Decrease In ΦPSII Research Articles

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation.

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO2 uptake, via phosphoenolpyruvate carboxylase (PEPC, C4 carboxylation), from the diurnal refixation by Rubisco (C3 carboxylation). At the end of the day (CAM‐Phase IV), when nocturnally stored CO2 has depleted, stomata reopen and allow additional CO2 uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO2 uptake via C3 and C4 carboxylation in phase IV in the CAM species Phalaenopsis “Sacramento” and Kalanchoe blossfeldiana “Saja.” Short blackout periods during phase IV caused a sharp drop in CO2 uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O2 indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.

Sub-Arctic alpine Vaccinium vitis-idaea exhibits resistance to strong variation in snowmelt timing and frost exposure, suggesting high resilience under climatic change

In tundra ecosystems, snow cover protects plants from low temperatures in winter and buffers temperature fluctuations in spring. Climate change may lead to reduced snowfall and earlier snowmelt, potentially exposing plants to more frequent and more severe frosts in the future. Frost can cause cell damage and, in combination with high solar irradiance, reduce the photochemical yield of photosystem II (ΦPSII). Little is known about the natural variation in frost exposure within individual habitats of tundra plant populations and the populations’ resilience to this climatic variation. Here, we assessed how natural differences in snowmelt timing affect microclimatic variability of frost exposure in habitats of the evergreen Vaccinium vitis-idaea in sub-Arctic alpine Finland and whether this variability affects the extent of cell damage and reduction in ΦPSII. Plants in early melting plots were exposed to more frequent and more severe frost events, and exhibited a more pronounced decrease in ΦPSII, during winter and spring compared to plants in late-melting plots. Snowmelt timing did not have a clear effect on the degree of cell damage as assessed by relative electrolyte leakage. Our results show that sub-Arctic alpine V. vitis-idaea is currently exposed to strong climatic variation on a small spatial scale, similar to that projected to be caused by climate change, without significant resultant damage. We conclude that V. vitis-idaea is effective in mitigating the effects of large variations in frost exposure caused by differences in snowmelt timing. This suggests that V. vitis-idaea will be resilient to the ongoing climate change.

Natural variation of photosynthetic efficiency in Arabidopsis thaliana accessions under low temperature conditions.

Low, but non‐freezing, temperatures have negative effects on plant growth and development. Despite some molecular signalling pathways being known, the mechanisms causing different responses among genotypes are still poorly understood. Photosynthesis is one of the processes that are affected by low temperatures. Using an automated phenotyping platform for chlorophyll fluorescence imaging the steady state quantum yield of photosystem II (PSII) electron transport (ΦPSII) was measured and used to quantify the effect of moderately low temperature on a population of Arabidopsis thaliana natural accessions. Observations were made over the course of several weeks in standard and low temperature conditions and a strong decrease in ΦPSII upon the cold treatment was found. A genome wide association study identified several quantitative trait loci (QTLs) that are associated with changes in ΦPSII in low temperature. One candidate for a cold specific QTL was validated with a mutant analysis to be one of the genes that is likely involved in the PSII response to the cold treatment. The gene encodes the PSII associated protein PSB27 which has already been implicated in the adaptation to fluctuating light.

Water relations and photo-protection mechanisms during drought stress in four coffee (Coffea arabica) cultivars from southwestern Saudi Arabia

This study investigated for the first time the effect of drought on water relations, gas exchange, osmotic adjustment, and photoprotection mechanisms of local Arabica coffee cultivars namely, Dawairi, Tufahi, Kholani and Tessawi grown in Southwestern Saudi Arabia. Two-year-old plants growing in pots were subjected to water stress by withholding watering for four weeks. Water relations and gas exchange measurements revealed that the severity of the stress varied among cultivars. The most contrasting behavior was between the two cultivars Dawairi (drought-tolerant) and Kholani (drought-sensitive). Tufahi and Tessawi, however, showed intermediate tolerance. Under drought stress, Dawairi trees were able to maintain higher leaf water content (WC), plant turgor potential (Ψp) and net photosynthetic assimilation rate (Anet) at a given predawn leaf water potential (Ψpd) than the other three cultivars. Irreversible wilting (turgor loss) occurred at leaf Ψpd of about −2.5 MPa for Kholani and about −3.5 MPa or lower for the other cultivars. The drought tolerant cultivar also showed the highest increase in instantaneous water-use efficiency (iWUE) under drought stress. All coffee cultivars were able to slow the onset of water stress by controlling transpiration (avoidance). But they did vary in their ability to avoid tissue dehydration under such stress. The reduction in Anet, transpiration rate (E) and stomatal conductance (gs) with decreasing Ψpd was earlier and more marked in Kholani and Tufahi than in Dawairi and Tessawi. Besides, the fluctuations in Ψpd and Ψmid revealed a contrasting behavior among the four cultivars. Under drought stress, Dawairi maintained the highest diurnal range of ∆Ψpd-mid and showed the highest daily re-saturation capability. Moreover, it had a greater capacity for osmotic adjustment through solute accumulation. In contrast, in Kholani and Tufahi the drought-induced accumulation of solutes was mainly passive due to dehydration of the leaves. Moreover, Dawairi and Tessawi accumulated more proline in their leaves than the two other cultivars under stress. Drought stress considerably altered total soluble sugar (TSS) concentration in most cultivars. Surprisingly, the decrease in TSS content was more pronounced in drought tolerant cultivars. Photoinhibition occurred in all four cultivars when subjected to drought stress albeit with variable severity. Moderate stress decreased the quantum efficiency of PSII (ΦPSII), photochemical fluorescence quenching (qP) and electron transport rate (ETR), and increased the non-photochemical quenching (NPQ); this implies dynamic photoinhibition in all cultivars. However, when drought became severe towards the end of the experiment, the decrease in ΦPSII, qP and ETR was accompanied by a decline in Fv/Fm indicating the occurrence of chronic photoinhibition in most cultivars. Still photoinhibition was more obvious in Kholani and Tufahi. Interesting genetic differences in drought tolerance appear to exist among these four economically important Arabica coffee cultivars.

Effect of PAR and temperature on the photosynthesis of the Japanese alga, Ecklonia radicosa (Laminariales), based on field and laboratory measurements

:The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a Japanese brown alga, Ecklonia radicosa, was determined by using dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry. The effective quantum yield (ΦPSII) of the sporophytes, in a natural population in shallow water, showed a characteristic noontime decline that indicated the influence of high PAR. However, ΦPSII of sporophytes in deeper water (−10 to −20 m) did not appear to be affected and maintained relatively high values of ΦPSII. In laboratory measurements of ΦPSII, after 12 h of continuous low and high PAR (100 and 1000 μmol photon m−2 s−1), a relatively greater decrease in ΦPSII in the high PAR treatment was observed. Furthermore, despite an overnight dark acclimation, the maximum quantum yield (Fv/Fm) under the high PAR treatment did not fully recover. A net photosynthesis–irradiance (P-E) curve was generated at 20°C, which revealed low compensation and saturation irradiances (Ec and Ek) of 14 and 45 μmol photon m−2 s−1, suggesting adaptation to a low PAR environment. Results from experiments to determine the net/gross photosynthesis and dark respiration rates revealed that the maximal gross photosynthetic rates occurred at 27.5°C and were 5.18 μg O2 gww−1 min−1; whereas, the dark respiration rate increased exponentially with a mean value of 1.35 μg O2 gww−1 min−1 at 22°C. Fv/Fm was relatively stable at low temperature, and the highest value (0.73) occurred at 16.5°C. The field survey and laboratory experiments revealed that E. radicosa is well adapted to relatively low PAR and water temperatures typical of warm temperate coastal regions in Japan. The poor response to high PAR and the inability of sporophytes to fully recover overnight from high PAR exposure indicates that the vertical distribution of this species is driven by the PAR environment.

The effect of irradiance and temperature on the photosynthesis of a native alga Sargassum fusiforme (Fucales) from Kagoshima, Japan

:The effect of irradiance and temperature on the photosynthesis of a Japanese native brown alga, Sargassum fusiforme, was determined by using dissolved oxygen sensors and pulse amplitude–modulated fluorometry. In situ measurements of the effective quantum yield (ΦPSII) decreased as irradiance increased at 12:00 PM and recovered in the late afternoon, indicating that a proportionately greater number of reaction centres were closed. Likewise, laboratory measurement after 6 hours of exposure to low and high (300 and 1000 μmol photons m−2 s−1, respectively) irradiances showed a relatively greater decrease in ΦPSII at high irradiance, and both treatments recovered overnight. A model of the net photosynthesis–irradiance relationship determined at 20°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 391 μmol photons m−2 s−1 [95% Bayesian credible interval (BCI) of 299–515 μmol photons m−2 s−1]. Gross photosynthesis and dark respiration rates determined over a range of temperatures (8–36°C) revealed that the maximal gross photosynthetic rate was 24.1 (BCI: 21.5–26.8) μg O2 gww−1 min−1 at 22.9°C (BCI: 19.8–25.8), and the dark respiration rate increased nonlinearly from a low of 0.5 μg O2 gww−1 min−1 at 8°C to a high of 8.9 μg O2 gww−1 min−1 at 36°C. The highest value of the quantum yield (Fv/Fm = ΦPSII at 0 μmol photons m−2 s−1) was estimated to occur at 22.9°C (BCI: 21.8–23.9) and was 0.72 (BCI: 0.70–0.73). The specimens examined appeared to be well adapted to the natural temperature range of the study site. Although the irradiances on the fronds were strong enough to induce a drop in quantum yield, our evidence clearly suggests that full recovery occurs overnight.

<b>Effect of salinity and priming on seedling growth in rapeseed (<i>Brassica napus</i> var <i>oleifera</i> Del.)</b> - doi: 10.4025/actasciagron.v35i4.17655

Experiments were performed to examine the effect of salt stress and GA3-priming on initial growth of two rapeseed cultivars, one tolerant and one sensitive to salt stress during germination. Seedlings from seeds germinated in salty (as NaCl) and non salty substrate were grown in salty and non salty hydroponics. Salt stress reduced seedling growth of the two genotypes consistently with their degree of stress tolerance during germination. Seedlings from stress sensitive seeds germinated under high salinity showed a rapid recover of growth in non stressing conditions. The effect of salt stress on shoot/root ratio was controversial, increased for lab and decreased for greenhouse experiments, probably due to different timing of stress application and additional experimental conditions. Salt stress decreased leaf photosynthesis and increased thermal dissipation in sensitive seedlings (decrease of ΦPSII and qP, increase of NPQ). The GA3-priming did not affect seedling growth of the stress sensitive cultivar subjected to stress, while it greatly improved the performance of the stress tolerant cultivar.

Bioinformatic and functional characterization of the basic peroxidase 72 from Arabidopsis thaliana involved in lignin biosynthesis

Lignins result from the oxidative polymerization of three hydroxycinnamyl (p-coumaryl, coniferyl, and sinapyl) alcohols in a reaction mediated by peroxidases. The most important of these is the cationic peroxidase from Zinnia elegans (ZePrx), an enzyme considered to be responsible for the last step of lignification in this plant. Bibliographical evidence indicates that the arabidopsis peroxidase 72 (AtPrx72), which is homolog to ZePrx, could have an important role in lignification. For this reason, we performed a bioinformatic, histochemical, photosynthetic, and phenotypical and lignin composition analysis of an arabidopsis knock-out mutant of AtPrx72 with the aim of characterizing the effects that occurred due to the absence of expression of this peroxidase from the aspects of plant physiology such as vascular development, lignification, and photosynthesis. In silico analyses indicated a high homology between AtPrx72 and ZePrx, cell wall localization and probably optimal levels of translation of AtPrx72. The histochemical study revealed a low content in syringyl units and a decrease in the amount of lignin in the atprx72 mutant plants compared to WT. The atprx72 mutant plants grew more slowly than WT plants, with both smaller rosette and principal stem, and with fewer branches and siliques than the WT plants. Lastly, chlorophyll a fluorescence revealed a significant decrease in ΦPSII and q L in atprx72 mutant plants that could be related to changes in carbon partitioning and/or utilization of redox equivalents in arabidopsis metabolism. The results suggest an important role of AtPrx72 in lignin biosynthesis. In addition, knock-out plants were able to respond and adapt to an insufficiency of lignification.

Photosynthesis, energy partitioning, and metabolic adjustments of the endangered Cistaceae species Tuberaria major under high temperature and drought

In view of predicted climatic changes for the Mediterranean region, study of high temperature and drought impacts on physiological responses of endangered species regains relevance. In this context, micropropagated plants of Tuberaria major, a critically endangered species, endemic of Algarve, were transferred to a controlled-environment cabinet with day/night temperatures set at 25/18°C (Reference) or 32/21°C (HT). After 15 days of HT acclimation, some plants were subjected to progressive drought followed by rewatering. The enhancement of temperature alone did not affect water relations and photosynthetic rates (PN) but the stomatal conductance (gs) exhibited a 3-fold increase in comparison with reference plants. The maximum quantum yield of photosystem (PS) II (Fv/Fm), the effective quantum yield of PSII photochemistry (ΦPSII), carotenoid (Car) and anthocyanin content enhanced, whereas the quantum yields of regulated (ΦNPQ) and nonregulated (ΦNO) energy dissipation decreased. Drought combined with HT reduced predawn leaf water potential to values of about −1.3 MPa, which had adverse effects on gas exchange and PSII activity. Values of PN and gs were 71 and 79% lower than those of HT plants. An impairment of photochemical activity was also observed: the decrease in ΦPSII and the increase of ΦNPQ. However, an irreversible photoinhibitory damage had not occurred. Carotenoid and anthocyanin content remained elevated and soluble sugars (SS) increased twice, whereas proline and MDA accumulation was not detected. On the first 24 h after water-stress relief, gs, PN, ΦPSII, and ΦNPQ did not recover, but SS returned to the reference level. Overall, T. major acquired an adequate capacity for a protection against the development of oxidative stress during drought and water recovery under HT. These findings suggest that T. major is prepared to deal with predicted climate changes.

Tolerance of photosynthesis to photoinhibition, high temperature and drought stress in flag leaves of wheat: A comparison between a hybridization line and its parents grown under field conditions

Photosynthesis and its tolerance to photoinhibition, high temperature and drought stress of flag leaves were investigated in a wheat (Triticum aestivum L.) hybridization line (1-12) and its parents (Jing-411 and Xiaoyan-54). From the beginning of flowering to the 10th day, light-saturated CO2 assimilation rate (Pmax) showed no significant decrease and Pmax of 1-12 was comparable to that of its parents. From the 20th day, Pmax decreased significantly and this decrease was much less in 1-12 than in its parents, whereas no decrease in chlorophyll content was observed in 1-12 and its parents on the 20th day, indicating that photo-oxidative damage occurred in 1-12 and its parents but 1-12 is more resistant to photo-oxidative damage than its parents. To further characterize photo-oxidative damage, tolerance to photoinhibition, high temperature and drought stress was compared in 1-12 and its parents. When exposed to high light (1400μmolm−2s−1), the maximal efficiency of PSII photochemistry (Fv/Fm) decreased significantly with increasing exposure time and such a decrease was much less in 1-12 than in its parents. When exposed to higher temperatures (30–45°C) for 15min, Fv/Fm started to decrease at 42°C in 1-12 and its parents. The greatest decrease in Fv/Fm was observed in Jing-411. 1-12 and Xiaoyan-54 showed a comparable decrease in Fv/Fm. Similar results were also observed in the actual PSII efficiency (ΦPSII), photochemical quenching (qP) and non-photochemical quenching (qN). During exposure of detached leaves to air under room temperature conditions, relative water content decreased with increasing exposure time. However, such a decrease was greatest in Jing-411 followed by Xiaoyan-54, and 1-12. There were neither changes in Fv/Fm nor qN during water loss in the line and its parents. However, there was a decrease in ΦPSII and qP and greatest decrease was observed in Jing-411 followed by Xiaoyan-54, and 1-12. Also, the decrease in Pmax was greatest in Jing-411, followed by Xiaoyan-54, and 1-12 during water loss. In addition, the activities of ribulose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, NADP-malate dehydrogenase and NADP-malate enzyme were significantly higher in 1-12 than in its parents. The results in this study suggest that high resistance to photo-oxidative damage of the flag leaves in 1-12 may be the physiological basis for its high yield when grown in north China. Our results also suggest that parents can be selected for improved biochemical and physiological traits and crossed to high yielding agronomically elite materials which can be selected for higher performance in yield.

The effect of drought stress on chlorophyll fluorescence in Lolium-Festuca hybrids

The effects of drought on photochemical efficiency of PSII in leaves of 22 hybrids of Festuca pratensis × Lolium multiflorum and Festuca pratensis × Lolium perenne and of Festuca pratensis cv. Skra were investigated. A significant decrease of electron transport efficiency (about 25%) in PSII (ΦPSII) was not found before 9 days of seedling growth in hydroponics with water potential (Ψw) equal to −0.8 MPa (simulated “soil drought”). The decrease of ΦPSII was similarly related to that of excitation energy capture by open PSII reaction centre (Fv’/Fm’) and also to the decrease of the proportion of oxidized to reduced QA (photochemical fluorescence quenching, qp). According to the drought prolongation, variation of all parameters of fluorescence between genotypes significantly increased. The seedlings of some genotypes were able to recover electron transport efficiency in PSII after increasing water potential in nutrient solution (removing the “soil drought”).

Long‐term plum pox virus infection produces an oxidative stress in a susceptible apricot,Prunus armeniaca, cultivar but not in a resistant cultivar

The effect of plum pox virus (PPV) infection on the response of some antioxidant enzymes was studied in two apricot cultivars, which behaved differently against PPV infection: cultivar Real Fino (susceptible) and cultivar Stark Early Orange (cv. SEO, resistant). In the susceptible cultivar, PPV produced a decrease inΦPSII,F′v/F′mandQp. PPV infection produced a drop inp‐hydroxy mercury benzoic acid (pHMB)‐sensitive ascorbate peroxidase, dehydroascorbate reductase and peroxidase in the soluble fraction from susceptible plants, whereas in the resistant apricot cultivar, pHMB‐insensitive ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase and superoxide dismutase increased. However, catalase decreased in the soluble fractions from both infected cultivars. Long‐term PPV infection also produced a decrease in the chloroplastic ascorbate–glutathione cycle enzymes only in the susceptible plants. As a consequence of PPV infection, an oxidative stress, indicated by an increase in lipid peroxidation and in protein oxidation, was produced only in the leaves from the susceptible cultivar which was also monitored by the diaminobenzidine peroxidase‐coupled H2O2probe. The loss ofΦPSII, indicative of activated oxygen species production, and the decrease in the levels of antioxidant enzymes in chloroplasts from susceptible plants could be responsible for the chlorosis symptoms observed. The results suggest that the higher antioxidant capacity showed by cv. SEO could be a consequence of a systemic acquired resistance induced by PPV penetration in stem tissue at the graft site and could be related, among other factors, to their resistance to PPV.

Changes in stem lignins (monomer composition and crosslinking) and peroxidase are related with the maintenance of leaf photosynthetic integrity during Verticillium wilt in Capsicum annuum.

• Verticillium dahliae is a soilborne pathogen that causes vascular wilt in pepper (Capsicum annuum var. annuum). Here we study to what extent changes in the lignification response of peppers condition tolerance of wilt. • For this, the quantum yield (ΦPSII ), the linear electron transport rate (ETR), and the lignification response (monomer composition and crosslinking) were studied in three C. annuum cultivars differing in degree of tolerance. • The results showed that in tolerant cultivars (Padrón and Yolo Wonder), both ΦPSII and ETR showed significantly higher levels at saturating photosynthetically active radiation values. This was not, however, the case for cv. Luesia, which showed a significant decrease in ΦPSII , ETR and nonphotochemical quenching values, suggesting that photochemical processes are strongly damaged in this cultivar as a consequence of the disease. The analysis of stem lignins in tolerant cultivars revealed that they were mainly composed of p-hydroxyphenyl units, while levels of β-O-4-linked coniferyl and sinapyl alcohols were significantly lower. • It is concluded that through the observed changes in stem lignins (monomer composition and crosslinking) peppers retard, since they maintain leaf photosynthetic integrity, but do not stop (since wilt symptoms are not avoided) V. dahliae fungal hyphae penetration.

Comparative Sensitivity of Seven Marine Microalgae to Cumulative Exposure to Ultraviolet-B Radiation with Daily Increasing Doses

A comparative study of the relative sensitivity to ultraviolet radiation (UVR) among seven marine microalgae from the classes Cyanophyceae (Synechococcus sp.), Rhodophyceae (Porphyridium cruentum), Cryptophyceae (Rhodomonas salina), Prymnesiophyceae (Isochrysis galbana), Bacillariophyceae (Chaetoceros sp.), Chlorophyceae (Dunaliella salina), and Prasinophyceae (Tetraselmis suecica), was carried out. Daily exposure for 4 h over a 4-day period with daily increasing doses of UVR (mainly ultraviolet-B, UV-B) led to a continuous decrease in the effective quantum yield (φPSII) in Rhodomonas salina, Chaetoceros sp. and Isochrysis galbana. In contrast, Tetraselmis suecica did not show a significant inhibition over the 4-day period of exposure. Synechococcus sp., Dunaliella salina and Porphyridium cruentum were able to restore, at least partially, the values of φPSII during the recovery period (white light only, 20 h), even though a significant decrease in φPSII was also measured in these algae after each exposure. Relative inhibition, defined here as the decrease in φPSII after exposure as compared to the value before exposure and normalised to the latter value (RI = ∆φ PSII /φ PSII ), was fitted against UV-B irradiance according to an equation of the type y = 1 ‐ (a x) c e -bx , where y is RI, x is the UV-B irradiance (W m ‐2 ), and a, b and c are constants determined from the fit, using unweighted and weighted irradiances. From the fit, the UV-B irradiance for 50% relative inhibition of φ PSII (RI = 0.5, E 50 [W m ‐2 ]), was calculated for each alga. Rhodomonas salina and Isochrysis galbana exhibited the lowest values of E50 (0.21 and 0.25, respectively, for unweighted irradiance), being the most sensitive algae to UV-B, while the highest value was calculated for Dunaliella salina (0.47 for unweighted irradiance). The E50 could not be calculated for Tetraselmis suecica since this alga showed an RI maximum of 0.1, and was therefore considered as a species which was highly tolerant of UV-B. A rise in optical density of each algal cell suspension at 680 and 750 nm, as well as in chlorophyll a content determined from methanolic extracts, was evident over the experimental period in all of the species except for Isochrysis galbana and Rhodomonas salina. In these two algae, in contrast to the other species, there was a notable increase in the carotenoid/chlorophyll a ratio, although this had negligible effects for the protection against deleterious UV-B. The phycobilin/chlorophyll a ratio showed a gradual decrease with time in the three phycobilin-containing algae, but it was particularly evident in R. salina after the third day of exposure.

Limitations to photosynthesis in Coffea canephoraas a result of nitrogen and water availability

Summary Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (gs), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in gs. As a whole, changes in internal CO2 concentration predominantly reflected changes in A rather than in gs. Under irrigated conditions, A, but not gs, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (ϕPSII) through slight decreases of photochemical quenching (qp) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in ϕPSII, due mainly to variations in qp. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.

Photosystem II photochemistry and its sensitivity to heat stress in maize plants as affected by nitrogen deficiency

Effects of nitrogen deficiency on photosystem II photochemistry and its sensitivity of heat stress (25–44 °C) were investigated in maize plants (Zea mays L.) grown under natural illumination. Maize plants were fertilized with 15 mmol/L nitrate (high N, control) versus 0.5 mmol/L nitrate (low N, N deficiency). Compared with the high-N plants, the low-N plants had lower values in the apparent quantum yield of photosynthesis and light-saturated CO2 assimilation capacity expressed either on a basis of leaf area or chlorophyll. In the light-adapted state, the low-N plants showed substantial decreases in the quantum yield of PSII electron transport (ΦPSII, 28%), the efficiency of excitation energy capture by open PSII reaction centers (Fv′/Fm′, 12%) and the photochemical quenching coefficient (qP, 19%), as well as a significant increase in the non-photochemical quenching coefficient (qN, 84%). In the dark-adapted state, the low-N plants showed a small but a significant decrease (4%) in the maximal efficiency of PSII photochemistry (Fv/Fm). These results suggest that N deficiency induced no substantial damage to PSII apparatus and the changes in PSII photochemistry in the light-adapted state can be seen as a regulatory response to down-regulate the quantum yield of PSII electron transport (ΦPSII) that would match with the decreased CO2 assimilation rate under N deficiency conditions. When exposed to high temperatures, a greater decrease in ΦPSII, Fv′/Fm′ and qP, as well as a larger increase in qN and the proportion of the QB-non-reducing PSII reaction centers were observed in the low-N plants than in the high-N plants, indicating that the responses of PSII to heat stress was significantly affected by N status, with N deficiency increasing the sensitivity of PSII to heat stress. These results suggest that nitrogen nutrition plays an important role in the protective adaptation of PSII to heat stress.

Photosynthetic electron transport and carbon-reduction-cycle enzyme activities under long-term drought stress in Casuarina equisetifolia Forst. & Forst.

The inhibition of photosynthetic electron transport and the activity of photosynthetic carbon reduction cycle (PCR) enzymes under long-term water stress after slow dehydration was studied in non-nodulated Casuarina equisetifolia Forst. & Forst. plants. Initially, drought increased the fraction of closed Photosystem II (PS II) reaction centres (lowered qP) and decreased the quantum yield of PS II electron transport (ΦPSII) with no enhancement of non-radiative dissipation of light energy (qN) because it increased the efficiency of electron capture by open PS II centres (F′v/F′m). As drought progressed, F′v/F′m fell and the decrease in ΦPSII was associated with an increased qN. The kinetics of dark relaxation of fluorescence quenching pointed to an increase in a slowly-relaxing component under drought, in association with increased contents of zeaxanthin and antheraxanthin. Total NADP-dependent malate dehydrogenase activity increased and total stromal fructose-1,6-bisphosphatase activity decreased under drought, while the activation state of these enzymes remained unchanged. Water stress did not alter the activity and the activation state of ribulose bisphosphate carboxylase oxygenase.