Effects Of Different Water Stress On Soybean Photosynthesis And Chlorophyll Fluorescence

In recent years,chlorophyll fluorescence has been widely used as a probe of photosynthesis in the research of water stress by virtue of its rapid and sensitive characteristics of reflecting the physiological state of vegetation.This study is based on the soybean drought experiments carried out at Nanjing University of Information Science and Technology from 2019 to 2020.The LI-6400XT portable analyzer was used to measure soybean gas exchange parameters,chlorophyll fluorescence parameters under different water stresses in each growth period.The effects of different droughts on photosynthesis and chlorophyll fluorescence of soybeans,and the relationships between them was explored.It aims to provide ground theoretical support for the currently developing chlorophyll fluorescence monitoring of vegetation photosynthesis,and further new monitoring methods for carbon cycle research.The main research conclusions are as follows:(1)Moderate water stress in the seedling and flowering stage was beneficial to promote the growth of soybeans,but when the water stress accumulated to a certain extent,the chlorophyll content will decrease.The effects of water stress in different growth periods on soybeans were not the same,and the most sensitive period was the flowering period.Under the drier conditions,the lower the N content of soybean leaves was.With the increase of time,the N content of soybean leaves gradually decreased,and the degree of defense and toughness of the leaves declined as well.(2)Under different water stress conditions,soybean leaf net photosynthetic rate(P_n),stomatal conductance(G_s),and transpiration rate(T_r)decreased with the increase of water stress.Drought led to a decrease in P_n,an increase in stomata closure,and a decrease in T_r,which also reduced the loss of leaf water and increases its own resistance.In the diurnal changes of photosynthetic parameters during the flowering period of soybean,P_n,G_s,and T_r all showed a bimodal curve,and the phenomenon of"noon break"appeared at noon,and in the order of the control>mild drought>moderate drought.The similar change curves of the three indicated that the change of T_r was related to the regulation of leaf stomata.The water stress from the flowering stage had less impact on P_n than the water stress from the seedling stage.(3)The photochemical efficiency(F_v'/F_m'),apparent electron transfer efficiency(ETR)and photochemical quenching coefficient(q _P)of Photosystem II(PS?)under different water stress treatments showed the order of control>mild drought>moderate drought.The more drought was,the lower the energy provided for photosynthesis was.The values of q _P and ETR during moderate drought from the maturity period were similar to those under the control group,indicating that soybean growth was not affected in this case.In the diurnal changes of the fluorescence parameters during the flowering period of soybean,F_v'/F_m',q _P,non-photochemical quenching coefficient(q _N)and ETR all showed a downward trend,and the overall performance was in the order of control>mild drought>moderate drought.The initial fluorescence(F_o')under light basically showed a decling trend first and then increased,and the overall performance was in the order of drought>control.This is a manifestation that the extent of damage to the PS?reaction center in soybean leaves increased with the increase of water stress.(4)P_n decreased with the increase of water stress.The effect of drought from the flowering period on soybean P_n was smaller than that from the seedling period,and drought from the mature period had almost no effect on soybean P_n.P_n has a negative correlation with F_o',and a positive correlation with F_v'/F_m',q _P,q _N,ETR,and the correlation coefficients of each growth period are mature stage>flowering stage>seedling stage.Soybeans at seedling stage were the least affected during the entire growth period,and photosynthetic electron transfer was also the least affected.As the growth time goes by,the damage to the leaf photosynthetic apparatus and the photosynthetic electron transfer rate were increasing.The impact was getting bigger and bigger.Water stress from the maturity period not only had little effect on soybeans,but also helped increase the opening ratio of PS?reaction centers and the rate of photosynthetic electron transfer.