CO2 assimilation, respiration and chlorophyll fluorescence in peach leaves infected by Taphrina deformans

Photosynthesis, respiration and chlorophyll fluorescence parameters were determined in peach ( Prunus persica L. cv. Dixired) leaves naturally infected by Taphrina deformans (Berk.) Tul. and in healthy leaves (controls), in two successive springs. A drastic decrease in net photosynthesis and an evident increase in respiration in curled leaves were noted. The instantaneous PSII fluorescence yield, with no (F₀) and with (F₀) quenching component, and steady state fluorescence yield (under actinic light, F_s) were essentially unchanged. Maximal fluorescence in dark-adapted (F_m) and illuminated (F'_m) leaves and the corresponding variable fluorescence (F_v and F_v) clearly decreased. The indicators of PSII quantum yield (F_v/F_m) in dark-adapted leaves, and the potential PSII excitation capture efficiency (F'_v/F'_m) and the quantum yield of PSII (q_p [F'_v/F'_m]) in the light were also significantly lower in curled leaves. Decreasing tendencies were also noted for the PSII photochemical yield (photochemical quenching, q_p) and in the energy status of the chloroplast (non-photochemical quenching, q_N, and Stern-Vollmer value, NPQ) although the differences were not always significant. In curled leaves the main alteration documented is the imbalance between the drastic inhibition of CO₂ fixation and the moderate decrease in photochemical reactions (i.e. F_v/F_m and ΔF/F'_m), indicating changes in the energy flux.     

Grassland species will not necessarily benefit from future elevated air temperatures: a chlorophyll fluorescence approach to study autumn physiology

Model ecosystems were grown in 12 sunlit, climate-controlled chambers to gain insight into the effects of elevated (+3°C) air temperature (T_air) on temperate grasslands. In this study, the hypothesis of delayed senescence in response to elevated T_air was tested for Rumex acetosa L. and Plantago lanceolata L. During the autumn of the first treatment year, frequent measurements were made of leaf chlorophyll a (Chl a ) fluorescence transients. Chl fluorescence images of individual leaves as well as digital colour images of these ecosystems were captured. Chl fluorescence variables, such as the maximum quantum yield of primary photochemistry (F_v/F_m), indicated a decreasing efficiency with time. Despite no treatment effect on F_v/F_m, other variables derived from the Chl fluorescence transients showed a strong trend towards a positive effect of a 3°C temperature increase on the photosynthetic performance of R. acetosa and P. lanceolata in the first year. After mid-September, the initial positive treatment effect disappeared for R. acetosa , strongly suggesting that leaf lifespan of this species was shortened by higher T_air. One possible explanation is more intense drought stress in the elevated compared to the ambient temperature treatments. Second-year measurements were possibly too limited in time to confirm this trend. These results show that temperate grassland species may take advantage of a future increase in T_air during autumn. This will ultimately depend on the species' degree of acclimation to a temperature change and on the resistance to drought stress.     

Photosynthesis and photoinhibition in leaves of chlorophyll b-less barley in relation to absorbed light

The response of photosynthesis to absorbed light by intact leaves of wild-type ( Hordeum vulgare L. cv. Gunilla) and chlorophyll b -less barley ( H. vulgare L. cv. Dornaria, chlorina-f2²⁸⁰⁰) was measured in a light integrating sphere. Up to the section where the light response curve bends most sharply the responses of the b -less and wild-type barley were similar but not identical. Average quantum yield and convexity for the mutant light response curves were 0.89 and 0.90, respectively, times those of the wild-type barley. The maximum quantum yield for PSII photochemistry was also 10% lower as indicated by fluorescence induction kinetics (F_v/F_m). Just above the region where the light curve bends most sharply, photosynthesis decreased with time in the mutant but not in the wild-type barley. This decrease was associated with a decrease in F_v/F_m indicating photoinhibition of PSII. This photoinhibition occurred in the same region of the light response curve where zeaxanthin formation occurs. Zeaxanthin formation occurred in both the chlorophyll b -less and wild-type leaves. However, the epoxidation state was lower in the mutant than in the wild-type barley. The results indicate that chlorophyll b -less mutants will have reduced photosynthetic production as a result of an increased sensitivity to photoinhibition and possibly a lowered quantum yield and convexity in the absence of photoinhibition.     

The consequences of freezing temperatures followed by high irradiance on in vivo chlorophyll fluorescence and growth in Picea abies

Picea abies (L.) Karst. plants, propagated by cuttings, were subjected to one night of freezing temperatures (-5°C), high irradiance (1 200 or 1 800 μmol m⁻² s⁻¹), or freezing temperatures followed by high irradiance. The treatments were applied at bud burst, at time of shoot elongation, and when the shoots had ceased to elongate. The maximum quantum yield of photosynthesis, F_v/F_m, dry weight of branches and needles, and length and survival of shoots were measured. F_v/F_m and growth decreased after a night of freezing temperatures followed by high irradiance, at the time of bud burst and shoot elongation. High irradiance alone influenced F_v/F_m, but not growth. Freezing temperatures affected F_v/F_m, and growth at the time of shoot elongation. F₀ increased after a night of freezing temperatures and decreased with age of the current-year needles. It was concluded that the use of short-term measurements of chlorophyll fluorescence induction to predict changes in growth after a night of frost and subsequent high light was not a reliable method.     

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO₂ (C_a) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO₂ assimilation rate at intercellular CO₂ pressures attained at C_a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO₂ (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O₂ evolution at saturating CO₂ was not affected. The maximum efficiency of PSII photochemistry (F_v/F_m) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g₋₁ dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m₋₂ s₋₁, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F'^v/F'^m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO² reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q_A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.     

ON-LINE MONITORING OF CHLOROPHYLL FLUORESCENCE TO ASSESS THE EXTENT OF PHOTOINHIBITION OF PHOTOSYNTHESIS INDUCED BY HIGH OXYGEN CONCENTRATION AND LOW TEMPERATURE AND ITS EFFECT ON THE PRODUCTIVITY OF OUTDOOR CULTURES OF SPIRULINA PLATENSIS (CYANOBACTERIA)

The saturating pulse fluorescence technique was applied to study photoinhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high oxygen and low temperature stress. Diurnal changes in maximum photochemical yield (F_v/F_m), photon yield of PSII (ΔF/F 'm), and nonphotochemical quenching (qN) were measured using a portable, pulse-amplitude–modulated fluorometer. When solar irradiance reached the maximum value, the F _v/F_m and ΔF/F 'm ratios of the Spirulina cultures grown under high oxygen stress decreased by 35% and 60%, respectively, as compared with morning values. The depression of the F_v/F_m and ΔF/F 'm ratios reached 55% and 84%, respectively, when high oxygen stress was combined with low temperature (i.e. 10° C below the optimal value for growth). Photoinhibition reduced the daily productivity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen–low temperature stress by 60%. Changes in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rate through the PSII (ΔF/F 'm × photon flux density). The results indicate that on-line chlorophyll fluorescence measurement is a powerful tool for assessing the photosynthetic performance of outdoor Spirulina cultures.     

A rapid, whole-tissue determination of the functional fraction of PSII after photoinhibition of leaves based on flash-induced P700 redox kinetics

Assaying the number of functional PSII complexes by the oxygen yield from leaf tissue per saturating, single-turnover flash, assuming that each functional PSII evolves one oxygen molecule after four flashes, is one of the most direct methods but time-consuming. The ratio of variable to maximum Chl fluorescence yield (F_v/F_m) in leaves can be correlated with the oxygen yield per flash during a progressive loss of PSII activity associated with high-light stress and is rapid and non-intrusive, but suffers from being representative of chloroplasts near the measured leaf surface; consequently, the exact correlation depends on the internal leaf structure and on which leaf surface is being measured. Our results show that the average F_v/F_m of the adaxial and abaxial surfaces has a reasonable linear correlation with the oxygen yield per flash after varied extents of photoinactivation of PSII. However, we obtained an even better linear correlation between (1) the integrated, transient electron flow (Σ) to P700⁺, the dimeric Chl cation in PSI, after superimposing a single-turnover flash on steady background far-red light and (2) the relative oxygen yield per flash. Leaves of C3 and C4 plants, woody and herbaceous species, wild-type and a Chl- b -less mutant, and monocot and dicot plants gave a single straight line, which seems to be a universal relation for predicting the relative oxygen yield per flash from Σ. Measurement of Σ is non-intrusive, representative of the whole leaf tissue, rapid and applicable to attached leaves; it may even be applicable in the field.     

Photoinhibition, xanthophyll cycle and in vivo chlorophyll fluorescence quenching of chilling-tolerant Oxyria digyna and chilling-sensitive Zea mays

The relationship between susceptibility to photoinhibition, zeaxanthin formation and chlorophyll fluorescence quenching at suboptimal temperatures was studied in chilling-sensitive maize and in non-acclimated and cold-acclimated Oxyria digyna , a chilling-tolerant plant of arctic and alpine habitats. In maize, zeaxanthin formation was strongly suppressed by chilling. Zeaxanthin formed during preillumination at 20°C did not protect maize leaves from photoinhibition during a subsequent high-light, low-temperature treatment, as judged from the ratios of variable to maximal fluorescence, F_v/F_m. However, such preillumination significantly increased non-photochemical quenching (q_N) at low temperatures, mainly due to an enhancement of the fast-relaxing q_N component (i.e., of energy-dependent quenching. q_E). In O. digyna , cold-acclimation resulted in an increased zeaxanthin formation in the temperature range of 2.5–20°C. Cold-acclimation substantially decreased the susceptibility towards photoinhibition at 4°C, but q_N remained nearly unchanged between 2 and 38°C, as compared to control plants. Effects of cold acclimation on photosynthesis, photochemical quenching and quantum efficiency of photosystem II were small and indicated a slight amelioration only of the function of the photosynthetic apparatus at suboptimal temperatures (2–20°Ct. I) is concluded, that the xanthophyll cycle is strongly influenced by cold acclimation, while effects on the photosynthetic carbon assimilation only play a minor role in O. digyna.     

Responses of photosynthesis to NaCl in gametophytes of Acrostichum aureum

Gametophytes of Acrostichum aureum were cultured in 0.0 to 1.0% NaCl solutions or in NaCl‐free solution and then transferred to 1.0% NaCl solution. Photosynthetic light‐response curves, efficiency of the primary photochemical reaction, relative electron transport rate, and photochemical and non‐photochemical quenching at steady state were determined by photosynthetic O₂ evolution and in vivo chlorophyll fluorescence. Results obtained showed that the chlorophyll fluorescence parameters, F_v/F_m and F'_v/F'_m and αO₂ (the initial linear slope of the photosynthetic light‐response curve) increased in gametophytes grown in NaCl. Linear electron transport rate was stimulated by NaCl. Based on the chlorophyll content, light‐saturated photosynthesis in gametophytes grown in 0.2 to 0.7% NaCl increased slightly; it decreased in gametophytes grown in 1.0% NaCl. Photochemical quenching decreased in NaCl‐grown gametophytes at all photosynthetic photon flux density (PPFD) levels measured, but there was no increase in non‐photochemical quenching. The chlorophyll a/b ratio increased with increasing NaCl concentration in culture solutions. These results indicated that NaCl enhanced photochemical efficiency of photosystem II (PSII) and photosynthetic linear electron transport, thus resulting in the development of an excitation pressure in PSII. Such excitation pressure might act as a signal for photosynthetic acclimation to salt stress, thus allowing the gametophytes to grow in their natural habitats.     

Photoinactivation of photosystem II in leaves of Capsicum annuum

Leaf discs of Capsicum annuum L. were illuminated in air enriched with 1% CO₂ in the absence or presence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. The loss of functional photosystem (PS) II complexes with increase in cumulative light dose (photon exposure), assessed by the O₂ yield per single-turnover flash, was greater in leaves of plants grown in low light than those in high light; it was also exacerbated in the presence of lincomycin. A single exponential decay can describe the relationship between the loss of functional PSII and increase in cumulative photon exposure. From this relationship we obtained both the maximum quantum yield of photoinactivation of PSII at limiting photon exposures and the coefficient k, interpreted as the probability of photoinactivation of PSII per unit photon exposure. Parallel measurements of chlorophyll fluorescence after light treatment showed that 1/F_o−1/F_m was linearly correlated with the functionality of PSII, where F_o and F_m are the chlorophyll fluorescence yields corresponding to open and closed PSII reaction centers, respectively. Using 1/F_o−1/F_m as a convenient indicator of PSII functionality, it was found that PSII is present in excess; only after the loss of about 40% functional PSII complexes did PSII begin to limit photosynthetic capacity in capsicum leaves.     

Photosynthetic activity of developing leaves of Zea mays is less affected by heat stress than that of developed leaves

Various physiological and biochemical characters of a leaf change with stages of its ontogeny. It is likely that the photosynthetic functions of leaves of different ontogeny have different levels of heat tolerance. This study was initiated to analyze the photosynthetic heat tolerance of fully-developed, nearly-developed (more than 2/3 expanded) and developing (10–12 cm visible) leaves of two maize genotypes, F223 and F250. The results indicate that the photosynthetic CO₂ assimilation rate (P_n) of developing leaves was less affected by heat stress (42°C in the dark for 90 min) than that of developed leaves. The impaired P_n recovered within 24 h in the developing leaves, while the P_n of developed and nearly-developed leaves did not reach the non-stress level, even after 72 h. The P_n of the developed leaves of genotype F250 was less affected by heat stress than that of genotype F223. After heat stress, the slightly affected P_n of the developing leaf was associated with the almost unchanged photochemical efficiency of photosystem II (F_v/F_m) and the quantum yield of photosystem II electron transport. The chlorophylls a and b were degraded by heat stress; the degradation was pronounced in the developed leaves. As a result of heat stress, the antheraxanthin and zeaxanthin of the xanthophyll cycle accumulated in both the nearly-developed and developed leaves but not in the developing leaves. Injury to the plasma membrane due to heat stress was much less severe in developing leaves than that in the developed leaves. From the physiological characters which we determined it would appear that the P_n functions of the developing leaves are more resistant to heat stress than those of nearly-developed and developed leaves.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

Photosynthesis of two poplar clones under long‐term exposure to ozone

The photosynthetic response was studied in two clones ( Populus deltoides × maximowiczii Eridano and Populus × euramericana I‐214), known for their differential response to ozone (O₃) in terms of visible symptoms, when exposed to O₃ (60 nl l⁻¹ 5 h day⁻¹, 7 and 15 days). The photosynthetic ability was tested using gas exchange and chlorophyll fluorescence analysis. O₃ caused a decrease in the CO₂ assimilation rate at light saturation level in mature leaves of both clones. Alterations of Chl fluorescence parameters, in particular the F_v/F_m ratio and non‐photochemical quenching were also observed. The effects were similar for both clones and it could not be concluded that differential effects on electron transport capacity were responsible for the observed reduction in photosynthesis. The reduction of photosynthetic rate in Eridano was due mainly to a reduced mesophyll activity, as evidenced by the increase in intercellular CO₂ concentration and the minimal changes in stomatal conductance. In contrast, in I‐214, stomatal effects were primarily responsible, although effects on the mesophyll cannot be excluded. Data obtained indicate that the effects observed at the mesophyll level may be attributed to indirect effects caused by membrane disorders.     

Photoprotection processes under water stress and recovery in Mediterranean plants with different growth forms and leaf habits

The response of photoprotection mechanisms to a short-term water stress period followed by rewatering, to simulate common episodic water stress periods occurring in Mediterranean areas, was studied in 10 potted plants representative of different growth forms and leaf habits. During water stress and recovery, relative water content, stomatal conductance, leaf pigment composition, electron transport rates, maximum quantum efficiency of PSII photochemistry (F_v/F_m), thermal energy dissipation and photorespiration rates (P_r) were determined. All the species analyzed proved to be strongly resistant to photoinactivation of PSII under the imposed water stress conditions. The responses of the analyzed parameters did not differ largely among species, suggesting that Mediterranean plants have similar needs and capacity for photoprotection under episodic water stress periods regardless of their growth form and leaf habit. A general pattern of photoprotection emerged, consisting in maintenance or increase of P_r at mild stress and the increase of the thermal energy dissipation at more severe stress. Adjustments in pigment pool sizes were not an important short-term response to water stress. The increase of thermal energy dissipation because of water stress depended mostly on the de-epoxidation state of xanthophylls, although the slope and kinetics of such relationship strongly differed among species, suggesting species-dependent additional roles of de-epoxidated xanthophylls. Also, small decreases in F_v/F_m at predawn during water stress were strongly correlated with maintained de-epoxidation of the xanthophylls cycle, suggesting that a form of xanthophyll-dependent sustained photoprotection was developed during short-term water stress not only in evergreen but also in semideciduous and annual species.     

Oxidative damage to thylakoid proteins in water-stressed leaves of wheat (Triticum aestivum)

The production of reactive oxygen species in the chloroplast may increase under water deficit. To determine if this causes oxidative damage to the photosynthetic apparatus, we analyzed the accumulation of oxidatively damaged proteins in thylakoids of water-stressed wheat ( Triticum aestivum L.) leaves. Water stress was imposed on 4-week-old plants by withholding watering for 10 days to reach a soil water potential of about −2.0 MPa. In thylakoids of water-stressed leaves there was an increase in oxidative damage, particularly in polypeptides of 68, 54, 41 and 24 kDa. High molecular mass oxidized (probably cross-linked) proteins accumulated in chloroplasts of droughted leaves. Oxidative damage was associated with a substantial decrease in photosynthetic electron transport activity and photosystem II (PSII) efficiency (F_v/F_m). Treatment of stressed leaves with l -galactono-1,4-lactone (GL) increased their ascorbic acid content and enhanced photochemical and non-photochemical quenching of chlorophyll fluorescence. GL reduced oxidative damage to photosynthetic proteins of droughted plants, but it reverted the decrease in electron transport activity and PSII efficiency only partially, suggesting that other factors also contributed to loss of photosystem activity in droughted plants. Increasing the ascorbic acid content of leaves might be an effective strategy to protect thylakoid membranes from oxidative damage in water-stressed leaves.     

Photoinhibition in Vitis californica: interactive effects of sunlight, temperature and water status

Abstract. In a series of factorial experiments with cultivated Vitis californica Benth. (California wild grape) growth outdoors in full sun, we examined the effects of sunlight, temperature and water status on net CO₂ uptake and PSH chlorophyll fluorescence at 77K. Exposure to either high light or high temperature caused reductions in PSH activity followed by partial or complete overnight recovery. Upon simulataneous exposure to high light and high temperature, PSH inhibition was severe and persistent. The maximum chlorophyll fluorescence (F_M) and the ratio of variable to maximum fluorescence (F_v/F_M) differed in their responses to combinations of light and temperature. At both low and high light. F_M declined with increasing temperature over a wide temperature range, while F_v/F_M exhibited a similar sensitivity to temperature only at high light. Net CO₂ uptake declined by mid-afternoon and recovered by the next morning in most leaves, regardless of incident light or temperature. However, high-light leaves exhibited severe and lasting declines if temperatures exceeded 45°C. Water-stressed leaves exposed to high light exhibited greater reductions of net CO₂ uptake than water-stressed leaves exposed to low light. However, the degree of light-dependent decline in PSH fluorescence (F_M and F_v/F_M) did not vary with water status, indicating that reduced PSH activity was not a primary cause of reduced carbon gain during water stress.     

The role of thylakoid lipids in the photodamage of photosynthetic activity

The effect of excess light at 10 or 30°C under aerobic or low O₂ condition on peroxidation of thylakoid lipids and primary photochemistry of photoinsynthesis was studied in wheat ( Triticum aestivum L. cv. HD 2329). Photoinhibitory damage to photosythesis was directly proportional to the peroxidation of thylakoid lipids. Photoinhibitory treatment given under low O₂ conditions resulted in significantly less peroxidation of the primary photochemistry of photosythesis measured using chlorophyll fluorescence and photosythetic electron trasport. Short term recovery of F_v/F_m ratio was fast while thylakoid lipids did not show much recovry. However, recovery (of F_v/F_m ratio and thylakoid lipids) was almost complete within 12 h after photoinhibition treatment. A possible relationship between peroxidation of thylakoid lipids and photodamage to photosynthesis is discussed.     

Role of light in changes in free amino acid and polyamine contents at chilling temperature in maize (Zea mays)

The effect of irradiance on changes in photosynthesis, free amino acids and polyamines was investigated. Two-week-old maize ( Zea mays L.) plants were chilled at 5°C in the light (250 μmol m⁻² s⁻¹ PAR) or dark. The chlorophyll fluorescence ratio, F_v/F_m, decreased in the light by ca 50% but did not change in the dark. Similarly to the F_v/F_m, there was no change in the transpiration rate or the stomatal conductance in the dark, while these parameters decreased by ca 55% in the light. The net photosynthesis rate declined in both cases, but to a far greater extent in the light (73%) than in the dark (40%). The intercellular CO₂ partial pressure increased by ca 50% in all cases. The free amino acid contents increased compared to the control during the cold treatment. In most cases this increase was more pronounced in the light than in the dark. There was a continuous increase in the putrescine level, which was more pronounced in the light than in the dark. The spermidine content increased one and a half times after one day in the light but decreased by 70% in the dark compared to the control values. From the second day a 50% decline in the spermidine content was observed in the light and an 80% decline in the dark. These results suggest that light has an influence not only on the photosynthetic processes during chilling stress but also on other stress markers such as polyamines and free amino acids.     

Drought and chlorophyll fluorescence in field-grown potato (Solanum tuberosum)

Light interception, stomatal conductance and chlorophyll fluorescence were measured in potato ( Solanum tuberosum L.) grown either irrigated, or droughted from the time of plant emergence. Compared with the irrigated treatment, drought reduced both light interception and stomatal conductance. In both treatments, the yields of variable fluorescence in the dark- and light-adapted states (F_y/F_m and F'_v/F'_m, respectively) were negatively correlated with photosynthetic photon flux density (PPFD) and mirrored daytime changes in PPFD. Photochemical quenching was positively correlated with PPFD, but the dominant effect of F'_v/F'_m resulted in a decrease in the quantum yield of photosystem II (PSII) electron transport with increasing PPFD.
Drought had no significant effect on the functioning of PSII and the balance between photochemical and non-photochemical quenching was unaffected. Non-photochemical quenching was not increased by drought and the quantum yield of PSII electron transport was unaffected. It is concluded that, in leaves of droughted plants, excess energy, resultant of stomatal limitation of photosynthesis, was dissipated by photochemical quenching such as increased photorespiration.     

Loss of the precise control of photosynthesis and increased yield of non‐radiative dissipation of excitation energy after mild heat treatment of barley leaves

The after effects of a short exposure of intact barley leaves to moderately elevated temperature (40°C, 5 min) on the induction transients and the irradiance dependencies of photosynthesis and chlorophyll fluorescence are presented. This mild heat treatment strongly reduced the oscillations in the rate of photosynthesis and in the yield of chlorophyll fluorescence. However, only a 25% irreversible inhibition of maximum photosynthetic capacity of photosystem II (PSII) measured by oxygen evolution was produced and the intrinsic quantum yield of PSII measured by the chlorophyll fluorescence ratio (F_m‐ F_o)/F_m decreased by only 15%. In contrast, the above treatment increased radiationless dissipation processes in PSII by a factor of two. In heat‐treated leaves, photosynthesis was not saturated even by strong light. Both ΔpH‐dependent quenching of excitons in PSII (including formation of zeaxanthin) and state 1/state 2 transition were found to be stimulated. Heat exposure enhanced the control of PSII activity by PSI, as evidenced by a significant increase in the quenching effect of far‐red light on the maximum yield of chlorophyll fluorescence. It was deduced that after mild heat treatment, the photosynthetic apparatus in leaves lacks the precise coordinating control of electron transport and carbon metabolism owing to the inability of PSII to support electron transport at a level adequate for carbon metabolism. This effect was not related to the small irreversible thermal damage to PSII, but was rather due to a significant increase in non‐photochemical quenching of excitation energy.