| High crop yield has always been one of the hot spots in the agricultural field at home and abroad.As the global population gradually increases and the area of cultivated land decreases year by year,how to increase crop yield is the core challenge of plant biology.Wheat is the third largest food crop in China,and its total output accounts for about 22%of total food production.It is an important ration crop and occupies a dominant position in China’s food security.For a long time,increasing yield has been the primary task of wheat genetic improvement.Shandong Province is China’s largest wheat planting province.It has experienced eight wheat variety replacements since the founding of the People’s Republic of China,and its yield per mu has increased significantly.It is a common research method in agronomic research to analyze the succession rules of important agronomic characteristics in variety replacement and find a feasible way for further genetic improvement.Photosynthesis provides a material basis for wheat plant growth and yield formation,and is a decisive factor for wheat yield.Therefore,the photosynthetic characteristics of wheat varieties over the years have attracted much attention.However,most of the previous studies only studied the maximum photosynthetic capacity of wheat leaves under saturated light intensity,and ignored the changes in light environment caused by changes in plant height and plant type during variety replacement.Changes in light receiving environment will significantly change the structure and function of leaf photosynthetic mechanisms.It is unclear how the photosynthetic environment on the surface of wheat leaves will change as the varieties change,and whether the photosynthetic mechanism of the leaves will fully adapt to this change.In order to explore these issues,this study used 26 varieties widely cultivated in eight varieties of Shandong as experimental materials（a more detailed study of the eight representative varieties）,systematically analyzed the agronomic characteristics of wheat,the light environment of the leaves,Succession law of maximum photosynthetic capacity under leaf saturated light,limiting light energy utilization efficiency under light environment,leaf photoinhibition resistance,and main photosynthetic related protein content.The main results are as follows:（1）With the change of wheat varieties,the ear number per ha increased significantly,plant biomass and grain yield increased significantly at the same seeding amount;wheat plant height,length of first internode,length of the second and third internode significantly decreased;there was no significant changes in areas of flag leaves,invered andinvered three leaves;the kernel number per ear increased significantly and the spike length shortened.The above results indicate that as the wheat cultivation density in the field gradually increases,this may lead to more severe shading between plants.Although the ear volume decreased,the shorter plant height and internode length made the ears closer to the leaves,which may cause the upper wheat ears to block the lower leaves more seriously.（2）In order to verify the speculation that the accumulated light intensity on the leaf surface weakened as the cultivar was replaced,we measured the 24-hour accumulated light of the above-ground organs of wheat cultivars in different years.The results showed that with the change of varieties,the accumulated light in the adaxial and abaxial of flag leaf and the invered second leaf of the wheat decreased significantly.Among them,the flag leaf and the invered second leaf had the most obvious decline in cumulative light,reaching 63.8%and61.6%,respectively.This indicates that the breeding work has significantly changed the light receiving environment of wheat leaves in the field.Compared with earlier varieties,the leaves of modern wheat varieties are in a weaker growth light environment.The structure and function of photosynthetic mechanisms are strongly affected by light intensity.In order to analyze the effect of weaker growth light intensity on leaf photosynthetic performance,we next analyzed the photosynthetic rate of flag leaves under different light intensities,PSII photoinhibition under strong light,and the content of leaf photosynthetic related proteins in wheat varieties of different ages.（3）With the change of variety,the net photosynthetic rate of flag leaf under saturated strong light(1600μmol·m-2·s-1)did not change significantly.We obtained this result in two years of field trials and trials of wheat seedlings under controlled indoor conditions.In addition,we found that the rate-limiting enzymes of photosynthetic carbon assimilation（Calvin cycle）in flag leaf,ribulose-1,5-bisphosphate carboxylase（Rubisco）,and his activating enzyme（RCA）content gradually changed with the change of varieties.The total nitrogen content of flag leaves did not change significantly with the succession of varieties.The above results show that in order to adapt to the low light growth environment,wheat has reduced the resource allocation to the Calvin cycle of the leaves.Rubisco is the largest nitrogen pool of plant leaves,and its content has decreased while the total nitrogen content of the leaves has not decreased,indicating that plants use more nitrogen in other metabolic processes.（4）Two years of field trials and trials of growing wheat under controlled indoor conditions have shown that the net photosynthetic rate of flag leaves under a limited light environment(400μmol·m-2·s-1)and the apparent quantum efficiency of the flag leaf obtained below 50μmol·m-2·s-1-200μmol·m-2·s-1.The chlorophyll content,chlorophyll a/b ratio,and carotenoid/chlorophyll content did not change significantly with the change of variety.The contents of the main proteins Lhcb1 and Lhcb2 of the light system II light-harvesting antenna complex,which are closely related to the absorption of light energy,remain constant during the replacement of the variety,while the core protein D1 protein of the PSII reaction center gradually decreases with the replacement of the variety,resulting in the ratio of the antenna to the response center increase.In recent years,wheat has not significantly increased its resource allocation to antennas,which has limited the improvement of its light energy absorption capacity.This may be one of the reasons for its lack of increase in light utilization efficiency.This study found that with the change of variety,the cumulative light intensity on the surface of wheat leaves gradually decreased due to changes in plant height and plant type.The photosynthetic apparatus of modern wheat varieties changed significantly to adapt to the weakening of light environment,but part of the adaptive response was significantly different from the typical weak light adaptive response.We speculate that this is the result of artificial selection.Artificial breeding makes wheat leaves to maintain the maximum photosynthetic capacity and the light resistance of PSII under the condition of weakened light environment and weakened Calvin cycle-related enzyme content,which is obviously beneficial to the efficient use of light energy in the field and the final yield.Of improvement.However,breed selection did not significantly improve the light energy use efficiency of leaves under low light,which is not conducive to the high yield of modern wheat varieties.In addition,because the current understanding of the relationship between photosynthesis and yield under complex environmental conditions is not comprehensive,and the complexity of photosynthesis itself,we cannot determine whether changes in leaf photosynthetic characteristics that occur during succession of wheat varieties are conducive to the use of light energy Rate and yield increase.Therefore,future research needs to pay attention to（1）how is this photosynthetic characteristic different from the typical weak light adaptive response formed?（2）Does this adaptation response benefit the efficient use of light energy and grain yield?In summary,this study proposes that in the future selection and breeding of wheat varieties,we should focus on improving the light energy utilization efficiency of leaves under low light,and provide a new perspective for building optimal wheat photosynthetic performance. |
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