The Mechanism Of Declined Photosynthesis And Hydraulic Conductivity Under PEG Induced Water Deficit Stress In Rice(Oryza Sativa L.)

Rice is life for more than half of global humanity, and ample amounts of fresh water is needed for rice production to meet the food requirement of increasing global population. Drought is a creeping catastrophe that spreads gradually and tapering irrigated land area with the passage of time, and global warming threatens the irrigated rice production. So, the water security and food security became global issues. The leaf is plant’s heart for life determining process of photosynthesis, so, leaf structural, anatomical as well as physiological studies gained the key importance in recent decades and a huge number of studies were conducted in higher plants as leaf economic spectrum. However, the studies related to these key aspects are at pregnancy stage in cereal crops, particularly in rice crop. Photosynthesis(A) is the key physiological process for crop production and very sensitive to environmental stresses. Plant hydraulic architecture regulates the gas exchange depending upon water availability, and leaves account a major resistance in whole plant hydraulic conductance. However, there is little information about(1) varietal difference in leaf venation architecture in cereal crops, such as rice plants;(2) the effects of PEGIWDS on leaf venation architecture development;(3) the coordination between leaf venation architecture and Kplant as well as Kleaf under PEG-IWDS;(4) whether leaf venation architecture can regulate photosynthesis via plant hydraulic conductivity, and whether their correlations can be affected by water deficit condition. In this study, eight or two rice varieties were grown hydroponically under well watered condition(WWC) and different PEG induced water deficit stresses(PEG-IWDS) conditions. Leaf photosynthetic parameters, leaf venation architecture, Kplant and Kleaf were measured to address above mentioned questions. The results obtained are listed as follows:(1) Leaf photosynthetic rate, stomatal conductance, leaf transpiration rate and plant hydraulic conductivity were significantly decreased under water deficit condition, while specific leaf weight, leaf nitrogen content, SPAD value, leaf vein density, and the minor to major vein ratio were significantly increased. Leaf photosynthetic rate, stomatal conductance, leaf transpiration rate and plant hydraulic conductivity were positively correlated with each other regardless of drought stress. Compared with well-watered condition, the decrease in photosynthetic gas exchange parameters revealed a significant and positive relationship with decrease in Kplant, while leaf vein density(LVD) had a significant but negative relationship with decrease in photosynthetic gas exchange parameters.(2) Leaf venation architecture exhibited significant varietal differences and PEG-IWDS significantly increased LVD while decreased vein thickness. PEG-IWDS sufficiently suppressed both Kplant and Kleaf, and the decrease was much higher in Kplant than Kleaf. There was a significant and positive correlation between LVD and Kleaf under both WWC and PEG-IWDS but the correlation between LVD and Kplant was only significant under WWC. Kleaf was significantly and positively correlated to Kplant under WWC but not under PEG-IWDS.(3) Photosynthesis, plant hydraulic conductivity, carboxylation efficiency(CE), maximum Rubisco carboxylation rate(Vcmax) and maximum electron transport rate(Jmax) decreased significantly under PEG-IWDS. Photosynthesis also revealed significant and positive correlations with stomatal conductance, mesophyll conductance and plant hydraulic conductivity.(4) PEG-IWDS significantly decreased A, gs, and Kplant in both varieties(Hanyou-3 and IR-64), but IR-64 was more sensitive to PEG-IWDS. Leaf vein density was significantly higher in IR-64 than in Hanyou-3. The A, gs and Kplant were significantly and positively related with each other, and they were negatively related to leaf vein density. The A, gs and Kplant were positively related to inter-vein distance and major vein thickness.Therefore, it is concluded that leaf venation architecture plays an important role in Kleaf, and Kleaf is a major determinant for Kplant under WWC but not under PEG-IWDS. The decreased photosynthesis under drought stress is more resulted from stomatal closure, although stomatal, mesophyll and biochemical impairment are also responsible for this decrease in photosynthesis. Photosynthesis is positively correlated with plant hydraulic conductivity, and improving leaf vein density can alleviate the negative effects of water deficit on photosynthesis.