Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep QA oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO2 fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.

Structural and functional alterations to the photosynthetic apparatus after growth at low temperature (5°C) were investigated in the green alga Chlorella vulgaris Beijer. Cells grown at 5'C had a 2-fold higher ratio of chlorophyll a/b, 5-fold lower chlorophyll content, and an increased xanthophyll content compared to cells grown at 27°C even though growth irradiance was kept constant at 150 pmol m-'s-l. Concomitant with the increase in the chlorophyll a/b ratio was a lower abundance of light-harvesting polypeptides in 5'C-grown cells as observed by sodium dodecyl sulfatepolyacrylamide gel electrophoresis and confirmed by western blotting. The differences in pigment composition were found to be alleviated within 12 h of transferring 5°C-grown cells to 27'C.Furthermore, exposure of 5°C-grown cells to a 30-fold lower growth irradiance (5 pmol m-'s-') resulted in pigment content and composition similar to that in cells grown at 27°C and 150 pmol temperature effects on COz-saturated Oz evolution, 5'C-grown cells exhibited light-saturated rates of O2 evolution that were 2.8-and 3.9-fold higher than 27'C-grown cells measured at 27°C and 5'C, respectively. Steady-state chlorophyll a fluorescence indicated that the yield of photosystem 11 electron transport of 5°C-grown cells was less temperature sensitive than that of 27°C-grown cells. This appears to be dueto an increased capacity to keep the primary, stable quinone electron acceptor of photosystem li (OA) oxidized at low temperature in 5°C-compared with 27°C-grown cells regardless of irradiance. We conclude that Chlorella acclimated to low temperature adjusts its photosynthetic apparatus in response to the excitation pressure on photosystem 11 and not to the absolute externa1 irradiance. We suggest that the redox state of Q,, may act as a signal for this photosynthetic acclimation to low temperature in Chlorella. m-2 s -1 . Although both cell types exhibited similar measuringUnder a given set of environmental conditions, photosynthetic organisms attempt to maintain a balance between energy supply through electron transport and energy consumption through carbon fixation. This balance is required to protect the organism from the detrimental effects of excess light while maintaining sufficient pools of ATP and NADPH for cellular metabolism. Sudden imbalances in the energy budget are countered by altering the efficiency of PSII photochemistry via alterations in the trans-thylakoid pH gradient (Foyer et al., 1990). In addition, environmental changes may 535induce structural and functional alterations to the photosynthetic apparatus, such as changes in photosynthetic unit size (Ley, 1986) or alterations in Rubisco activity (Mortain-Bertrand et al., 1988), to maintain the energy balance.A major environmental variable that can perturb the equilibrium between energy input and energy consumption and induce photosynthetic alterations is low temperature. Any phenotypic adjustment of functional or structural properties of the photosynthetic apparatus that can be modulated by envi...

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 -qp (photochemical quenching). We established that Chlorella vulgaris exposed to ei- than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results i n an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.

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The photosynthetic response to light can be accurately defined in terms of (1) the initial slope (quantum yield); (2) the asymptote (light-saturated rate); (3) the convexity (rate of bending); and (4) the intercept (dark respiration). The effects of photoinhibition [which damages the reaction centre of photosystem II (PSII)] on these four parameters were measured in optically thin cultures of green plant cells (Chlamydomonas reinhardtii). The convexity of the light-response curve decreased steadily from a value of 0.98 (indicating a sharply bending response) to zero (indicating Michaelis-Menten kinetics) in response to increasing photoinhibition. Photoinhibition was quantified from the quantum yield of inhibited cells relative to that of control cells. The quantum yield was estimated by applying linear regression to low-light data or by fitting a non-rectangular hyperbola. Assuming the initial slope is linear allowed comparison with earlier work. However, as the convexity was lowered this assumption resulted in a significant underestimate of the true quantum yield. Thus, the apparent level of photoinhibition required for a zero convexity and the initial decrease in light-saturated photosynthesis depended upon how the quantum yield was estimated. If the initial slope of the light response was assumed to be linear the critical level of inhibition was 60%. If the linear assumption was not made, the critical level was 40%. At the level of inhibition where the convexity reached zero, the light-saturated rate of photosynthesis also began to decrease, indicating that this level of inhibition caused photosynthesis to be limited at all light intensities by the rate of PSII electron transport. At this level of inhibition the Fm-Fi signal (where Fm is maximal chlorophyll fluorescence and Fi is intermediate chlorophyll fluorescence of dark adapted cells; Briantais et al. 1988) from the fluorescence induction curve was zero and the Fi-Fo signal (where Fo is initial chlorophyll fluorescence of dark adapted cells) was 30% of the control, indicating dramatic reduction or complete elimination of one type of PSII. These data do not contradict published mathematical models showing that the ratio of the maximum speed of electron transport in PSII relative to the maximum speed of plastoquinone electron transport can determine the convexity of the photosynthetic response to light.

No abstract

Under conditions of iron-stress, the Photosystem II associated chlorophyll a protein complex designated CP 43', which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of isiA, encodes the protein flavodoxin, which can functionally replace ferredoxin under conditions of iron stress. We have constructed two cyanobacterial insertion mutants which are lacking (i) the CP 43' apoprotein (designated isiA (-)) and (ii) flavodoxin (designated isiB (-)). The function of CP 43' was studied by comparing the cell characteristics, PS II functional absorption cross-sections and Chl a fluorescence parameters from the wild-type, isiA (-) and isiB (-) strains grown under iron-stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pigment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype suggests an adequate acclimation to low iron even in absence of CP 43' and/or flavodoxin. Under both iron conditions, no differences were detected between the three strains in the functional absorption crossection of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43' is not part of the functional light-harvesting antenna for PS II. In the wild-type and the isiB (-) strain grown under iron-deficient conditions, CP 43' was present in the thylakoid membrane as an uncoupled Chl-protein complex. This was indicated by (1) an increase of the yield of prompt Chl a fluorescence (Fo) and (2) the persistence after PS II trap closure of a fast fluorescence decay component showing a maximum at 685 nm.

A basic requirement of all photosynthetic organisms is a balance between overall energy supply through temperature‐independent photochemical reactions and energy consumption through the temperature‐dependent biochemical reactions of photosynthetic electron transport and contiguous metabolic pathways. Since the turnover of photosystem II (PSII) reaction centers is a limiting step in the conversion of light energy into ATP and NADPH, any energy imbalance may be sensed through modulation of the redox state of PSII. This can be estimated in vivo by chlorophyll a fluorescence as changes in the redox state of PSII, or photosystem II excitation pressure, which reflects changes in the redox poise of intersystem electron transport carriers. Through comparisons of photosynthetic adjustment, we show that growth at low temperature mimics growth at high light. We conclude that terrestrial plants, green algae and cyanobacteria do not respond to changes in growth temperature or growth irradiance per se, but rather, respond to changes in the redox state of intersystem electron transport as reflected by changes in PSII excitation pressure, We suggest that this chloroplastic redox sensing mechanism may be an important component for sensing abiotic stresses in general. Thus, in addition to its role in energy transduction, the chloroplast may also be considered a primary sensor of environmental change through a redox sensing/signalling mechanism that acts synergistically with other signal transduction pathways to elicit the appropriate molecular and physiological responses.