UV-B treatment of 2 h was found to cause increment of about 6% in Ci as compared to control whereas Ci of 3 h treated plants was more or less similar to that of control plants. Under conditions of stomatal limitation, either a decrease in CO2 fixation or an increase in respiration rate can enhance Ci. In the present study, induced expression of RBCS1B and RBCL genes involved in CO2 fixation (Fig. 4) (Discussed in section 3.4) suggests that enhanced Ci here could probably be due to an increase in respiration, which is in-tune with the observations of Cechin et al [26]. Gwyann-Jones [3] had also reported stimulated respiration rate as a short-term impact of UV-B radiation.
In the present study, chlorophyll fluorescence parameters such as Fo, Fm, Fv and Fv/Fm responded differentially to different short-term UV-B treatments (Fig. 3). Fo and Fm enhanced under UV-B exposure. Increase in Fo was most pronounced and occurred at the greatest rate after 4 h of UV-B exposure. The increases in Fo were greater than the increases in Fm, resulting in the decline in Fv/Fm. After 1, 2, 3 and 4 h of UV-B treatment, Fv/Fm was observed to be 0.79, 0.78, 0.78 and 0.72 respectively, whereas, control plants had Fv/Fm value of 0.8. Chlorophyll fluorescence parameters are thought to be very accurate indicators of the state of photosynthetic apparatus and indicate the change in photosynthetic performance. Fv/Fm is a measurement ratio, representing the maximum potential quantum efficiency of PSII and a decline in Fv/Fm was widely reported as a good indicator of photoinhibition [27].
Fig. 3. Effects of different durations of UV-B radiation on chlorophyll fluorescence parameters (Fo, Fm, Fv, Fv/Fm) of A. annua. Separate vertical bars indicate mean value ± ...
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...conclusion, major early responses of A. annua to UV-B radiation were regulatory rather than damaging. Impedance of gas exchange caused reduced transpiration as well as photosynthesis up to 3 h of UV-B exposure. A stable Fv/Fm value up to 3 h of UV-B exposure may indicate that A. annua plants were not experiencing any serious stressful conditions and various ways making it possible could be overexpression of PSBA and PSBO gene transcripts, for rapid D1 protein turnover and well maintenance of Manganese-cluster of PSII respectively, ultimately gauging the PSII health. Furthermore consistent expression of LHCB1, overexpression of RBCS1B, RBCL and CHS as well as potential acclimation mechanisms such as elevated accumulation of flavonoids, phenolics and anthocyanins that filter out UV-B photons, were also contributing effectively to withstand the UV-B impact in A. annua.