Diurnal And Seasonal Variation And Influcing Factors Of Rice Canopy Photosynthesis

Rice canopy photosynthesis is determined by meteorological factors,photosynthetic capacity of aboveground organs and canopy structure together.These components have obvious diurnal and/or growing season changes.Clarifying the temporal various characteristics of rice canopy net photosynthetic rate(Ac_ground,μmol m^-2ground area s^-1)and its influencing factors will provide new ideas for improving rice canopy light use efficiency(LUE,μmol CO₂μmol^-1 photon)and yield.The daily and seasonal variation characteristics of rice Ac_ground were mainly measured by ¹³C mark,3D canopy photosynthetic model and Eddy covariance,Howerver,the effects of rice varieties and management measures on temporal various of canopy photosynthesis are still unclear.In this study,a field experiment was carried out in 2021.The conventional rice Huanghuazhan（HHZ）and Nangeng9108（NG9108）,which were planted in large areas in production were selected.Tow transplanting densities of 20 cm×13.3 cm and 20 cm×26.7 cm were set respectively.From panicle initiation stage to physiological maturity stage,the multi-channel canopy photosynthesis measurement system was used to continuously conduct in-situ measurement of rice Ac_ground in the field,and the related agronomic traits and single leaf net photosynthetic rate were simultaneously measured.The purposes of this study were to explore:1)The diurnal or seasonal variation characteristics of Ac_ground,canopy CO₂ daily net assimilation and LUE in rice canopy,and the differences of them between varieties and planting densities;2)The major environmental factors affecting the diurnal and seasonal variation of photosynthesis in the rice canopy.The main results are as follows:1.In diurnal variation,rice Ac_ground increases with the increase of photosynthetically active radiation(PAR,μmol m^-2 s^-1),when the PAR at the top of the canopy decreases to about260μmol m^-2 s^-1 on average,Ac_ground is close to 0.There are significant differences in canopy light compensation points in different growth stages and varieties.The canopy light compensation points of HHZ were 131,262,and 218μmol m^-2 s^-1,respectively at three different growth stages which are panicle initiation stage,heading stage and physiological maturity stage.The canopy light compensation points at three different growth stages of NG9108 were 151,422 and 381μmol m^-2s^-1,respectively at three different growth stages which are panicle initiation stage,heading stage and physiological maturity stage.On the other hand,the diurnal variation tendency of Ac_ground existed diversity in different weather conditions and growth stages.Around noon on a sunny day,during panicle initiation stage,HHZ and NG9108 reached light saturation point of about 1900μmol m^-2s^-1 and 1800μmol m^-2s^-1 respectively.Ac_ground decreases by about 7.2%because of the phenomenon named“midday depression”.The diurnal variation characteristics of Ac_ground vary among varieties,weather and growth periods.2.The diurnal variation characteristics of rice LUE were not significantly different between density and varieties,but were significantly distinct at different growth stages and weather conditions.During panicle initiation stage,the LUE showed an obvious double-peak pattern with two peaks growing at about 8:30 am and 16:30 pm.The peak values of diurnal variation were quite different in sunny and cloudy weather,with the mean values of0.0369 and 0.0445μmol CO₂μmol^-1 photon,respectively.During heading stage and physiological maturity stage,the diurnal variation characteristics of LUE was not obviously different,which was basically maintained at 0.0255 and 0.0138μmol CO₂μmol^-1 photons.From panicle initiation stage to physiological maturity stage,LUE reached maximum in the day when the PAR intercepted by the canopy was between 800 and 1000μmol m^-2 s^-1.3.During one day,the correlation coefficient between Ac_ground and PAR radiation is above 0.98,and the correlation coefficient between Ac_ground and temperature is only0.12～0.51.The influence of PAR on Ac_ground is much more obvious than that of temperature.However,in different growth periods and weather conditions,the response characteristics of Ac_ground to PAR are different in one day.In the growth season,the correlation coefficient between Ac_ground and PAR decreased to 0.47～0.83.The seasonal variation of Ac_groundresponds less sensitively to PAR than that of the diurnal variation,and the photosynthetic ability and light interception ability of leaves also played an important role in different growth stages.4.In different planting densities,there were differences in Ac_ground,canopy CO₂ daily net assimilation and LUE.From panicle initiation stage to physiological maturity stage,Ac_ground,canopy CO₂ daily net assimilation and LUE measured treated with low density were slightly higher than those measured in rice treated with low density.The variation trends of canopy photosynthesis with the growth period were similar between the density and varieties,and the Ac_ground and canopy CO₂ daily net assimilation reached the maximum between the panicle initiation stage and the heading stage and then gradually decreased.The LUE was the largest at about 55 days after seeding with an average of 0.0396μmol CO₂μmol^-1 photons.The LUE decreased to 0.0257μmol CO₂μmol^-1photons at heading stage.5.For different varieties,canopy CO₂ daily net assimilation and Ac_groundof HHZ measured treated were higher than NG9108 at around 70 days after seeding.During this period,the canopy light interception rate of HHZ in high and low density reached 85.7%and 81.6%,respectively,and that of NG9108 was only 75.5%and 67.4%.At the late growth stage,the Ac_ground,canopy CO₂ daily net assimilation and LUE of NG9108 were higher than those of HHZ,while NG9108 maintained higher leaf photosynthetic capacity.In summary,the diurnal variation law of canopy photosynthesis in different rice varieties and planting densities is similar.The main influencing factor of diurnal variation is photosynthetically active radiation,and seasonal variation is not only affected by photosynthetically active radiation,but also plays an important role in leaf photosynthetic ability and canopy light interception rate.In terms of canopy light use efficiency,there is large space for improving in high-light environment and after the heading stage.In addition,improving the light interception ability of the pre-growth canopy light interception capability and maintaining the photosynthetic potential of leaf in the post-growth period are conducive to improving the canopy photosynthesis and light use efficiency of trice.