Effects of shading on photosynthetic characteristics of the medicinal and edible plant Angelica keiskei

Objective This study aimed to investigate effects of different shading treatments on photosynthetic pigment content and photosynthetic characteristics of Angelica keiskei， thereby providing a scientific reference for the understory planting of Angelica keiskei and evaluation of its shade tolerance mechanism.Method The 6-month-old Angelica keiskei seedlings were used as the test materials to carry out the pot experiment. Four treatments were set up： no shading control， mild shading （shading rate of 30%）， moderate shading （shading rate of 50%） and severe shading （shading rate of 70%）. The photosynthetic pigment content of functional leaves was measured after 30， 60 and 90 d of shading， and the photosynthetic characteristics of leaves were measured after 90 d of shading to analyze the changes of each index and their correlations.Result With the increase of shading degree， the chlorophyll a （Chla） content， chlorophyll b （Chlb） content， total chlorophyll content， and chlorophyll a/chlorophyll b （Chla/Chlb） of the three shading treatments increased first and then decreased， and the chlorophyll content under moderate shading treatment was significantly higher than that under other treatments （P<0.05， the same below）. The change trends of light response parameters were different： the apparent quantum yield （AQY） and light saturation point （LSP） increased first and then decreased； the maximum net photosynthetic rate （P_max） increased first， then decreased and then increased； the light compensation point （LCP） continued to decrease； and the dark respiration rate （R_d ） decreased first， then increased and then decreased. The diurnal variation of net photosynthetic rate （P_n） showed a single-peak curve； the stomatal conductance （G_s） and transpiration rate （T_r） increased first and then decreased； the diurnal variation of intercellular CO₂ concentration （C_i） showed a V-shape curve； the diurnal variation of stomatal limitation （L_s） was inconsistent across treatments. The results of correlation analysis showed that P_n had a highly significant positive correlation with L_s （P<0.01， the same below）， but P_n had highly significant negative correlations with C_i， and AQY， significantly positive correlations with LSP and Car； photosynthetic pigment content had highly significant negative correlations with photosynthetic active radiation （PAR）， T_r， G_s， LCP， and R_d， but it had a highly significant positive correlation with AQY； other parameters were correlated to different extents.Conclusion Appropriate shading conditions can enhance the ability of Angelica keiskei to capture and utilize weak light by increasing chlorophyll content， so as to adapt to low light environment， showing a strong light energy utilization and photosynthetic capability of Angelica keiskei. The diurnal variation curves of P_n under different shading treatments are single-peak， showing no indicating no midday depression of photosynthesis caused by strong light inhibition， therefore， the increase of C_i and decrease of L_s are the main reasons for the decrease of P_n， which mainly limit the photosynthesis of Angelica keiskei.