Mechanisms Underlying The Effects Of Leaf And Root Structures On Crop Photosynthesis

Photosynthesis is the basis of crop biomass and yield.It has become the consensus of scientists that improving crop photosynthesis is the most promising approach to further increase crop yield in the future,especially considering that the harvest index is close to the theoretical maximum.However,how to improve crop photosynthesis remains to be explored,especially what leaf structural traits could be used to improve crop photosynthesis is still unclear.Increasing the nitrogen application can improve crop photosynthesis and thus increase crop biomass and yield.However,the nitrogen utilization efficiency will gradually decrease and the environmental pollution will increase with the increment of nitrogen application.Therefore,in the context of reducing the nitrogen application,it is very important to explore the leaf and root structural traits that are associated with high nitrogen use efficiency of crops.In addition,photosynthesis is extremely sensitive to external environmental factors such as temperature.Under the background of global warming,how to understand and predict crop growth largely depends on how crop photosynthesis responds to temperature.Therefore,this study mainly attempts to explore the above three questions from the perspective of leaf and root structures.According to the research purpose,this study mainly includes three parts:(1)explore the possible ways to improve crop photosynthetic efficiency through meta-analysis and rice pot experiment;(2)investigate the traits that underlying the high nitrogen use efficiency of wheat plants under both high and low nitrogen growth conditions and(3)examine the impacts of leaf structure on the response of mesophyll conductance to temperature among C3 crops.The main results are as follows:(1)Both during the processes of plant evolution and domestication,leaf photosynthetic capacity has been significantly improved.Compared with gymnosperms,area-based leaf photosynthetic rate,mesophyll conductance,stomatal conductance and Rubisco maximum carboxylation rate of non-crop angiosperms increased by 44.8%,68.2%,82.2%and 39.4%,respectively;Compared with non-crop angiosperms,the above parameters of crops increased by 130.7%,129.9%,195.1%and 30.3%,respectively,indicating that the improvement of photosynthesis in the processes of plant evolution and domestication is mainly caused by the increment of CO2 transport capacity,while increment of photosynthesis has little relationship with the leaf biochemical capacity.In addition,from ferns and fern allies to gymnosperms,the chloroplast surface area facing intercellular airspace and cell wall thickness increased by 158.0%and 32.1%,respectively,while there was no significant difference in mesophyll conductance between the two plant groups;From gymnosperms to non crop angiosperms,the chloroplast surface area facing intercellular airspace and cell wall thickness decreased by 32.6%and 35.6%,respectively,though the mesophyll conductance increased significantly,indicating that cell wall thickness plays a more important role than chloroplast surface area facing intercellular airspace in regulating mesophyll conductance.Therefore,our present results suggested that increased crops photosynthesis could be achieved by modifying cell wall thickness and/or properties.(2)Larger xylem conduits contributed to higher leaf hydraulic conductance and stomatal conductance,which consequently resulted in higher leaf photosynthetic rate in rice plants.In addition,larger phloem size was also associated with higher leaf photosynthetic rate,which might be caused by the increased transport capacity of photosynthetic products.(3)Mass-based leaf photosynthetic rate,stomatal conductance and mesophyll conductance increased with specific leaf area across different wheat varieties both under high and low nitrogen treatments.However,positive relationship between photosynthetic nitrogen use efficiency and specific leaf area was only observed under high nitrogen treatment,which suggested that increasing specific leaf area could synergistically improve leaf photosynthetic rate and photosynthetic nitrogen use efficiency in wheat plants.Additionally,nitrogen can significantly affect the relationships between plant traits.Under low nitrogen treatment,high shoot biomass was associated with low specific leaf area and specific root length but large root diameter,which contributed to a high tolerance of wheat plants to nitrogen deficiency.Thus,in areas which require low or limited nitrogen input,wheat species with thicker root could contribute to higher shoot biomass accumulation.(4)There were large variations in the response of mesophyll conductance to temperature among different C3 crops.Mesophyll conductance increased significantly with temperature in rice,cotton,soybean and sunflower,while that of spring wheat,winter wheat,barley,broad bean and rape showed less responsiveness to temperature variations.The larger the chloroplast surface area facing intercellular airspace is,the more sensitive of mesophyll conductance to temperature.Using two-component model,this study found that the temperature response of mesophyll conductance was co-determined by the ratio of membrane conductance to liquid conductance and the response of membrane conductance to temperature.Although leaf vein density could impact the response of mesophyll conductance to temperature by affecting leaf water potential,effect of which was limited.Our results indicated that leaf anatomical traits could impact the temperature response of mesophyll conductance among different C3 crops.