Application Of Chlorophyll A Fluorescence Transient In Study Of Plant Physiology Under Stress Conditions

Chlorophyll fluorescence emitted by plants varied moment by moment. When plants were exposed to light from dark adaption, the chlorophyll fluorescence increases firsly, then decreases. The increase is called chlorophyll fluorescence transient. Currently, the chlorophyll a fluorescence transient can be attained by using a portable PEA or Handy-PEA with a time resolution of 10μs. Chlorophyll a fluorescence transient is a rich and complex signal. It mainly reflects changes in primary photochemical reactions of PSⅡand structure and state of photosynthetic apparatus. Recently, based on theory of energy flux in biomembranes, Strasser and Strasser(1995) developed a method called JIP-test to quantitatively analyze the chlorophyll a fluorescence, which provide us an usful tool to study the primary photochemical reactions in photosynthetic apparatus. In recent years, the simultaneous measurements of chlorophyll a fluorescence transients of PSⅡand PSⅠby using PEA-Senior have been developed, which enrich the information on primary photochemical reactions of the two photosystems. The aim of the study is to explor the availability of chlorophyll a fluorescence transients and JIP-test in study of plant physiology under different stress conditions.In this study, (1) effects of high light stress on photosynthetic apparatus in red, reddish and green Berberis thunbergii leaves and the following dark recovery,(2) different high temperatures on the primary photochemical reactions in soybean(Glycine max L.) leaves, (3) damages to the primary photochemical reactions in Berberis thunbergii green leaves under combined stress of high light and high temperature as well as under the each individual stress, (4) regulation of photosynthetic electron transport during leaf(Lonicera maackii (Rupr.) Maxim.) dehydration, (5) responses of photosynthetic electron transport chain during leaf senescence of Salvia splendens plant, were investigated by using chlorophyll a fluorescence transient measurements and the JIP-test.The main results are as follows:1. Before high light treatment, the primary photochemical reactions were different among the three group leaves of Berberis thunbergii plants. During high light treatment, (1) the donor and acceptor sides of PSⅡwas not damaged in the three group leaves; (2) the maximum quantum yield of PSⅡ(φ_Po) and the density of active reaction centers per excited cross-section (RC/CS) both decreased more pronouncedly in the green leaves than those in the red and reddish leaves; (3) heat dissipation per excited, cross-section(DI_O/CS) and per active reaction centers (DI_O/RC) both increased more shapely in the green leaves than those in the red and reddish leaves; (4) no differences were observed inφ_Po, RC/CS, DI_O/CS and DI_O/RC between the red and reddish leaves. These results suggest that anthocyanins could protect photosynthetic apparatus against high light stress, but the protecting ability of anthocyanins was not positively correlated with anthocyanin content in the leaves. In addition, the results also exhibited that, after 2 h recovery in the dark, the decreasedφ_Po and RC/CS and increased DI_O/CS and DI_O/RC caused by high light treatment recovered similarly in the three group leaves, implying that the anthocyanins did not affect the dark recovery of the photosynthetic apparatus after high light stress. We suggest that anthocyanins could only act as light attenuator to protect the photosynthetic apparatus from high light stress.2. 48℃treatment damaged the primary photochemical reactions, especially the maximum quantum yield of photosystemⅡ(φ_Po), in soybean leaves more severely than 45℃treatment. However, with an identical decrease inφ_Po, the shapes of O-J-I-P transients in leaves treated with 48℃and 45℃were significantly different. The parameters derived from the O-J-I-P transients were analyzed, which demonstrated that, with an identical decrease inφ_Po after the treatments, 48℃decreased the density of photosystemⅡ(PSⅡ) active reaction centers per excited cross-section and the ratio of Q_B-reducing reaction centers to Q_A-reducing reaction centers more markedly, and damaged the donor and acceptor sides of PSⅡmore pronouncedly than 45℃treatment. Furthermore, it was also proved that, with identical damage to the acceptor sides of PSⅡat the two high temperature treatments, the donor side of PSⅡwas damaged more severely at the higher temperature.3. When whole primary photochemical reactions were damaged similarly in the leaves of Berberis thunbergii treated with high light and high temperature, damage target to the primary photochemical reactions was inclined to the trapping efficiency of the absorbed light by Q_A reduction; whereas, under the high temperature treatment, the target was inclined to the donor and acceptor sides of PSⅡ, especially the donor side. The combination of high light and high temperature stresses exacerbated the damage to the whole primary photochemical reactions compared to the each stress applied individually. Nevertheless, with an identical decrease in the maximum quantum yield of PSⅡ(φ_Po), though damages to the whole primary photochemical reactions were similar under the three treatments, the donor and acceptor sides of PSⅡwere damaged more pronouncedly under the combined stress than that under the individual high light stress, but less than that under the individual high temperature stress. These results demonstrate that under cross stresses, damages to the primary photochemical reactions were not the accumulation of those damages under individual stress.4. PhotosystemⅠ(PSⅠ), the maximum quantum yield of PSⅡ, and the donor and acceptor sides of PSⅡwere all damaged by leaf dehydration with or without light. At the early stage of leaf dehydration, a linear correlation between the efficiency that a trapped exciton can move an electron into the electron transport chain beyond Q_A^- and the relative variable fluorescence transmission at 820 nm was observed, which indicates that the reduction in photosynthetic electron transport at that time was a positive down-regulation. However, at the late stage, the photosynthetic electron transport between the two photosystems was passively damaged. Compared with leaf desiccation in the dark, leaf dehydration in the light reduced the trapping efficiency of all absorbed light by Q_A reduction more significantly, as a result, damages to PSⅠand to the donor and acceptor sides of PSⅡwere alleviated. We speculate that this might be a protecting mechanism in plants.5. The maximum quantum yield of PSⅡ, donor side of PSⅡ, acceptor sides of PSⅡespecially the electron transport chain before plastoquinone(PQ) pool, and PSⅠwere all damaged gradually with the progression of senescence. However, during leaf senescence, the electron transport chain before PQ pool at the acceptor side of PSⅡwas damaged more severely than the donor side, whereas, the PSⅡwas damaged more pronouncedly than PSⅠat the late stage of leaf senescence though the two photosystems matched perfectly at the early stage, of senescence. With the progression of leaf senescence, the light absorption per active PSⅡreaction center was raised gradually, but the fraction of active PSⅡreaction centers and capacity of photosynthetic electron transport were declined step by step. Consequently, the excitation pressure per active PSⅡreaction center would be increased, enhancing the probability of AOS production at PSⅡ.Demonstrated by the aboved results, the chlorophyll a fluorescence transient is a powerful tool in study of stress plant physiology, it is convenient and fast to detect complicated responses of PSⅡand PSⅠand the photosynthetic electron transport chain between them under different conditions in different plants.