Effects Of Ozone Stress On Yield Formation Of Different Rice Cultivars And Its Physiological Mechanism

The tropospheric rising ozone has a serious impact on the growth and yield formation of rice,but the causes of yield loss and lodging resistance changes,genotype differences and their physiological basis are not clear.By using glasshouse-type fumigation chambers,eight rice cultivars(YL900,YL1998,Y538,Y15,NJ9108,WY27,HD5 and YD6)were grown in 2016 and 2017 at two ozone levels:Low ozone concentration as control(10 nL L-1)and high ozone concentration as elevated ozone treatment(100 nL L-1)from around a week after transplanting until maturity.The effects of ozone stress on the growth,yield formation,dry matter production and distribution,leaf morphology and physiology traits,element absorption and utilization and stem lodging resistance of different rice cultivars were studied.The purpose of this study was to clarify the genotypic differences and physiological basis of rice yield loss under ozone stress,so as to provide a scientific basis for the selection of ozone-tolerant rice cultivars to the increase of surface ozone concentration.Results showed as follows:1.The heading stage of YL900,YL1998 and Y15 were reached by 0.5-4.0 days earlier in high ozone concentration than the control,while that of the other five cultivars were prolonged 1.5-6.5 days.Ozone stress decreased plant height and tiller number at different growth stages,and the ozone-induced reduction increased gradually with time,and the maximum decrease were 7.0%and 12.4%respectively.Significant interactions were detected between ozone and year or cultivar for plant at all growth periods,but there was almost no interaction for tiller number.2.Ozone stress significantly decrease panicle number,spikelet number per panicle,fully-filled grain percentage and fully-filled grain weight by 10.0%,15.3%,13.0%and 5.7%,but significantly increased empty grain percentage and incomplete-filled grain percentage by 70.7%and 79.5%,respectively.There were significant interactions between ozone and year,cultivar(except panicle number)or grains position(except fully-filled grain percentage and incomplete-filled grain percentage)for above parameters,and the ozone-induced reduction in these yield traits almost followed the order 2016>2017,or IS(inferior spikelet)>MS(medium spikelet)>SS(superior spikelet).3.Ozone stress significantly decreased grain yield of all cultivars by 36.0%on average(range from 28.8%to 48.8%),and by 40.6%and 31.5%in 2016 and 2017,and by 22.9%,35.3%and 54.7%in SS,MS and IS,respectively.Significant interactions were detected between ozone and year,cultivar or position for grain yield4.Ozone stress significantly decreased above ground dry weight at tillering stage,jointing stage,heading stage and maturity by 14.2%,22.6%,22.0%and 29.2%,respectively,and the ozone-induced reduction in leaf dry weight was much lower than that in stem or panicle.In terms of dry matter distribution,ozone stress significantly increased the ratio of leaf to above ground dry weight at tillering stage,jointing stage,heading stage and maturity by 7.6%,18.2%,34.4%and 35.0%,respectively,but the ratio of stem or panicle to above ground dry weight showed a decline trend.Ozonexyear has a significant impact on the above parameters,and significant interactions were detected between ozone and cultivar for the dry matter distribution at all stages and dry matter production at maturity.5.Ozone stress caused obvious injury symptoms of top rice leaves,and significantly decreased the SPAD value and chlorophyll content of the top three leaves of all cultivars,and the decline were increased with the prolonging fumigation time and the lowering leaf position Ozone stress had no significant effect on the top three leaves area,dry weight,length and dry weight per unit area at heading stage,20 days after heading and maturity in 2017.Significant interactions were almost detected between ozone and year,cultivar or leaf position(only SPAD value of top three leaves)for the the green degree and morphological traits of top leaves at 20 days after heading.6.Ozone stress significantly decreased the leaf net photosynthetic rate of all cultivars at tillering stage,heading stage and 20 days after heading by 24.1%,24.8%and 30.1%.stomatal conductance by 23.9%,38.0%and 38.5%,respectively,but had almost no significant effect on transpiration rate or internal cellular CO2 concentration.Significant interactions were almost detected between ozone and year or cultivar for these photosynthetic parameters at heading stage and 20 days after heading.7.Ozone stress significantly increased the leaf malondialdehyde(MDA)content of all cultivars at tillering stage,heading stage and 20 days after heading by 23.4%,21.5%and 36.1%,and leaf ascorbic acid content and total phenolics also showed the increasing trend,but significantly decreased leaf carotenoid content by 12.9%,5.2%and 16.5%,respectively.The effect of ozone stress on the leaf ascorbic acid content and the activity of antioxidant enzymes(SOD,POD and CAT)increased first and then decreased with the growth period.Significant interactions were almost detected between ozone and year or cultivar for MDA and carotenoid content of rice leaf at all stages.8.Ozone stress significantly decreased the pushing resistance of the lower part of a single plant and stem of all cultivars at maturity by 39.0%and 31.4%,and decreased the breaking resistance of the third,fourth and fifth internode from the top of rice stem by 22.5%,24.0%and 32.6%,respectively.In terms of internode traits,ozone stress significantly decreased the cross-sectional area of the third,fourth and fifth internode by 16.0%,22.7%and 23.3%,respectively,and the dry weight per unit length and culm wall thickness of basal internode also showed a decline trend.Ozone-induced reductions in above parameters at 20 days after heading were similar to those at maturity.Almost no significant interaction was detected between ozone and year for all above parameters,but there were significant interactions between ozone and cultivar.The ozone-induced reduction in the breaking resistance was closely related to the decrease of basal internode traits,especially the thinning and filling of internode.9.Ozone stress increased N,P,K,Ca,Mg,Mn,Fe,Cu and Zn concentrations of straw by 28.7%,11.6%,8.3%,2.6%,27.4%,20.7%,8.8%,27.6%and 9.9%,while the uptake of all elements decreased by 3.3%,15.9%,18.5%,21.8%,4.4%,6.9%,17.6%,2.5%and 16.9%,respectively.In most cases,the ozone-induced changes in elements concentrations and uptake in stems were greater than those in leaves.Ozone stress increased elements allocation(except Ca and Cu)to leaves by 9.6-34.9%,while decreased those in stems by 8.1-16.0%.There were almost significant interactions between ozone and cultivar for element concentrations,uptake or allocation in straw and its components.10.In terms of yield components,ozone-induced reduction in yield was mainly related to the decrease of spikelet number per panicle(especially spikelets located at middle or lower parts of a panicle),followed by the decrease of panicle number and grain filling capacity.From the dry matter production and distribution,ozone stress significantly decreased above ground dry weight,while increased the dry matter distribution to leaves and decreased the assimilate distribution ratio in the stems or panicles,which led to yield loss and increase the potential lodging risk.Among all the leaf traits,in the process of defending against ozone damage,which not only consumed a lot of material and energy of rice leaves,but also ozone caused obvious damage to leaf photosynthetic and assimilate transport system,as indicated by decreasing the photosynthetic capacity and producing photosynthesis feedback inhibition.With the extension of ozone exposure time,the membrane lipid peroxidation of rice leaves was intensified,which led to cause leaves yellowing and premature senescence,and final reduce the biology and grain yield.Ozone-induced reduction in yield was closely related to the response of the green degree of leaf(SPAD value and chlorophyll),morphological traits(area,length and dry weight)and physiological characteristics(net photosynthetic rate and malondialdehyde)of top leaves at 20 days after heading,which can be used as screening parameters in ozone-tolerant rice breeding.In summary,the ozone concentration of 100 nL L-1 seriously inhibited rice growth,as indicated by caused leaves yellowing,decreased leaf photosynthesis,increased membrane lipid oxidation and decreased the mechanical strength of the rice stem,which led to substantial final biological and grian yield losses.The ozone-induced changes in rice growth and physiological parameters were mostly affected by year(temperature and illumination),cultivar or grain position,and the responses of indica cultivars(especially YD6)were almost greater than those of japonica ones,and those of 2016 were greater than those of 2017.The grain yield losses of different rice genotypes under ozone stress was mainly related to the formation of spikelets(especially inferior spikelets)and the traits of top leaves at the middle grain filling stage.It is of great significance to select high-quality ozone tolerant rice cultivars(japonica type)and to combine with appropriate cultivation measures to reduce the damage of ozone stress on spikelet formation at middle growth stage and maintain the vitality of top leaves at the later growth stage,which in order to minimize the impact of ozone pollution on rice productivity in the near future high ozone conditions.