Effects Of Output And Accumulation Of Photosynthates On Photosynthetic Compensation Of Maize Under Heterogeneous Light

Plants are usually exposed to heterogeneous light in field and natural environment,that is,different parts of the plant are exposed to different light conditions.For example,maizes are planted in wide and narrow rows in the maize-soybean intercropping system,the light condition of the plants in wide rows is better than narrows rows.However,the light energy of narrow-row side is limited due to mutual shading which lead to the photosynthetic capacity of leaves was reduced.The photosynthetic capacity of wide-row side leaves is usually enhanced under this bilateral heterogeneity light which can compensate narrow-row side leaves for the decreased photosynthetic capacity,which is called photosynthetic compensation phenomenon.This is considered to be an important way for plants to make full use of light resources to adapt to heterogeneous light environment.Previous studies have found that different plants may show significant differences in photosynthetic compensation ability under heterogeneous light environment,and this difference is closely related to photosynthates,but the mechanism is not clear.In this study,two maize varieties "Rongyu 1210"(RY)and "Zhongdan 808"(ZD)with different photosynthetic capacity in intercropping were selected.Heterogeneous light treatment was performed on the plants(LH).That is,the two side blades are unshaded side(US)and shaded side(S).Normal light of the whole plant was the positive control(PC).And shading of the whole plant was the negative control(NC).The effects of the accumulation and transport of photosynthates on the photosynthetic compensation of maize under heterogeneous light were analyzed through the detection of photosynthetic indices,carbohydrates,and related enzymes and genes.The main research results are as follows:(1)The net photosynthetic rate(Pn)increased by 16.4% in RY-unshaded side(RY-US)and decreased by 16.2% in ZD-unshaded side(ZD-US).These results indicate that the difference of photosynthetic compensation capacity between the two varieties mainly occurr in the unshaded side.Further study showed that the stomatal conductance(Gs)of RU-US leaves increased by 25.3%,and the intercellular CO2 concentration(Ci)decrease by 22.8%.However,in ZD-US leaves,the Gs remain unchanged and the Ci increase by15%.This indicate that the fixation of CO2 in RY-US leaves is enhanced,while that in ZD-US leaves is weakened.In conclusion,the photosynthetic compensation capacity of RY is enhanced due to the increase of carbon assimilation capacity,while the photosynthetic compensation capacity of ZD is weakened due to the decrease of carbon assimilation capacity.(2)In RY-S leaves,the level of sucrose content and starch content is increased by16% and 36.6%,while in ZD-S leaves,they are increased by 23.9% and 57.1%.However,the levels of photosynthate compounds in unshaded leaves show different behaviors.The content of sucrose and starch in unshaded leaves are decreased by 39.4%and 28.1% in RY-US leaves.Which are increased by 35.5% and 44.9% in ZD-US leaves.In conclusion,the difference of photosynthate between the two varieties mainly appeared in the unshaded side,which showed a decrease in RY and an increase in ZD.(3)The protein level of sucrose transporter(SUTs)in RY-S leaves decrease by 25%and the SUT1 gene decrease by 14.3%,while the levels of SWEET13 a and SWEET13 b genes increase by 76.0% and 3%.The SUTs protein level and SUT1 gene decrease by28.4% and 14.3% in ZD-S leaves.Meanwhile,the levels of SWEET13 a and SWEET13 b genes increased by 137.8% and 100%.These results indicate that the phloem loading in the shading side leaves of both varieties have similar performance,and the ability of sucrose transport to the apoplast is inhibited.At this time,the levels of photosynthate in leaves is increased significantly,indicating that the accumulation of photosynthate in the shading leaves of both varieties is due to the output inhibition.In RY-US leaves,the level of SUTs and SWEETs is increased significantly by 44% and 16.5%,while the relative expressions of SUT1,SWEET13 a and SWEET13 b genes are increased by 99.3%,62.7% and 48.2%.In RY-US leaves,the sucrose loading capacity of phloem is enhanced.At the same time,the photosynthate compounds in RY-US leaves are decreased significantly,so it is speculated that the increase of phloem loading capacity keep the photosynthate compounds at a low level in leaves.In ZD-US leaves,the protein levels of SUTs and SWEETs are decreased significantly by 27% and 24.5%,while the relative expressions of SUT1,SWEET13 a and SWEET13 c genes are decreased by 43%,34.9%and 68.3%.indicating that the sucrose loading capacity of the phloem is impaired in ZD-US leaves.At the same time,photosynthate compounds increased significantly in ZD-US leaves,so the accumulation of photosynthate compounds in ZD-US leaves is due to the limited loading capacity of phloem.In addition,the relative expression level of TST2 gene is increased by 147.9% and that of SUT2 gene decreased by 80.8% in ZD-US leaves,indicating that excess sucrose is stored in vacuole in ZD-US leaves.(4)The sucrose level in RY-US leaves decreased,and the Tre6 P level have a decreasing trend.The Sucrose level is increased in ZD-US leaves,but are stored in vacuoles in large quantities,and the Tre6 P level is also decreased.Therefore,in the unshaded side leaves of the two varieties,Tre6 P have a tendency to regulate the increase of sucrose synthesis,and the up-regulation tendency is greater in the RY-US leaves.The level of Trehalose 6-phosphate(Tre6P)in RY-S leaves did not change significantly,nor did the activities of Trehalose 6-phosphate synthetase(TPS)and Trehalose 6-phosphate phosphatase(TPP).Which indicate that heterogeneous light have little effect on the Tre6P-sucrose pathway in RY-S leaves.In addition,in ZD-S leaves,The Tre6 P content is increased significantly by 39.1%,the TPS enzyme activity is increased by 27.2% and the TPP enzyme level decreased by 21.3%.In addition,protein level and expression levels of related genes in SWEETs are increased significantly.Therefore,it is speculated that high levels of Tre6 P may be related to loading of SWEETs protein in ZD-S leaves.In conclusion,the photosynthetic capacity of maize is decreased due to the weaken of carbon sequestration ability and the inhibited output of photosynthates under heterogeneous light in shading side leaves.Unshaded leaves with strong photosynthetic compensation ability could improve their photosynthetic compensation ability by enhancing sucrose synthesis and output.On the contrary,in plants with reduced photosynthetic compensation capacity,a large amount of sucrose was transported and stored in vacuoles due to reduced carbon assimilation capacity and reduced phloem loading capacity in unscreened leaves.In addition,Tre6 P has a potential regulatory effect on sucrose synthesis and output in leaves under heterogeneous light,but the specific mechanism needs to be further studied.This study provid reference for further understanding of physiological changes of plants under heterogeneous light,and is helpful for guiding maize varieties breeding.