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 . Gwyann-Jones  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 .
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 ± SE having five replicates each. Means with different letters are significantly different (P< 0.05).
Our results suggest that only modest inhibition of Fv/Fm occurred up to 3 h of UV-B exposure and the values were quite near to the range (0.79 to 0.84) set for healthy plants , indicating the absence of any major PSII photoinhibition and the subsequent photodamage atleast up to 3 h of UV-B exposure in A. annua. UV-B exposure for 4 h suddenly dropped the Fv/Fm ratio to 0.72, revealing UV-B-mediated photoinhibition upon long exposure.
These results indicate that in A. annua, short-term UV-B exposure up to 3 h has a little negative impact on photosynthetic efficiency. Though it reduced photosynthetic rate but was mainly associated with stomatal limitations, as reflected by the impaired stomatal conductance and leaf transpiration as well as maintained PSII efficiency up to 3 h of UV-B treatment. Reduced photosynthesis due to stomatal factors, in response to short-term UV-B radiation was also recently reported in grapevine leaves .
3.4 Expression analysis of photosynthesis related genes under UV-B radiation
A total of five genes related to photosynthesis (LHCB1, RBCS1B, RBCL, PSBA and PSBO) were analyzed for their...