Patterns of chlorophyll fluorescence kinetics in relation to growth and expansion in cucumber leaves

Photochemical development was studied in developing cucumber (Cucumis sativus L. cv Hokushin) leaves to determine if the spatial pattern coincided with relative growth rates of expanding leaves, intercalary cell division, or position relative to the vascular tissue. Both leaf surfaces undergo a series of similar changes in chlorophyll a fluorescence kinetics, but the upper surface more quickly achieved the characteristic response. Imaging of fluorescence showed an individual developing leaf has four regions differing in kinetics, but these regions do not coincide with areas of increasing relative growth rate. Two of these regions lie at the leaf edge and their divergent kinetics may be related to structural and physiological features present at this position. A third area with different kinetics, in the basal region of the leaf, is spatially consistent with primordial regions that are clonal during development. The correspondence between areas of clonal growth and specific fluorescence kinetics indicates that cells of common ancestry show functional uniformity. No evidence was found that the proximity of the vascular tissue influenced development of photochemical function.     

Photosynthetic characterization of Jerusalem artichoke during leaf expansion

Gas exchange, chlorophyll a fluorescence and modulated 820 nm reflection were investigated to explore the development of photosynthesis in Jerusalem artichoke (Helianthus tuberosus L.) leaves from initiation to full expansion. During leaf expansion, photosynthetic rate (Pn) increased and reached the maximal level when leaves were fully expanded. The same change pattern was also found in the stomatal conductance and chlorophyll content. Lower Pn could not be ascribed to the higher stomatal resistance in developing leaves, as intercellular CO₂ concentration was not significantly lower in these leaves. Lower Pn partly resulted from the lower actual photochemical efficiency of PSII in developing leaves, as more excited energy was dissipated through non-photochemical quenching. The development of primary photochemical reaction and electron transport in the donor side of PSII was completed in the initiating leaves. However, the development of electron transport in the acceptor side of PSII was not accomplished until leaves were fully expanded, indicated by the change in probability that an electron moves further than primary quinone (ψo). PSI activity changed in parallel with ψo suggesting that PSI cooperated well with PSII during leaf expansion. It should be stressed that the development of carbon fixation process was later than primary photochemical reaction but earlier than photosynthetic electron transport during leaf expansion. The later development of photosynthetic electron transport may reduce the production of reactive oxygen species from Mehler reaction, particularly under low carbon fixation.     

Age-dependent changes in the functions and compositions of photosynthetic complexes in the thylakoid membranes of Arabidopsis thaliana

Photosynthetic complexes in the thylakoid membrane of plant leaves primarily function as energy-harvesting machinery during the growth period. However, leaves undergo developmental and functional transitions along aging and, at the senescence stage, these complexes become major sources for nutrients to be remobilized to other organs such as developing seeds. Here, we investigated age-dependent changes in the functions and compositions of photosynthetic complexes during natural leaf senescence in Arabidopsis thaliana. We found that Chl a/b ratios decreased during the natural leaf senescence along with decrease of the total chlorophyll content. The photosynthetic parameters measured by the chlorophyll fluorescence, photochemical efficiency (F _v/F _m) of photosystem II, non-photochemical quenching, and the electron transfer rate, showed a differential decline in the senescing part of the leaves. The CO₂ assimilation rate and the activity of PSI activity measured from whole senescing leaves remained relatively intact until 28 days of leaf age but declined sharply thereafter. Examination of the behaviors of the individual components in the photosynthetic complex showed that the components on the whole are decreased, but again showed differential decline during leaf senescence. Notably, D1, a PSII reaction center protein, was almost not present but PsaA/B, a PSI reaction center protein is still remained at the senescence stage. Taken together, our results indicate that the compositions and structures of the photosynthetic complexes are differentially utilized at different stages of leaf, but the most dramatic change was observed at the senescence stage, possibly to comply with the physiological states of the senescence process.     

Increased photosynthetic activities and thermostability of photosystem II with leaf development of elm seedlings (Ulmus pumila) probed by the fast fluorescence rise OJIP

Experiments were conducted to investigate the photosynthetic activity and thermostability of photosystem II (PSII) in elm seedling (Ulmus pumila) leaves from initiation to full expansion. During leaf development, photosynthesis, measured as CO₂ fixation, increased gradually and reached a maximum value when leaves were fully developed. In parallel with the increase of carbon assimilation, chlorophyll content increased. The chlorophyll a fluorescence measurements showed that the maximum quantum yield of PSII primary photochemistry (φ_po), the efficiency with which the energy of trapped excitons is converted into the electron transport beyond Q_A (Ψ_o) and the quantum yield of electron transport beyond Q_A (φ_Eo) increased gradually. The low light experiments confirmed these results independently. When subjected to heat stress, young leaves exhibited progressively lower φ_po and maximal fluorescence (F_m) values with considerably higher minimal fluorescence (F_o) than mature leaves, demonstrating that PSII in newly initiating leaves is more sensitive to heat stress. Further analysis revealed that PSII structure in newly initiating leaves showed a robust alteration under heat stress, which was reflected by the clear K phase in the OJIP curves. Therefore, we suggest that the enhanced thermostability of PSII in the case of leaf growth might be associated with an improvement of the stability of the oxygen-evolving complex (OEC) to heat stress during leaf development.     

Photosynthesis of soybean under the action of a photosystem II-inhibiting herbicide

Photosynthesis, the fundamental physiological process of plant responsible for the growth and yield of crops, is strongly affected by environmental stresses. Several methods have been used to study changes in the physiological parameters of plants exposed to stresses. The work aimed to study physiological parameters related to photosynthesis in leaf discs of soybean plants exposed to a photosystem II-inhibiting herbicide. Soybean leaf discs obtained from mature leaves of plants in the vegetative stage immersed in bentazon herbicide solutions at concentrations of 0, 100, 250 or 500 μM were evaluated. In experiment I, the effect of the herbicide on chlorophyll a fluorescence transient was measured using a portable fluorometer. In the second experiment, the effect of the herbicide on modulated chlorophyll a fluorescence and gas exchange were evaluated, with the latter being measured with an infrared gas analyzer. The evaluations of transient and modulated fluorescence provided additional information on the photosynthetic activity of soybean leaf discs exposed to the action of bentazon. For the fluorescence transient analysis, performance indices were the parameters most sensitive to the action of bentazon, showing a decrease of approximately 70 % at a dose of 500 μM. For the modulated fluorescence analysis, the photochemical quenching coefficient, the electron transport rate, the photochemical efficiency of photosystem II and the net assimilation rate, decreased in response to herbicide application, with values that were almost equal to zero at a dose of 500 µM, which are the parameters that showed the greatest sensitivity to bentazon in soybean.     

Comparison of chlorophyll fluorescence emission characteristics of wheat leaf tissue and isolated thylakoids as a function of excitation wavelength

Abstract. Chlorophyll fluorescence emission spectra and the kinetics of 685 mm fluorescence emission from wheat leaf tissue and thylakoids isolated from such tissue were examined as a function of excitation wavelength. A considerable enhancement of fluorescence emission above 700 nm relative to that at 685 nm was observed from leaf tissue when it was excited with 550 nm rather than 450 nm radiation. Such excitation wavelength dependent changes in the emission spectrum occurred over an excitation spectral range of 440–660 nm and appeared to be directly related to the total quantity of radiation absorbed at a given excitation wavelength. Experiments with isolated thylakoid preparations demonstrated that changes in the fluorescence emission spectrum of the leaf were attributable to the optical properties of the leaf and were not due to the intrinsic characteristies of the thylakoid photochemical apparatus. This was not the case for the observed excitation wavelength dependent changes in the 685 nm fluorescence induction curve obtained from leaf tissue infiltrated with DCMU. Excitation wavelength dependent changes in the ratio of the variable to maximal fluorescence emission and the shape of the variable fluorescence induction were observed for leaf tissue. Isolated thylakoid studies showed that such changes in the leaf fluorescence kinetics were representative of the way in which the photochemical apparatus in vivo was processing the absorbed radiation at the different excitation wavelengths. The results are considered in the context of the use of fluorescence emission characteristics of leaves as non-destructive probes of the photochemical apparatus in vivo.     

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in non-photochemical quenching and a decrease in photo-chemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of QB-non-reducing PSII reaction centres and a decrease in the rate of QA reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant to thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was down-regulated by an increase in the proportion of QB-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.Key words: Chlorophyll fluorescence, gas exchange, maize (Zea mays L.), photochemical and non-photochemical quenching, photosystem II photochemistry.      

Limiting Factors in Photosynthesis: VI. Regeneration of Ribulose 1,5-Bisphosphate Limits Photosynthesis at Low Photochemical Capacity

Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO₂ concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.     

Comparative Studies of Fluorescence from Mesophyll and Guard Cell Chloroplasts in Saxifraga cernua: Analysis of Fluorescence Kinetics as a Function of Excitation Intensity

The chlorophyll fluorescence induction curves from mesophyll and guard cell chloroplasts of Saxifraga cernua, including both the fast (O to P, the transients involved in the rise in variable fluorescence) and slow (P to steady state fluorescence due to quenching) components, were characterized over a range of excitation intensities using microspectrophotometry (with epi-lumination) equipped with apertures designed to eliminate cross contamination of the fluorescence signal between the two chloroplast types. At low excitation intensities, the fast fluorescence kinetics from guard cell plastids showed an extended I to D phase and a more rapid appearance of P while minimal quenching from P to steady state fluorescence was observed compared to the transients from mesophyll chloroplasts suggesting a lower activity of photochemical (electron movement via carriers between donor and acceptor sites) and nonphotochemical (such as membrane conformational changes) events which regulate the fluorescence induction curve kinetics. As the excitation intensity was increased, the quenching rates of guard cells were faster at initiating conditions for photophosphorylation and the fast and slow fluorescence kinetics from guard cells resembled those of the mesophyll cells.

Guard cell chloroplasts of S. cernua from intact epidermal peels showed a low temperature (77 K) fluorescence emission spectrum having three major peaks (at 685, 695, and 730 nanometers when excited at 440 nanometers) which were qualitatively similar to those in the spectrum obtained from mesophyll tissue.

These data suggest that S. cernua guard cell chloroplast photosystems I and II contribute to light-dependent stomatal activity only at high light intensities.

     

Effects of low temperature acclimation upon photosynthetic induction in barley primary leaves

Oxygen evolution and chlorophyll fluorescence were measured in cold-hardened and unhardened leaves of barley ( Hordeum vulgare L. cv. Asa) during the induction period of photosynthesis. The lag phase of light-saturated photosynthesis was increased and steady-state rates of photosynthesis were higher in cold-hardened than in unhardened barley leaves. Fluorescence was quenched more rapidly during the first minutes of induction in hardened than unhardened leaves, largely because of greater energy-dependent quenching (q_E). Also, slow fluorescence transients through the M peak were delayed and less pronounced in cold-hardened than in unhardened leaves. Based upon the combined fluorescence and oxygen evolution data it was concluded that cold-hardening delayed light activation of the energy consuming carbon reduction cycle, thereby delaying the use of ATP and NADPH formed in the light reaction. Measurements of oxygen evolution and fluorescence kinetics during photosynthetic induction under oxygenic and anoxygenic conditions suggest that oxygen photoreduction is important for additional ATP generation during both the onset of photosynthetic carbon assimilation and during steady-state photosynthesis.     

A quantitative determination of photochemical and non-photochemical quenching during the slow phase of the chlorophyll fluorescence induction curve of bean leaves

Estimations of the changes in the reduction-oxidation state of Photosystem II electron acceptors in Phaseolus vulgaris leaves were made during the slow decline in chlorophyll fluorescence emission from the maximal level at P to the steady-state level at T. The relative contributions of photochemical and non-photochemical processes to the fluorescence quenching were determined from these data. At a low photon flux density of 100 μmol · m^?2 · s^?1, non-photochemical quenching was the major contributor to the fluorescence decline from P to T, although large charges were observed in photochemical quenching immediately after P. On increasing the light intensity 10-fold, the contribution of photochemical processes to fluorescence quenching was markedly diminished, with nearly all the P-to-T fluorescence decline being attributable to changes in non-photochemical quenching. The possible factors responsible for changes in non-photochemical quenching within the leaves are discussed.     

水淹导致皇冠草光合机构发生变化并加剧其出水后光抑制

通过气体交换和叶绿素荧光等方法研究了水淹及胁迫解除后皇冠草不同功能叶的光合特性及光抑制的变化.结果表明:与对照相比,气生叶(全淹组淹水前形成的功能叶)在水淹条件下叶片大小和气孔没有明显变化,但沉水叶(全淹组淹水后新生的功能叶)的叶面积增加,气孔变小,上表皮气孔密度增加.水淹导致气生叶碳同化能力、光化学效率和叶绿素含量下降.沉水叶在发育过程中碳同化能力、光化学效率和叶绿素逐渐升高.气生叶和沉水叶出水后其活体叶片在强光下的相对含水量急剧下降,发生明显的光抑制;而弱光下无明显光抑制发生.出水后离体叶片强光照射下6h后两种功能叶均发生严重光抑制,且弱光下不能恢复.因此,可以认为淹水条件下,沉水叶上表皮气孔密度的增加使其蒸腾速率提高;沉水叶较强的碳同化能力和增加的叶面积是确保其植株水下生存的重要因素;强光使气生叶和沉水叶出水后均发生严重光抑制,导度和蒸腾速率提高导致的叶片失水则加剧了这一过程,两者共同作用导致自然条件下两种功能叶的出水死亡.     

Salinity treatment shows no effects on photosystem II photochemistry,but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemistry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5 degrees C, air temperature). CO(2) assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F(v)/F(m)), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F(v)'/F(m)') and the actual PSII effciency (Phi(PSII)), photochemical quenching (q(P)), and non-photochemical quenching (q(N)) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO(2) assimilation rate decreased significantly and no net CO(2) assimilation was observed at 47.5 degrees C. Salinity treatment had no effect on the response of CO(2) assimilation to high temperature when temperature was below 40 degrees C. At 45 degrees C, CO(2) assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO(2) assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 degrees C, the declines in F(v)/F(m), Phi(PSII), F(v)'/F(m)', and q(P) were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O(2)-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F(i)-F(o))/(F(p)-F(o)) which is an expression of the proportion of the Q(B)-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O(2)-evolving complex and the reaction centres of PSII to high temperature.     

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

The synergistic effect of drought and light stresses in sorghum and pearl millet

The effects of drought stress and high irradiance and their combination were studied under laboratory conditions using young plants of a very drought-resistant variety, ICMH 451, of pearl millet (Pennisetum glaucum) and three varieties of sorghum (Sorghum bicolor)—one drought-resistant from India, one drought-tolerant from Texas, and one drought-sensitive variety from France. CO₂ assimilation rates and photosystem II fluorescence in leaves were analyzed in parallel with photosynthetic electron transport, photosystem II fluorescence, and chlorophyll-protein composition in chloroplasts isolated from these leaves. High irradiance slightly increased CO₂ assimilation rates and electron transport activities of irrigated plants but not fluorescence. Drought stress (less than −1 megapascal) decreased CO₂ assimilation rates, fluorescence, and electron transport. Under the combined effects of drought stress and high irradiance, CO₂ assimilation rates and fluorescence were severely inhibited in leaves, as were the photosynthetic electron transport activities and fluorescence in chloroplasts (but not photosystem I activity). The synergistic or distinctive effect of drought and high irradiance is discussed. The experiments with pearl millet and three varieties of sorghum showed that different responses of plants to drought and light stresses can be monitored by plant physiological and biochemical techniques. Some of these techniques may have a potential for selection of stress-resistant varieties using seedlings.     

Changes in the activities of nitrogen assimilation enzymes ofLolium perenne L. during regrowth after cutting

The activities of key enzymes involved in N assimilation were investigated after defoliation of 6-week-old ryegrass plants grown in water culture conditions. In a first experiment, nitrate reductase, glutamine synthetase and glutamate dehydrogenase activities were measured in roots, stubble and leaves on the day of cutting and at 7-day intervals over the following 5-week period of regrowth. Ammonia assimilation enzymes showed little change whereas the nitrate reductase activity sharply decreased 2 weeks after clipping. In a second experiment, the nitrate reductase activity was measured at 2- or 3-day intervals 1 week before and 3 weeks after clipping.In vivo andin vitro assays both showed an increasing activity in leaves up to 8 days after cutting while root activity decreased. The opposite changes then occurred and both organs recovered their initial nitrate reductase activity levels after 12–14 days of regrowth. These fluctuations in nitrate reductase activity were considered to be related to the capacity for C assimilation and the nitrate availability.     

Effets du rapport RC:RS sur les jeunes feuilles de trèfle blanc. Assimilation nette de CO2, activité photochimique et morphologie foliaire

Young leaves of white clover are subjected to low irradiance and low red to far-red (R:FR) ratio within canopies. The objectives were to investigate the consequences of low R:FR ratio on morphology, net CO₂ assimilation and photochemical activity of leaves developed under simulated light environment of canopy. We used far-red (FR) light emitting diodes to modify the R:FR ratio only at the developing leaf under a low irradiance. Net CO₂ assimilation rate, stomatal conductance and leaf morphology were not affected by low R:FR ratio. FR exposure slightly reduced the photochemical quantum yield of PSII but there were no consequences on electron flow through photosystem II. The carbon fixation by the leaf was therefore not modified by light quality. However, low R:FR ratio decreased the leaf chlorophyll content by 21 %. Those effects were only attributed to just unfolded leaves as they were not persistent in mature leaves and there were no consequences on plant biomass accumulation.     

Photochemical activities and organization of photosynthetic apparatus of C3 and C4 plants grown under different light intensities

Changes in the photochemical activities, influenced by variation in the growth light intensity, were followed in typical C₃ (Phaseolus, Ipomoea) and C₄ (Amaranthus, Sorghum) plants. Progressive decrease in the growth light intensity accelerated the O-P fluorescence induction in whole leaves. Such acceleration of the fluorescence kinetics was found to be not due to enhanced photosystem II activity but possibly a result of reduced rate of electron flow between the two photosystems. This is supported by 4 lines of evidence: (1) by the Hill activity determined in the presence of electron acceptors functioning before and after plastoquinone; (2) the photosynthetic unit size determined after flash excitation showing variations that were apparently too small to account for the changes observed fluorescence induction; (3) modification of the kinetics of secondrange light-induced absorbance changes at 520 nm; and (4) absence of significant changes in the ratio of P₇₀₀/total chlorophyll ratio. The P₇₀₀/cytochrome f ratio, however, increased from the usual 1–1.5 to 3–4 in plants grown under 9% sunlight. Increase in the P₇₀₀/cytochrome f ratio was found to be due to a decrease in the cytochrome f/chlorophyll ratio, and this was due to perhaps to a simultaneous increase in chlorophyll and decrease in cytochrome content.     

Zeaxanthin and the Heat Dissipation of Excess Light Energy in Nerium oleander Exposed to a Combination of High Light and Water Stress

Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the carotenoid zeaxanthin. During the imposition of water stress, the zeaxanthin content of leaves increased at the expense of violaxanthin and β-carotene as a water deficit developed over a period of several days. The increase in zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna chlorophyll as calculated from the characteristics of chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form zeaxanthin at the expense of both violaxanthin and β-carotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between β-carotene and the three carotenoids of the xanthophyll cycle.     

Photochemical activities and organization of photosynthetic apparatus of C3 and C4 plants grown under different light intensities

Changes in the photochemical activities, influenced by variation in the growth light intensity, were followed in typical C₃ (Phaseolus, Ipomoea) and C₄ (Amaranthus, Sorghum) plants. Progressive decrease in the growth light intensity accelerated the O-P fluorescence induction in whole leaves. Such acceleration of the fluorescence kinetics was found to be not due to enhanced photosystem II activity but possibly a result of reduced rate of electron flow between the two photosystems. This is supported by 4 lines of evidence: (1) by the Hill activity determined in the presence of electron acceptors functioning before and after plastoquinone; (2) the photosynthetic unit size determined after flash excitation showing variations that were apparently too small to account for the changes observed fluorescence induction; (3) modification of the kinetics of second-range light-induced absorbance changes at 520 nm; and (4) absence of significant changes in the ratio of P₇₀₀/total chlorophyll ratio. The P₇₀₀/cytochrome f ratio, however, increased from the usual 1–1.5 to 3–4 in plants grown under 9% sunlight. Increase in the P₇₀₀/cytochrome f ratio was found to be due to a decrease in the cytochrome f/chlorophyll ratio, and this was due to perhaps to a simultaneous increase in chlorophyll and decrease in cytochrome content.