely association with photosynthetic and chlorophyll fluorescence characteristics of rice,and could be used as the potential indicators to identify the shading tolerance of rice.4.In general,there was no significant difference in grain development,size,colour,and grain filling degrees of caryopsis between top and middle grains.However,the grain development in bottom part of panicle was later than that of top and middle grains.For IIyou 498,most of the chalky grains were abdominal white;and the development of starch granules at dorsal and central sections of grain were superior to ventral side,by which the starch granule had polyhedron crystal morphology,with even and close arrangement.While for Yixiangyou 2115,most of the chalky grains were core white types,the transport distance of carbohydrate was close to ventral side,which resulted in similar development of starch granule at dorsal and ventral sides.Under shading stress,the development of bottom grains was seriously distorted,leading to the reduction in cell adhesion and uneven arrangement of starch granule,as well as the decrease in milled and head rice percentages,and eating and cooking quality.5.The sucrose,starch,and soluble sugar accumulations of panicle were significantly reduced by shading stress,which resulted in markedly decrease in dry matter accumulation in panicle.However,the effectiveness of shading stress on dry matter accumulation of panicle was varied to the spikelet position and rice variety.The reduction in soluble sugar and starch in top panicle part was lower than these of middle and bottom part,while the decline of dry matter accumulation in bottom part could be as high as 60%.These suggested that shading stress markedly reduced the dry matter accumulation of panicle,mainly by decreasing the dry matter accumulation in bottom grains,which led to the significant decrease in spikelet filling and grain weight.Furthermore,the reduction in soluble sugar and starch accumulations of IIyou 498 was lower than of Yixiangyou 2115,preventing the drastic decline in fresh and dry matter weight of panicle,and suggesting the stronger tolerance of Ilyou 498 to shading stress.6.The contents of amylose and starch in grain yield were reduced with the decreasing spikelet position.The correlation analysis indicated that the amylose content of rice was significantly(P<0.01)positive associated with the hold viscosity,final viscosity,setback,and pasting temperature,while negative related to breakdown.Shading stress markedly increased the blue value of amylopectin,but reduced that of amylose,as well as the contents of amylose and starch.Moreover,the effectiveness of shading stress on the bottom grain was stronger than the top and middle grains.The reduction in proportion of short chains and increase in proportion of long chains of amylopectin were observed by shading stress.Shading stress obviously decreased the short chains(DP,degree of polymerization<20)of amylopectin,but improved the long chains(20 ≤ DP ≤ 60)in IIyou498.Meanwhile,the short chains(DP S 12)of amylopectin in top and middle grains of Yixiangyou2115 was reduced by shading stress,while the 13<DP<36 chain was increased. |
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