Studies On The Mechanisms For The Effects Of Nitrogen And Water Deficit On Dynamic Photosynsthesis In Rice Crops

As one of the most important staple food crops,rice is the main source of energy and nutrition for people.With the increasing world population and the declining rice production area,crop yield should be dramatically improved to ensure food security.By breeding high-yield varieties and improving cultivation management,harvest index and radiation intercepting efficiency have been significantly improved since 1960s.This finally lead to a dramatic increase in grain yields.Photosynthesis is the basis for biomass production,improving leaf photosynthesis is one of the most important approaches to further increase rice yield.At present,studies on photosynthesis are mostly conducted under steady-state conditions,such as saturatiing light,optimum temperature and humidity.However,steady-state conditions is rare under natural conditions,especially for the irradiance.It can suddendly increased from 1 to 2000?mol m^-2 s^-1 within several seconds or minutes.The light intensity received by the leaves in canopy,especially for the understory of the canopy,changes rapidly due to wind,cloud cover,dayly variation in incoming solar irradiance,and self-shading of the upper leaves.It is reported that the recovery of photosynthesis lags behind the change of light intensity.Total carbon gain of plant under dynamic light environment were significant lower than these under steady-state light environment.Nitrogen is one of the essential nutrients for plant growth,it is also a major component for many important organic compounds in plants.Nitrogen can increase the steady-state photosynthetic rate of rice.However,it is not clear whether nitrogen can affect the dynamic photosynthesis of rice.Therefore,the present study will investigate the effect of nitrogen on the dynamic photosynthesis in rice.In addition,water deficit is considered to be another major environmental factors limiting photosynthesis and crop growth.Under water deficit,photosynthetic rate decreases as a consequence of stomatal closure,decline in mesophyll conductance and biochemical capacity.However,it is not known whether water deficit can affect the response rate of photosynthesis to dynamic irradiance.Therefore,two experiments were conducted in present study:?1?Pot experiments supplied with two nitrogen concentrations were conducted,to study the efffect of nitrogen on dynamic photosynthesis in rice.The objective is to investigate the limitation of Rubisco activity and stomatal conductance to dynamic photosynthesis;?2?Hydroponic experiments were conducted by supplying three water statues:well-water condition,15%PEG?m/v,polyethylene glycol,6000 Da?and 20%PEG induce water deficit conditions.The objective is to investigate the effects of water deficit on dynamic photosynthesis,and to verify whether the negative effects of water deficit on photosynthesis are deteriorated under flecked irradiance.The major results are listed as follows:1.Firstly,the induction states?IS%?of photosynthetic rate and stomatal conductance declined with increasing low light period.The IS%of photosynthetic rate under HN was significantly higher than LN after exposing to 5 and 10 mins of low light.This suggested that nitrogen can improve the activation state of photosynthesis system during dark periods.Secondly,nitrogen can improve the recovery rate of photosynthetic rate(T_90%A)in rice,which was 10.9 min under HN in comparison with 17.9 mins under LN.Thirdly,the maximum photosynthetic rate under flecks(A_max–fleck)was significantly higher in HN treatment than LN.The relative higher A_max–fleck and lower T_90%A finally contribute to a less carbon loss under HN?25.6%?than under LN treatment?41.45%?.Fourthly,the Rubisco activation rate calculated from a photosynthesis model was significantly lower under HN?4.4 mins?than under LN treatment?6.0 mins?.After a transition from low to a high light level,the limitation of biochemical process for photosynthesis was larger than that of stomata.Therefore,nitrogen can improve the rapid response of photosynthesis under flecks,and the rate of Rubisco activation is the most important factor affecting dynamic photosynthetic in rice.2.Compared with well watered condition,the maximum photosynthesis under lightflecks(A_max–fleck)in Champa was decreased by19.2%and 43.5%,respectively,under under 15%and 20%PEGtreatments.And it was decreased by 11.7%and 32.1%,respectively,in YLY6.Secondly,the maximum electron transport capacity(J_max)and the maximun Rubisco carboxylation capacity(V_cmax)were significantly decreased under 20%PEG treatment in Champa,but water deficit had no significant effect on them inYLY6.Therefore,the inhibition of photosynthesis by water stress is larger in Champa than in YLY6.In addition,IS%of photosynthetic rate and IS%of stomatal conductance were highly correlated in both cultivars,and they were both significantly decreased under severe water deficit.The minimum photosynthetic rate under lightflecks(A_min–fleck)was significantly lower than steady-state photosynthetic rate under the same light intensity(A_initial)in both two cultivars,and the difference was larger under water deficit condition in comparison with well watered condition.In comparison with steady-state irradiance,carbon assimilation under flecked irradiance was significantly decreased.The lower IS%of photosynthesis udner water deficit condition,and the larger difference between A_initialnitial and A_min–fleck under water deficit resulted in a larger carbon loss than under well watered contion.Therefore,water deficit can ehanced the inhibition of flecks on photosynthesis in rice.