Effects Of Different Light Intensities On Photosynthetic And Physiological Characteristics Of Bletilla Striata

As a traditional medicinal plant in China,Bletilla striata has been widely used in the fields of medicine and chemical industry.In order to explore the mechanism of photosynthesis and physiological response of Bletilla striata under different light intensities,three gradients of light intensity?50%,70%,90%shading rate?were set through the shading network in this study,and the full sunlight was taken as the control?CK?.The dynamic changes of morphological structure,leaf structure,photosynthetic characteristics and physiological indexes were studied,The main results are as follows:?1?Morphological structure:different light intensities had no significant effect on leaf number.Compared with CK,50%,70%and 90%shading treatments increased plant height,leaf length,leaf width and leaf area,under 50%shading treatment,the increase of plant height,leaf length and leaf area was the most prominent.?2?Change of the structure of leaves:the stomatal density decreased gradually with the decrease of light intensity,and the stomatal length,stomatal width and stomatal area showed the rule of CK>90%>50>70%.The results of leaf cross-cutting showed that different light intensities had no significant effect on the cuticle thickness of upper and lower epidermis,and thickness of upper epidermis.50%and 70%treatments increased the leaf thickness,mesophyll thickness and lower epidermis thickness,reduced the distance of adjacent vascular bundles.90%treatment reduced the leaf thickness and mesophyll thickness,and increased the distance of adjacent vascular bundles.The results of chloroplast ultrastructure showed that,50%treatment increased the number of grana and the degree of thylakoid stacking in chloroplast.70%treatment also increased the number of grana,but at the same time increased the number of osmiophilic particles.90%treatment reduced the number of grana and the degree of stroma stacking of grana thylakoid,which was not conducive to chloroplast development.?3?In terms of photosynthetic characteristics,the diurnal variation of photosynthesis shows,the diurnal variation of net photosynthetic rate?Pn?of Bletilla striata leaves under CK treatment showed a“double-peak”curve,shading alleviated the"lunch break"phenomenon under strong light,50%and 70%treatments increased the average diurnal variation of Pn and stomatal conductance?Gs?.90%treatment increased intercellular CO₂ concentration?Ci?,but decreased Pn.Strong light environment produces light inhibition on Bletilla striata leaves of CK,and reduced the average diurnal variation of Pn,Ci and Gs.The results of correlation analysis show that,photosynthetic active radiation?PAR?was positively correlated with Pn in three shading treatments,Pn was negatively correlated with Ci in most months,Pn was positively correlated with Gs from May to August,and Gs was positively correlated with transpiration rate?E?.Effect of the light response parameters:from May to June,under 50%and 70%treatments,the maximum net photosynthetic rate(Pn_max),apparent quantum efficiency?AQY?,light saturation point?LSP?were maintained at a high level,while the light compensation point?LCP?was kept at a low level.Under 90%treatment,LCP are lower,but Pn_max and LSP are also lower.From July to September,Pn_max and LSP increased under CK and 50%treatments.Difference of the perspective of chlorophyll fluorescence parameters:shading treatments increased Fv/Fm;q P and ETR decreased with the decrease of light intensity,while the q N of CK treatment was higher,and there was no significant difference among the three shading treatments.In terms of chlorophyll contents,from May to August,shading treatments significantly increased the contents of Chl a,Chl b and Chl a+b in Bletilla striata leaves,while decreased Chl a/b.In September,under 90%treatment,the contents of Chl a,Chl b and Chl a+b were significantly lower than those in CK treatment,which shows that low light treatment is not conducive to the synthesis of chlorophyll.Rubisco activity:from May and June,the activity of ribulose-1,5-diphosphate carboxylation/oxygenase?Rubisco?in Bletilla striata leaves gradually decreased with the decrease of light intensity.In July,the activity of Rubisco was higher in 50%and70%treatments,but there was no significant difference in CK,50%and 70%treatments from August to September.Compared with CK,90%treatment significantly decreased Rubisco activity in Bletilla striata leaves.?4?In terms of physiological indexes,the changes of antioxidant enzyme activities:from May to September,the activity of SOD increased first and then decreased under 90%treatment,while the activity of SOD in other three treatments increased gradually as a whole,and the activity of SOD in 50%treatment was higher from June to September.The CAT activity decreased gradually from May to September,and the CAT activity of 50%and 70%treatments was higher from May to June.From July to August,the CAT activity of 50%and 70%treatments was not significantly different of CK.In September,the CAT activity of CK and 50%treatments was higher.From May to August,the POD activity was higher under 50%and 70%treatments;in September,CK and 50%treatments had higher POD activity.The activity of APX decreased gradually with time,there was no significant difference in APX activity between CK,50%and 70%treatments,while 90%treatment decreased APX activity.The change of photosynthetic products:the contents of soluble sugar and soluble protein in Bletilla striata leaves decreased gradually with the decrease of light intensity,which indicated that the leaves regulated cell osmotic pressure by increasing the content of osmotic substances under high light intensity.The starch content in Bletilla striata leaves increased gradually from May to September,and in September the starch content of 50%and 70%treatments was the highest,which was beneficial to the improvement of water level of Bletilla striata production,while 90%treatment was not conducive to starch synthesis.To sum up,50%shading treatment improve the development of leaves morphology and chloroplasts,improved photosynthetic efficiency,chlorophyll contents and antioxidant system activity.70%shading increased the photosynthetic rate,but long-term treatment was not conducive to the improvement of quantum efficiency and grana development.90%shading treatment were not conducive to the development of photosynthetic organs,decreased the Rubisco activity and antioxidant enzyme activity,decreased the photosynthetic capacity.Under full light,plants had light inhibition.It is suggested that 50%shading treatment in May to August is beneficial to the growth and development of Bletilla striata.