Studies On The Mechanisms Of The Responses Of Stomatal Conductance And Photosynthesis To Different Light Environments In Oryza

Rice is the staple food for more than half of the world population.With the continuous decline in rice cultivated area and the increase in world population,it is important to increase rice yield to achieve the food security.In order to improve the plant biomass and crop yields,the improvment in the conversion efficiency of solar radiation intercepted by leaves has received much attention in recent years.The conversion efficiency of solar radiation is mainly determined by leaf photosynthesis,therefore,the increment in the photosynthetic rate is one of the major targets for future high-yield breeding.At present,most studies in photosynthesis are conducted under steady-state and saturated light conditions.However,steady-state are rare in nature,irradiance is intrinsically heterogeneous in time and space received by leaves.Therefore,studying the mechanisms underlying the responses of photosynthesis to different light environments is important to improve the conversion efficiency of solar radiation.Thus,the objective of present study was to explore the mechanisms underlying the responses of photosynthesis to different light environments.Based on the purposes mentioned above,the experiments of present study include three parts.Firstly,the gas exchange and leaf hydraulic conductance were measured under different light intensity in two rice genotypes growing in shading and normal light conditions,to explore the limiting factors of photosynthesis under different light environments;Secondly,with the measurements of the dynamic photosynthesis in different species and rice relatives,the mechanisms underpin the dynamic photosynthesis were studied;Thirdly,stomatal conductance,photosynthesis and stomata anatomical traits were measured under different light environments to evaluated the effect of stomatal size on stomatal conductance in different rice relatives.The main results were as follow.?1?Shading significantly affected the developments of leaf veins and stomata in two rice varieties.In response to shading,the densities of leaf xylem conduits and stomata were decreased by 10-32%and 23-43%,respectively,while stomatal size was increased by 10-43%.Shading significantly decreased leaf hydraulic conductance and mesophyll conductance in HHZ and YY12,with the average decrease of 28.4±8.4%and 51.9±0.8%,respectively,for leaf hydraulic conductance and mesophyll conductance.However,Shading had no significant effect on stomatal conductance in both genotypes.Stomatal conductance and leaf hydraulic conductance were curvilinearly increased with an increase in Photosynthetic photon flux density?PPFD?from 200 to 2000?mol m^-2 s^-1.Mesophyll conductance was significantly increased from 200 to 500?mol m^-2 s^-1,but it did not show any further increase thereafter.Generally,the transport capacities of CO₂and H₂O in leaves were partly coordinated under different light environments.The coordination between mesophyll conductance and leaf hydraulic conductance under different long-term light treatment may be caused by the changes of leaf structures;and under different short-term light intensities,the coordination is only present between stomatal conductance and leaf hydraulic conductance.?2?Compared with normal light condition,photosynthetic rate decreased by 15-31%at different light intensities under shading treatment.However,shading had no effect on the light response of photosynthesis.Photosynthetic rate was curvilinearly increased from200 to 2000?mol m^-2 s^-1,which suggested that a PPFD of 2000?mol m^-2 s^-1is not saturated for A in these two genotypes.There were distinct differences in the limiting factors of photosynthesis between the long-term and short-term light intensity treatments.Under same measurement light intensity,the photosynthetic rate was positively correlated with mesophyll conductance,but not correlated with stomatal conductance.However,under different measurement light treatments,stomatal conductance was positively correlated with leaf photosynthetic rate,but not correlated with mesophyll conductance.This suggested that the response of stomatal conductance was adjusted to meet the demand of CO₂ assimilation under short-term changes of light intensity.?3?After a step change in irradiance from low to high intensity,the photosynthetic response rate varied greatly across the selected Oryza genotypes;the time to reach 50%of the steady-state photosynthetic rate ranged from 125±28 s in E16-21 to 639±133 s in YLY6.The stomatal size was positively correlated with the photosynthetic response rate.The kinetic of photosynthesis is influenced by both stomatal and Rubisco enzyme response rates.Light-induced stomatal opening includes an initial time lag followed by an exponential increase.Smaller stomata had a larger maximum increasing rate during the exponential increase phase,but showed a longer time lag and a lower initial stomatal conductance at low light,thus impeding the stomatal response in early phase.Meanwhile,with a lower initial stomatal conductance and a larger lag time,small stomata showed a faster decrease of intercellular CO₂ concentration during the induction process,which may lead to a slower apparent Rubisco activation rate.?4?Large variations in nocturnal stomatal conductance were observed among the Oryza genotypes.Nocturnal leaf conductance showed a significant circadian rhythm,it gradually increased by 58%from 20:30 to 04:30.On the average,the ratio of nocturnal to day-time transpiration rate was 14.4±6.3%in the selected genotypes.Nocturnal stomatal conductance was positively correlated with stomatal size,the distance between major veins,leaf thickness,leaf area,and leaf width,while was negatively correlated with stomatal density.Stomatal size should be the driver for the correlations between nocturnal stomatal conductance and stomatal anatomy and leaf morphology.Nocturnal transpiration had a significant effect on leaf temperature.Leaves with large stomata had lower nocturnal leaf temperature by maintaining higher nocturnal stomatal conductance and transpiration rate.Besides,leaf morphology,especially leaf width,may also affect nocturnal leaf temperature.