Diurnal Variation of Photosynthetic Physiological Characteristics of Kadsura coccinea (Lem.) A. C. Smith

Zongyou CHEN Shengfeng CHAI Xiaoxue QI Yujing Wei Huafei CEN

Abstract [Objectives] This study was conducted to investigate the diurnal variations of photosynthetic and physiological characteristics in 3-year-old Kadsura coccinea （Lem.） A. C. Smith plants.

[Methods]A Li-6400 portable photosynthetic system was used to study the diurnal variations of the photosynthetic physiological characteristics of K. coccinea.

[Results] The diurnal variation of the net photosynthetic rate （Pn） of K. coccinea leaves showed a "double peak" curve， and there was a lunch break at noon. The maximum value of Pn [4.51 mmol/（m2·s）] appeared at 10：00， and the daily average Pn was 3.98 mol/（m2·s）; and the stepwise regression analysis showed that photosynthetically active radiation （PAR） and stomatal conductance （Gs） were the main factors affecting Pn. The partial correlation analysis and path analysis showed that the order of effects on Pn was Gs>PAR.

[Conclusions]This study provides a reference for further research on the development and utilization of K. coccinea.

Key words Kadsura coccinea （Lem.） A. C. Smith; Net photosynthetic rate; Daily variation; Photosynthesis

Received： May 5， 2021  Accepted： July 6， 2021

Supported by Guangxi Key R&D Program Project （GKAB18221091）; Guilin Scientific Research and Planning Science and Technology Key Project （20160223-1）.

Zongyou CHEN （1980-）， female， P. R. China， associate researcher， devoted to research about introduction， domestication and molecular ecology of characteristic economic plants.

*Corresponding author. E-mail： qixiaoxue@126.com.

Kadsura coccinea （Lem.） A. C. Smith， belonging to Kadsura in Magnoliaceae， also known as Lengfantuan， Guoshanlongteng， Dazua， Renmianguo （Longsheng， Guangxi）， Bufuna （Miao language）， is an evergreen woody vine wild medicinal plant and wild fruit. It is native to the virgin forests of southwestern China， mainly distributed in the hillsides and valleys of Guangxi， Yunnan， Hunan， Sichuan， Guizhou， Guangdong and other provinces. It is often entangled on trees. For a long time， K. coccinea has been excavated， collected and used as a rhizome medicinal plant， and plus the deforestation and the destruction of vegetation， its wild resources have been on the verge of extinction.

With the development of science and technology and the improvement of peoples living standards， the medicinal[1-13] and health care value[14-18] of K. coccinea is constantly being explored. It is no longer limited to root medicine， and its fruit is developed and utilized as a characteristic fruit[14-19]. At present， K. coccinea is grown as a fruit nationwide， and the planting area is constantly increasing. However， the existing problems are particularly prominent， such as the survival rate of K. coccinea seedlings after planting， the length of the production cycle from seedling stage to fruiting stage， the yield and quality of K. coccinea fruit， etc. In response to these problems， in 201 we conducted a study on the diurnal variations of the photosynthetic physiological characteristics of adult K. coccinea plants grown in the K. coccinea germplasm resource garden of the Guangxi Institute of Botany， Chinese Academy of Sciences， Guangxi Zhuang Autonomous Region.

Materials and Methods

Experimental materials

The materials used in the experiment were adult K. coccinea plants cultivated in the K. coccinea germplasm nursery of Guangxi Institute of Botany， Chinese Academy of Sciences， Guangxi Zhuang Autonomous Region.

Experimental methods

The Li-6400 portable photosynthetic system （Li-Cor， Inc， USA） was used to measure the daily photosynthetic progress of K. coccinea leaves. The upper leaves of the plants growing out in the very year were selected for photosynthesis determination. Three trees that grew normally were selected， and one leaf was measured for each plant. The spatial orientation and angle were as consistent as possible. All the leaves were westward and basically parallel to the ground.

A typical sunny day （October 29， 2012） was chosen， and the leaves were measured once every 1.5 h from 8：30 to 17：30， with four repetitions each time. The measurement items included the net photosynthetic rate （Pn）， transpiration rate （Tr）， stomatal conductance （Gs） and intercellular CO2 concentration （Ci） of plants， etc. The environmental factors included photosynthetically active radiation （PAR） and air CO2 concentration （Ca）， air temperature （Ta）， leaf temperature （Tl）， relative air humidity （RH）， etc.

Data processing and analysis

Statistical analysis of the measured parameters was performed with SPSS13.0 （SPSS Inc.， USA）， and Sigmaplot 9.0 （SPSS Inc.， USA） was used for plotting.

Results and Analysis

Diurnal variations in environmental factors

The daily variations of PAR， Ca， Ta and RH in the microenvironment of the sample plot are shown in Fig. 1. From 8：30 to 17：30， the PAR changed from 29.8 to 571.5 μmol/（m2·s）， reaching 166.0 μmol/（m2·s） at 8：30 in the morning， and it was above 550 μmol/（m2·s） from 11：30 to 13：00， and decreased rapidly after 17：30. The variation range of Ca was 396.8-436.7 μmol/mol， with the maximum appearing at 8：30 in the morning， and the minimum appearing at 14：30 in the afternoon. Ta varied from 22.2 to 32.9 ℃， and the temperature difference during the day was large. The temperature was the lowest at 8：30， reached 32.86 ℃ at 13：00， and then gradually decreased. The RH varied from 35.0% to 62.6%， and was the highest at 8：30 in the morning and the smallest at 13：00 in the afternoon.

Diurnal variations of photosynthetic characteristic parameters

Diurnal variations of net photosynthetic rate and intercellular CO2 concentration

The diurnal variation of Pn in K. coccinea showed a "double peak" curve （Fig. 2）， and there is a "noon break" phenomenon. The maximum value of Pn appeared at 10：00， reaching 5.96 μmol/（m2·s）， and PAR was 314.5 μmol/（m2·s） at this time. After that， Pn gradually decreased until 13：00， when it reached a trough， at 4.43 μmol/（m2·s）， while PAR reached its maximum value of the day， which was 571.5 μmol/（m2·s）， indicating that the photosynthesis of K. coccinea was photoinhibited at noon. At 14：30 in the afternoon， Pn increased slightly， after which Pn decreased rapidly due to the decrease in light intensity. The daily average Pn of K. coccinea was 3.98 μmol/（m2·s）. The diurnal variation of Ci was in the shape of "V". At 8：30 in the morning and 17：30 in the afternoon， Ci was at high levels， 369.0 and 363.8 μmol/mol， respectively. By 13：00 at noon， Ci was at the lowest level in the whole day， at 171.0 μmol/mol.

Diurnal changes in stomatal conductance and transpiration rate

The stomatal conductance almost showed a downward trend throughout the day. At 8：30 in the morning， Gs reached the maximum [0.109 mmol/（m2·s）]， and then gradually decreased. By 13：00 at noon， Gs reached a trough [0.034 mmol/（m2·s）]， and at 14：30， it rose slightly， and then gradually decreased， reaching its minimum value [0.013 mmol/（m2·s）] at 17：30. The diurnal variation of transpiration rate showed a "double peak" curve. With the increase of light and temperature， Tr gradually increased， reaching the first peak at 11：30 [1.41 mmol/（m2·s）]， and thereafter it decreased slightly， reaching another peak at 14：30 [1.18 mmol/（m2·s）] （Fig. 3）.

Correlation analysis between environmental factors and photosynthetic characteristic parameters

In order to further study the relationship between the photosynthetic characteristic parameters of K. coccinea and its environmental factors， the correlation analysis was carried out. It can be seen from Table 1 that Pn had a significant positive correlation with Tr， an extremely significant positive correlation with Gs， and no significant correlation with other factors. Moreover， among the various physiological and ecological factors， PAR had a significant positive correlation with Tl， an extremely significant positive correlation with Tr， and a significant negative correlation with RH and Ci; Ta was in an extremely significant positive correlation with Tl， and a significant or extremely significant negative correlation with Ca， RH， and Ci; Ca was significantly positively correlated with RH; and RH was significantly positively correlated with Ci.

In order to determine the degrees of influence of various physiological and ecological factors on the photosynthetic characteristics of K. coccinea， a stepwise multiple regression method was used to select the factors that had a greater impact on the photosynthetic rate to establish an optimal regression equation. After calculation， the stepwise regression equation was Pn=-0.883+0.004*PAR+ 90.986*Gs （r2=0.993）， and PAR and Gs were the main factors affecting Pn. The partial correlation analysis showed that the effects of PAR and Gs on Pn reached a significant level， and the path analysis showed that the order of the degrees of influence on Pn was： Gs>PAR （Table 2）.

Conclusions and Discussion

Photosynthesis is a very complex physiological process. The photosynthetic efficiency of leaves is not only closely related to its own factors such as chlorophyll content， leaf thickness， and leaf maturity， but also affected by external factors such as light intensity， temperature， air relative humidity， and soil moisture content[20]. In this study， the diurnal variation of the net photosynthetic rate of K. coccinea showed a "double peak" curve， and there was a lunch break at noon. The maximum value of Pn [4.51 mmol/（m2·s）] appeared at 10：00， and the daily average Pn was 3.98 μmol/（m2·s）. It can be seen that K. coccinea is intolerant to strong light， which is consistent with the results of Qi et al.[21]. If the light intensity is above 550 μmol/（m2·s）， its photosynthesis is inhibited. The correlation analysis of the net photosynthetic rate （Pn） of K. coccinea and the physiological and ecological factors showed that the stomatal conductance （Gs） and transpiration rate （Tr） were significantly correlated with the net photosynthetic rate （Pn）. The stepwise regression analysis showed that PAR and Gs had greater impacts on Pn， and the effects ranked as Gs>PAR.

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