Photosynthetic performance of Vigna radiata L. leaves developed at different temperature and irradiance levels
Introduction
Mungbean (Vigna radiata L.) is one of the most important legumes in the tropics and subtropics. The optimum average temperature for potential yield of mungbean lies between 28 and 30 °C [1]. It is grown in summer when the temperature and light irradiance fluctuate frequently. In some mungbean growing areas of the tropics the early summer is characterized by high temperature (often exceed 40 °C) and cloudy sky, while the late summer has high temperature and bright sunshine. Because of tropical monsoon the irradiance shows regular fluctuation. During the same day it may drop from full sunshine to a minimum level. The day temperature is normally high throughout the long summer but shows fluctuation.
High temperature affects photosynthetic functions of plants by its effect on the rate of chemical reactions and on structural organization. The physicochemical properties as well as the functional organization of thylakoid membrane are changed, reversibly or irreversibly, by high temperature [2], [3], [4]. PSII, particularly, is the most heat-sensitive component of the photosynthetic system, whereas PSI activity, stromal enzymes, or chloroplast envelope are comparatively thermostable [2], [5]. The thermolability of PSII is reported to vary substantially by the influence of various environmental factors [6]. However, the role of light on thermotolerance is less clear. In some reports light has been shown to markedly reduce damage to PSII during heat stress [3], [7], [8], while some others pointed out the injurious effect of light on photosynthetic machinery during heat stress [9], [10]. The quantum efficiency of photosynthesis of a plant is largely reduced (photoinhibition) when it is exposed to excess light level. Excess light induced photoinhibition of photosynthesis, as determined by the chlorophyll fluorescence parameter Fv/Fm [11], is the net result of a complex set of interacting cellular and leaf level processes [12], [13], [14]. However, in most of the studies, related to the role of light on the thermotolerance of photosynthetic apparatus, heat stress was applied either in presence or absence of light. The effects of light intensities on high temperature acclimation of photosynthetic apparatus during heat stress have been insufficiently studied.
Mungbean growing environment (high temperature+clear/or cloudy sky) shows dramatic fluctuations in minutes to hours. The photosynthetic process of this crop has, thus, to be adjusted accordingly. In the present study we have investigated the performance of photosynthetic apparatus of mungbean (Vigna radiata L.) leaves developed under different temperature and light conditions, measuring several Chl a fluorescence parameters and photosynthetic CO2 assimilation rates at different temperature and light levels. In addition, the susceptibility of leaf tissue, developed under such conditions, to photoinhibition of photosynthesis was investigated exposing the tissue to strong light at different temperatures.
Section snippets
Plant material and growth conditions
Mungbean (Vigna radiata L.) seeds var. BARImung-3 collected from Bangladesh Agricultural Research Institute were germinated in petri-dishes in the dark at room temperature. After emergence of radical, the germinated seeds were transferred to 15-l plastic pots containing 4 kg powdered kaolinitic hyperthermic ultisol. The soil was prepared by mixing with NPK fertilizers (14:14:14) at 0.5 g kg−1 soil. The pots were divided into three groups and all the plants were initially grown in three growth
Photosynthetic CO2 assimilation at different light intensities
The photosynthetic CO2 assimilation rate (Pn, μmolCO2 m−2 s−1) increased with increasing PFD, irrespective of levels of temperature (Fig. 1). At both 30 and 40 °C (measurement temperatures) leaves of all the three treatments showed a similar Pn at the lowest PFD. Afterwards, leaves developed at LT+HI (30/25 °C+700 PFD) showed higher Pn at all PFDs when measured at 30 °C and over 700 PFDs when measured at 40 °C. At 40 °C, leaves developed at HT+HI showed a similar Pn to those at LT+HI till 700 PFDs.
Discussion
Both growth temperature and light intensity influenced the photosynthetic efficiency of Vigna radiata L. leaves. At measuring temperatures 30 and 40 °C, Pn of the plants grown at LT+HI was significantly higher at the high PFDs than that of the plants grown under high temperature. Similarly, the leaves developed at 40/25 °C+HI showed remarkably higher Pn than that developed at the same temperature but with low light (LI). Our findings confirm the earlier reports that high temperature reduced
Acknowledgements
We gratefully acknowledge Bangladesh Agricultural Research Institute for providing us mungbean seeds. The senior author is grateful to Japan International Research Center for Agricultural Sciences (JIRCAS) for providing him a fellowship during his study in JIRCAS Okinawa Subtropical Station and a wonderful stay in Ishigaki.
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