Studies On The Mechanisms Of The Varietal Differences In Leaf Photosynthesis In Oryza Sativa L.

Leaf photosynthesis is the basis for biomass formation,the improvement of leaf net photosynthetic rate?A?is one of the most promising approaches to further boost rice yield in future.Studying the limiting factors of the photosynthetic capacity in rice can provide theoretical evidences for high-yield rice breeding.In the present study,four pot experiments were conducted to investigate the mechanisms of the varietal differences in photosynthetic characteristics and the associated traits:?1?The intraspecific variations in leaf nitrogen contents,leaf gas exchange parameters,leaf mass per area?LMA?,leaf thickness?LT?and leaf density?LD?in Oryza sativa L.as well as the coordination between parameters were investigated using 121 rice germplasm materials;?2?The varietal differences in leaf venation traits and their effects on gas exchange were investigated using 6 rice lines showing a large variation in stomatal conductance?g_s?;?3?The varietal differences in leaf anatomical traits,leaf chemical composition and their effects on gas exchange were investigated using 8 rice lines showing large variation in LMA;?4?The effects of nitrogen application rates on leaf anatomical traits,leaf chemical compositions were investigated using 4 rice lines showing large variation in LMA.The main results are as follows:1. The mass-based(N_mass)and area-based(N_area)leaf nitrogen contents,A,g_s,leaf transpiration rate?E?,LMA,LT and LD all showed great varietal differences.Allparameters were normally distributed,and all varied by 2-folds among variates.Aranged from 15.5-32.6mmol m^-2 s^-1.The A of the 4 widely grown rice cultivars inChina–Huanghuazhan,Liangyoupeijiu,Shanyou 63 and Yangliangyou 6,were atthe average level among the 121 lines,and were significantly lower than the A ofNancay PA which possessed the highest A.This suggested that leaf photosynthesisof the widely grown rice cultivars can be largely improved and,hence to furtherincrease the yield.A was positively related to N_area across the 121 rice lines.However,A was not correlated with N_area when the N_area were larger than 1.29 g m^-2,which indicated that N_area was not the main reason for the varietal differences inA when N_area was above this threshold.g_s was positively correlated with both N_massand N_area.A and E were positively correlated with g_s across the 121 rice lines.Thevarietal difference in g_s was one of the reasons for the varietal difference of A.Photosynthetic nitrogen use efficiency?PNUE?was positively related to A acrossthe 121 rice lines.Nancay PA possessed the highest PNUE and A among the 121rice lines,which can be used as a high photosynthetic capacity and highphotosynthetic efficiency material for rice breeding.2. Total leaf vein density,minor leaf vein density and the portion of minor vein length to total leaf vein length showed no varietal differences,but the major leaf veindensity showed a great varietal difference.Rice lines with high total and minor veindensities showed a high leaf hydraulic conductance and,hence,a higher g_s and A.3. LT was positively correlated with LMA,while the fraction of the intercellular airspace was not correlated with LMA.The mesophyll cell surface area exposed tointercellular airspace per leaf area?S_m?and the surface area of chloroplasts exposedto intercellular airspace per leaf area?S_c?were positively correlated with LT andLMA.Cell wall thickness was positively correlated with LMA.Mesophyllconductance?g_m?was negatively correlated with both cell wall thickness and LMA.As the main structural compositions of the leaf,cell wall compounds accounted for71.5%of the leaf dry mass,and non-structural carbohydrates?NSC?accounted for10.0%of the leaf dry mass.Rice lines with a high LMA invested more leaf mass tocell wall compounds and less to leaf compositions associated with photosynthesis,including leaf nitrogen and chlorophyll.The mass-based leaf net photosynthetic ratewas negatively correlated with LMA.4. N_mass,N_area and A were increased with increasing nitrogen application,however,different nitrogen supplies had no effects on g_s,g_m and PNUE.Nitrogen applicationrates had no significant effects on LMA and cell wall thickness.The responses ofLT and LD to nitrogen supplies were inconsistent in different rice materials.LT ofthe tested materials Sab Ini and Huanghuazhan were larger under LN than underHN treatments,while LT of N22 was lower under LN than under HN treatments.The LD of Sab Ini was lower under LN than under HN treatments,but the LD ofN22 and Yongyou 12 were higher under LN than under HN treatments.Except forthe soluble sugar in Huanghuazhan,nitrogen application rates showed nosignificant effects on mass-based chemical compositions?pectic substance,hemicellulose,cellulose,lignin,soluble sugar,starch and NSC?.The area-basedsoluble sugar and NSC contents of Sab Ini were lower under LN than under HNtreatments,and the area-based cell wall content of Yongyou 12 was higher underLN than under HN treatments.Conclusions:Photosynthetic characteristics and associated traits including leaf nitrogen contents,LMA?etc.?all showed significant varietal differences in 121 Oryza Sativa L.germplasms under same nitrogen supply;The variations in stomatal conductance is one of the reasons for the varietal difference in A.High vein density especially high minor vein density promoted the leaf hydraulic conductance,and ultimately led to high g_s and high A.High LMA rice genotypes invested more leaf mass to cell wall and less to inclusions associated with photosynthesis like leaf nitrogen and chlorophyll,and showed a low g_m and A_mass.Nitrogen application rate impacted A greatly,while had no effect on other photosynthetic characteristics?g_s,g_m and PNUE?;nitrogen application rate didn't impact cell wall thickness;nitrogen application rate showed inconsistent effects on different rice genotypes considering their leaf anatomical traits and leaf chemical compositions,the underlying mechanisms should be further investigated in future.