Abstract
The tolerance to chilling temperature is one of the most important factors in determining photosynthetic yield of plants. The chilling temperature may lead to (1) a decrease in the fluidity of thylokoid membrane; (2) an inhibition of photosystem II (PSII) and/or photosystem I (PSI); and (3) slowing down of the dark reaction. Former studies concerning with photosynthesis in chilling temperature were almost long-term experiments (several hours, several days and several months), thus we can only figure out the middle and long term effects, while the information about plants’ early responses are still sparse. In this study, we choose a cold-tolerant plant, pea (Pisum sativum L.), and a cold-sensitive plant, cucumber (Cucumis sativus L.), to investigate their early response of photosynthesis to chilling temperature. The photosynthesis of pea and cucumber, including light and dark reactions were monitored by chlorophyll fluorescenc. Enzymatic assay was used to measure Rubisco activity. We found that when facing a sudden lowing of temperature (5℃), the electron transport rate (ETR) of pea droped but with a rapid recovery while that of cucumber was more seriously inhibited and with a recovery to much less extent. We also found that the inhibition under 5℃ could be alleviated if we pre-treated leaves sample with methyl viologen, which is an efficient electron acceptor of PSI. In addition, high CO2 concentration would make the inhibition more seriously, while reducing O2 concentration would ease it. In the enzyme-linked assay, we found the activation state of Rubisco paralleled with the change of electron transport rate. From the chlorophyll fluorescence experiment, we found that qN of pea rose following the recovery of Yield and qP. We also found that most of its qN relaxed in ten minutes. Thus we infer that the major component of qN is QT (state transition). It means that PSII of pea may transfer excess radiation energy to PSI, and with the protection mechanism pea can keep well photosynthesis under chilling stress. On the contrary, qN of cucumber didn’t rise too much so that we could observe its photosynthetic yield still inhibited under chilling stress. We thus suggest that (1) Dark reaction is the major inhibition site under chilling stress. (2) The photosynthesis rate of cold-tolerant plant like pea decreases in response to a sudden chilling stress, but it recovers within l minutes. On the other hand, cold-sensitive plant like cucumber doesn’t possess thus kind of capability. The phenomenon was not observed by former long-term experiments. (3) The ability of pea to keep normal photosynthetic yield in chilling temperature may be related to the Rubisco adjustment and the ability to mediate the excess radiation energy (QT), and which is absent in cucumber. Owning to the complexity of photosynthesis, the fluorescence detected is always a result of effects of multi-factors. We still need more experimental data to find out the roles played by various parts of photosynthesis under chilling stress.