Chronic Photoinhibition Research Articles

Effects of artificial canopy shading on vegetative growth and ripening processes of cv. Nero d'Avola (Vitis vinifera L.).

The biology of the grapevine (Vitis vinifera L.) is clearly influenced by the climatic conditions of the growing environment, where temperature and light play a major role in modifying plant physiology. In the scenario of climatic changes, radiative excess, correlated to the increase in temperature, can concretely subject the photosynthetic apparatus to a condition of light saturation and cause a drastic reduction in photochemical efficiency, giving rise to chronic photoinhibition phenomena. Undoubtedly, the ripening behavior also undergo evident alterations; the problem of sugar ripening, which is often strongly accelerated, is induced not only by high temperatures but also by the excess concentration of carbon dioxide (CO2), which results in a higher ripening. In addition, high berry temperatures favor a reduction in the concentration of organic acids. The reported trends indicate that the need for urgent action is closely linked to the future environmental impacts that these changes could have on the entire wine sector. In recent years, shade treatments have been applied to the vine canopy to overcome this issue. The objective of this study was to determine how artificial canopy shading affects the vines vegetative growth and the ripening processes of Vitis vinifera cv. Nero d'Avola during the 2019-2020 vegetative seasons. Three treatments were established: shading treatment with a green net (shade factor 27%), shading treatment with a white net (shade factor 32%), and untreated vines, thus naturally exposed to light radiation. Artificial shading, applied at full fruit set, interfered with the microclimate of the vines, causing partial effects on the grape ripening processes. At harvest, significant differences were found between the treatments in terms of sugars, also shading treatments increased must acidity and decrease pH. Results obtained on vegetative parameters, suggest that the shading treatment delays leaf fall, with potentially positive effects on the starch accumulation on perennial reserve organs to be exploited at the following season's sprouting. Shading significantly reduced berry size, with obvious consequences on bunch weight and yield per vine. In 2020, shaded plants showed a delay in all the phenological phases. The total anthocyanins content was not changed by the shading treatment. The results obtained confirm the importance of net coverage on the microclimate of the vines, vegetative-productive activity, and grapes quality. From this point of view, the net covering technique can be a tool for controlling grapes ripening dynamics in the context of climate change.

Expression of nuclear-encoded, haptophyte-derived ftsH genes support extremely rapid PSII repair and high-light photoacclimation in Karenia brevis (Dinophyceae)

Karenia brevis, a neurotoxic dinoflagellate that produces brevetoxins, is endemic to the Gulf of Mexico and can grow at high irradiances typical of surface waters found there. To build upon a growing number of studies addressing high-light tolerance in K. brevis, specific photobiology and molecular mechanisms underlying this capacity were evaluated in culture. Since photosystem II (PSII) repair cycle activity can be crucial to high light tolerance in plants and algae, the present study assessed this capacity in K. brevis and characterized the ftsH-like genes which are fundamental to this process. Compared with cultures grown in low-light, cultures grown in high-light showed a 65-fold increase in PSII photoinactivation, a ∼50-fold increase in PSII repair, enhanced nonphotochemical quenching (NPQ), and depressed Fv/Fm. Repair rates were among the fastest reported in phytoplankton. Publicly available K. brevis transcriptomes (MMETSP) were queried for ftsH-like sequences and refined with additional sequencing from two K. brevis strains. The genes were phylogenetically related to haptophyte orthologs, implicating acquisition during tertiary endosymbiosis. RT-qPCR of three of the four ftsH-like homologs revealed that poly-A tails predominated in all homologs, and that the most highly expressed homolog had a 5′ splice leader and amino-acid motifs characteristic of chloroplast targeting, indicating nuclear encoding for this plastid-targeted gene. High-light cultures showed a ∼1.5-fold upregulation in mRNA expression of the thylakoid-associated genes. Overall, in conjunction with NPQ mechanisms, rapid PSII repair mediated by a haptophyte-derived ftsH prevents chronic photoinhibition in K. brevis. Our findings continue to build the case that high-light photobiology—supported by the acquisition and maintenance of tertiary endosymbiotic genes—is critical to the success of K. brevis in the Gulf of Mexico.

The decline of Fraxinus angustifolia Vahl in a Mediterranean salt meadow: Chlorophyll fluorescence measurements in long-term field experiment

The ecological transition zones also known as ecotones are specific ecosystems but how global change will affect their biodiversity and functioning is poorly understood. In the case of the areas at the interface between riparian forest and salt meadows the biodiversity conservation remains a major issue. The aim of this work was to evaluate the effects of a salty environment on Fraxinus angustifolia a non-halophyte species. For this, the photochemistry of PSII was in situ explored during the summer between 2007 and 2016. The predawn maximum quantum efficiency of PSII (Fv/Fm) was always greater than 0.8 which indicates an absence of chronic photoinhibition induced by salinity. During these years, the light kinetic of the actual quantum yield of PSII (ΦPSII) was studied. It appears that this yield is generally higher in salty environments than in the riparian forest. Multivariate analysis, showed that the dissipation regulated light energy faction (ΦNPQ), the photochemical quenching (qP) and the Fd/Fs ratio constitute the main explanatory variables of ΦPSII changes. The subsequent two-way-ANCOVA carried out demonstrated that ΦNPQ was weaker in saline environments, suggesting a better efficiency of photosystem collecting antennas. Meanwhile, the analysis of photosynthetic pigments did not exhibited a salt effect except the significant accumulation of chlorophyll a in 2016. In order to address the question of the salt-related toxicity, sodium ion contents in the leaves were found to be identical in the two sites, suggesting the establishment of an energy-dependent mechanism of salt exclusion/retention in roots. The growth rings of F. angustifolia in saline areas highly follows the annual water deficit changes. Between 2007 and 2016, the biomass in the salt meadow was higher with a peak in 2011 representing 60% of the biomass of control individuals. But, since 2011 a continuous decline of growth was recorded. Water conditions seem to constitute a limiting factor even if the photosynthetic apparatus generates the excess metabolic energy necessary to cope with the saline toxicity. This study thus highlighted the plasticity of the photochemistry of the PSII depending to the environmental conditions of the salt meadows. It also shows the vulnerability of an ecotone through the decline in the productivity of a glycophyte with climate change. This augurs for changes in the composition of plant communities which is in favor of strict halophytes.

Effects of increased irradiance on biomass, photobiology, nutritional quality, and pigment composition of Arctic sea ice algae

Ice algae are key contributors to primary production and carbon fixation in the Arctic, and light availability is assumed to limit their growth and productivity. We investigated photo-physiological responses in sea ice algae to increased irradiance during a spring bloom in West Greenland. During a 14 d field experiment, light transmittance through sea ice was manipulated to provide 3 under-ice irradiance regimes: low (0.04), medium (0.08), and high (0.16) transmittances. Chlorophyll a decreased with elevated light availability relative to the control. Maximum dark-adapted photosynthetic efficiency (ΦPSII_max) showed an initially healthy and productive ice algae community (ΦPSII_max > 0.6), with ΦPSII_max decreasing markedly under high-light treatments. This was accompanied by a decrease in the light utilization coefficient (α) and photosynthetic capacity (maximum relative electron transfer rate), and a decrease in the ratio of mono- to polyunsaturated fatty acids. This was partly explained by a corresponding increase of photoprotective pigments (diadinoxanthin and diatoxanthin), and a development of mycosporine-like amino acids as identified from a distinctive spectral absorption peak at 360 nm. After 14 d, in situ fluorescence imaging revealed significant differences in ΦPSII_max between treatments of dark-adapted cells (i.e. those sampled before sunrise and after sunset), during diel cycles, with clear chronic photoinhibition in high and medium treatments. Data demonstrate the high sensitivity of spring-blooming Arctic sea ice algae to elevated irradiance caused by loss of snow cover. The predicted loss of snow cover on landfast ice will negatively impact ice algae, their potential primary production, and nutritional quality for higher trophic levels.

Effect of light and temperature on photosynthesis of a cultivated brown alga, Saccharina sculpera (Laminariales), from Japan

ABSTRACT Understanding the physiological responses of kelp to environmental parameters is essential for the conservation of natural kelp beds and increased cultivation yield. We investigated the effects of photosynthetically active radiation (PAR) and temperature on photosynthesis of a subarctic edible brown alga, Saccharina sculpera (Laminariales), using pulse amplitude modulation (PAM)–chlorophyll fluorometry and optical dissolved oxygen sensors. Photosynthesis–PAR experiments at 8 °C, 15 °C, and 22 °C revealed the highest net photosynthetic rates at 22 °C. Values for compensation (Ec) and saturation (Ek) indicated a high affinity for a light environment that reflected the natural vertical habitat of this alga. The photosynthetic response of S. sculpera to temperature indicated that the optimum temperature for oxygenic photosynthesis () was 22.9 °C, which was consistent with the highest seawater temperature in Japanese distribution range for this subarctic species. The alga survived for 168 h at 24 °C in darkness. Continuous 6 h exposure to PAR of 100 µmol (low) and 1000 µmol (high) photons m–2 s–1 at 8 °C, 15 °C, and 22 °C induced chronic photoinhibition in the alga, with greater declines in effective quantum yields of photosystem II (ΦPSII). Failure to recover Fv/Fm after dark acclimation at low and high PAR was observed, except for low PAR at 8 °C. The physiological performance of S. sculpera indicates an affinity for a low light environment. We expect that our results will lead to an improvement in aquaculture techniques through the regulation of cultivation depth.

The temperature and light responses on the photosynthesis of two freshwater red algae, Virescentia helminthosa and Sheathia arcuata (Batrachospermaceae), from Japan

The temperature and light responses of photosynthesis in two freshwater red algae, Virescentia helminthosa and Sheathia arcuata (Batrachospermaceae), were determined by a pulse amplitude modulation (PAM) chlorophyll fluorometer and dissolved oxygen sensors. Net oxygenic photosynthesis–photosynthetically active radiation (PAR) models of V. helminthosa and S. arcuata revealed similar low PAR-adapted responses, with a compensation PAR (Ec) of 6.95 and 11.5 μmol photons m−2 s−1 (5.58–8.42 and 9.10–11.42, 95% Bayesian prediction interval, BPI) and saturating PAR (Ek) of 18.8 and 17.7 μmol photons m−2 s−1 (14.5–24.7 and 13.0–23.90, BPI), respectively. A temperature-dependent model of net photosynthesis and dark respiration for two species also showed similar temperature responses, and the gross photosynthetic rate (GPmax), 1.79 and 1.19 μg O2 gww−1 min−1 (1.62–1.96 and 1.08–1.29, BPI), was highest at 26.4 and 30.3 °C (23.9–28.7 and 28.3–32.1, BPI). The maximum quantum yields (Fv/Fm) for two species also had similar responses with respect to temperature; however, it was generally stable at low temperatures (8–20 °C) with the highest value of 0.52 and 0.56 (0.49–0.54 and 0.54–0.58, BPI) occurring at 18.5 and 20.9 °C (17.1–19.7 and 19.8–21.9, BPI). Continuous exposure (12 h) to PAR of 100 (low) and 1000 (high) μmol photons m−2 s−1 at 12, 16, and 24 °C revealed greater declines in their effective quantum yield (ΦPSII) in the two species under high PAR. Nevertheless, their Fv/Fm mostly recovered after a subsequent 12-h dim-light acclimation, suggesting the potential of recovery from daytime chronic photoinhibition.

Photosynthetic Performance in Pinus canariensis at Semiarid Treeline: Phenotype Variability to Cope with Stressful Environment

Low temperatures represent the most important environmental stress for plants at the treeline ecotone; however, drought periods at the semiarid treeline could modify photosynthetic performance patterns. Gas exchange, chlorophyll fluorescence, photosynthetic pigments, and α-tocopherol were measured in a Pinus canariensis forest located at a semiarid treeline forest at 2070 m altitude over a whole year. The level of summer drought, caused by an extended period without rain and very low previous rainfall, was remarkable during the study. Furthermore, the cold season showed extraordinarily low temperatures, which persisted for five months. All of these factors combined made the study period an extraordinary opportunity to improve our understanding of photosynthetic performance in a drought-affected treeline ecotone. A high dynamism in all the measured parameters was detected, showing robust changes over the year. Maximum photosynthesis and optimal values were concentrated over a short period in spring. Beyond that, fine regulation in stomatal closure, high WUEi with a great plasticity, and changes in pigments and antioxidative components prevented dehydration during drought. In winter, a strong chronic photoinhibition was detected, and α-tocopherol and β-carotene acquired a main role as protective molecules, accompanied by morphological variations as changes in specific leaf areas to avoid freezing. The recovery in the next spring, i.e., after these extreme environmental conditions returned to normal, showed a strategy based on the breakdown of pigments and lower photosynthetic functions during the winter, and rebuilding and regreening. So, a high level of plasticity, together with some structural and physiological adaptations, make P. canariensis able to cope with stresses at the treeline. Nevertheless, the carbon gain was more limited by drought than by low temperatures and more extended droughts predicted in future climate change scenarios may strongly affect this forest.

Starch accumulation does not lead to feedback photosynthetic downregulation in girdled coffee branches under varying source-to-sink ratios

Leaves were forced to accumulate starch as much as possible but maintained relatively low soluble sugar levels with no evidence of photosynthetic feedback downregulation. Here, we examined whether the regulation of photosynthesis in coffee depends on sink activity or carbohydrate build-up in source leaves and how the coffee tree adjusts its photosynthetic performance and primary metabolism to varying source-to-sink ratios. We designed a field experiment by girdling coffee branches that were further manipulated by controlled defoliation and/or defruiting such that three highly varying source-to-sink ratios were created. Under markedly high source-to-sink ratios, photosynthesis rates were primarily limited by diffusional factors, and feedback downregulation contributed a maximum of 7% of the photosynthetic reduction. Such responses were accompanied by a relatively enormous capacity for starch accumulation coupled with the maintenance of low levels of soluble sugars. Chronic photoinhibition and photodamage could be avoided by increases in nonphotochemical energy dissipation, photorespiration and respiration rates, thus diminishing the excitation pressure on photosystems. Overall, the activities of key enzymes (and their transcript abundance) associated with carbon metabolism varied only marginally across treatments. Leaf metabolic adjustments were more evident under high sink demand conditions, and nitrogen metabolism was more affected than carbon metabolism. In conclusion, our results offer novel insights into the high coordination between the source supply and sink demand in coffee, with a minor role of photosynthetic downregulation even under dramatically low sink conditions.

Photosynthetic performance of Saccharina angustata (Laminariales, Phaeophyceae) at the southern boundary of subarctic kelp distribution in Japan

ABSTRACTEffects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a subarctic edible brown alga, Saccharina angustata (Laminariaceae, Laminariales), from Hokkaido, Japan, were determined using a pulse amplitude modulation (PAM)–chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–PAR experiments at 8, 16, and 20 °C revealed highest net photosynthetic rates at 16 °C; whereas, the temperature effect on maximum quantum yield (Fv/Fm) showed the highest Fv/Fm at 7.0 to 8.4 °C. These temperatures corresponded to the optimum temperatures for growth of Japanese Saccharina species. The maximum gross photosynthetic rate from gross photosynthesis–temperature curve was at 21.2 °C. Though consistent with the highest seawater temperature in the study site, it is close to the limit for thermal inhibition, because Fv/Fm above 20 °C was compromised after 48–72 h of incubation. Continuous 6-h exposures to PAR of 200 µmol (low) and 1000 µmol (high) photons m−2 s−1 at 8, 16, and 20 °C caused chronic photoinhibition in the alga, with greater declines in effective quantum yields of photosystem II (ΦPSII) and failure of recovery in post-dark acclimation Fv/Fm under high PAR. This physiological performance provides a basis for understanding the persistence of S. angustata near its southern boundary in the western Pacific.

Ecophysiological responses of a threatened red alga to increased irradiance in an in situ transplant experiment

The red alga Gelidium corneum is a dominant foundation species in the south-eastern Bay of Biscay, where a decline in its populations has been documented in the few last decades. We investigated the ecophysiological responses of G. corneum to different light conditions by means of an in situ transplant experiment. We found that the stress response measured by physiological and biochemical approaches was higher in G. corneum at higher irradiance levels, for both transplanted and control specimens, than under lower light intensities. In the former case the specimens showed a decrease in maximum quantum yield (Fv/Fm), maximum electron transport rate (ETRmax), photosynthetic efficiency (αETR), photosynthetic pigment contents, nitrogen content and thallus length, whereas the C:N ratio, MAAs and bleaching cover increased. In general terms, these responses were more evident in the apical parts of the thallus than in middle ones. Our results suggest that high light stress at depths of 3 m triggered photobiological changes in G. corneum, involving ineffective photoprotection and the occurrence of chronic photoinhibition. Therefore, considering the upward trend in summer mean surface solar radiation in the study area since the 80s, high light conditions may have played a role in the declines observed in G. corneum beds from the south-eastern Bay of Biscay.

Salinity alleviates zinc toxicity in the saltmarsh zinc-accumulator Juncus acutus

The potential importance of Juncus acutus for remediation of Zn-contaminated lands has been recognized, because of its Zn tolerance and capacity to accumulate Zn. Since it is also a halophyte, the extent to which salinity influences its Zn tolerance requires investigation. A factorial greenhouse experiment was designed to assess the effect of NaCl supply (0 and 85 mM NaCl) on the growth, photosynthetic physiology and tissue ions concentrations of plants exposed to 0, 30 and 100 mM Zn. Our results indicated that NaCl supplementation alleviated the effects of Zn toxicity on growth, as Zn at 100 mM reduced relative growth rate (RGR) by 60% in the absence of NaCl but by only 34% in plants treated also with NaCl. This effect was linked to a reduction in Zn tissue concentrations, as well as to overall protective effects on various stages in the photosynthetic pathway. Thus, at 85 mM NaCl plants were able to maintain higher net photosynthesis (AN) than in the absence of added NaCl, although there were no differences in stomatal conductance (gs). This contributed to preserving the trade-off between CO2 acquisition and water loss, as indicated by higher intrinsic water use efficiency (iWUE). Hence, AN differences were ascribed to limitation in the RuBisCO carboxylation, manifested as higher intercellular CO2 concentration (Ci), together with dysfunction of PSII photochemistry (in term of light harvest and energy excess dissipation), as indicated by higher chronic photoinhibition percentages and variations in the photosynthetic pigment profiles in presence of Zn under non-saline conditions.

Assessment of photosynthetic performance in the two life history stages of Alaria crassifolia (Laminariales, Phaeophyceae)

SUMMARYUnderstanding of the physiological responses of kelp to environmental parameters is crucial, especially in the context of environmental change that may have contributed to the decline of kelp forests all over the world. The current study presents the photosynthetic characteristics of the macroscopic sporophyte and microscopic gametophyte stages of the brown alga Alaria crassifolia from Hokkaido, Japan, as determined by examining their photosynthetic responses over a range of temperature and irradiance using dissolved oxygen and chlorophyll fluorescence measurements. Net photosynthetic rates of the sporophyte were consistently higher than those of gametophyte across temperature gradients and irradiance levels. Photosynthesis–irradiance curves at 8°C, 16°C, and 20°C revealed similar initial slopes (α = 0.4–0.9) on the two life history stages, but higher compensation (E c = 4–7 μmol photons m−2 s−1) and saturation irradiances (E k = 53–103 μmol photons m−2 s−1) for the sporophyte than for the gametophyte (E c = 0–7 μmol photons m−2 s−1; E k = 7–10 μmol photons m−2 s−1). Both stages exhibited chronic photoinhibition, as shown by the failure of recovery in their maximum quantum yields (F v/F m) following high irradiance stress, with greater possibility of photodamage at low temperature. Gametophytes were less sensitive to low temperatures than sporophytes, given their relatively stable F v/F m response. Nevertheless, temperature optima for photosynthesis of both stages coincide with each other at 20–23°C, which correspond to the growth and maturation periods of A. crassifolia in Japan. This species is also likely to suffer from thermal inhibition as both GP rates and F v/F m decreased above 24°C.

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.

A native parasitic plant affects the performance of an introduced host regardless of environmental variation across field sites.

Increasing evidence from glasshouse studies shows that native hemiparasitic plants can significantly impact the performance and growth of introduced host plants. We investigated the effect of the native Australian hemiparasite Cassytha pubescens R.Br. on the introduced shrub Ulex europaeus L. at three field sites in South Australia. Parasite infection significantly decreased midday PSII efficiency (ΦPSII) and the maximum electron transport rates (ETRmax) of U. europaeus across sites. The impact of C. pubescens on the photosynthetic performance of U. europaeus may have been caused by infected plants having significantly lower N and K, but higher Fe and Al than uninfected plants at all sites. Significant Al and Fe enrichment in infected plants may be possibly due to the parasite indirectly inducing rhizosphere acidification. At two sites, C. pubescens significantly affected host Fv/Fm, indicating chronic photoinhibition in response to infection. The impact of infection on Fv/Fm was greatest at the wettest site, in line with an experiment where C. pubescens had more impact under high water availability. At this site, infected plants also had the highest foliar Fe and Al. The C isotope (δ13C) of infected plants was significantly lower than that of uninfected plants at only one site. Unusually, the δ13C of the parasite was the same as or significantly higher than that of the hosts. There were no site effects on parasite Fv/Fm or ΦPSII; however, ETRmax and δ13C varied across sites. The results suggest that this native parasite has negative effects on U. europaeus in the field, as was found for glasshouse studies. The abundance of this introduced weed in Australia could be negatively affected by C. pubescens infection.

Leaf acclimation strategies to contrasting light conditions in saplings of different shade tolerance in a tropical cloud forest.

To study the acclimation responses of the leaves of saplings of six tree species when changed to low or high levels of irradiance, we carried out a light exposure experiment. Species representative of contrasting shade tolerance groups were identified across a light gradient in the understorey of a Venezuelan Andean cloud forest. Measured traits included gas exchange, chlorophyll fluorescence, and morphoanatomical, biochemical and optical properties. Saplings were grown for 6 months in a shade-house receiving 20% photosynthetic photon flux (PPF) of full sunlight. Plant samples were then moved to shade-houses receiving low PPF (4%) or high PPF (65%). A factorial model (species×PPF), with repeated measurements (0, 15 and 120 days) was designed. Our results showed that morphological and anatomical traits were more plastic to PPF changes than photosynthetic traits. All species were susceptible to photoinhibition (15 days): shade-intolerant species showed dynamic photoinhibition (120 days), whereas shade-tolerant species presented chronic photoinhibition and the consequent inability to increase C assimilation rates under high PPF. The partially shade-tolerant species showed mixed responses; nonetheless, they exhibited larger adjustments in morphoanatomical and optical properties. Thus the acclimation responses of these species when subject to contrasting light conditions could help to explain their distribution along the light gradient in the understorey.

Photosynthetic acclimation of Grammatophyllum speciosum to growth irradiance under natural conditions in Singapore

Grammatophyllum speciosum, a native species to Singapore, have become extinct mainly due to habitat loss. Recently, Singapore has reintroduced G. speciosum into the natural environment under the orchid conservation programme. In this study, leaves of G. speciosum grown under low light (LL) under natural conditions had faster expansion rate and higher specific leaf area than leaves grown under intermediate light (IL) and high light (HL). All leaves had more than 95% midday relative water content. Although midday Fv/Fm ratios were lower in HL leaves than in IL and LL leaves, none of them exhibited chronic photoinhibition. HL leaves had upregulated their light utilization through higher photochemical quantum yield (ΔF/Fm′) and greater electron transport rate. HL leaves also had higher non-photochemical quenching, indicating that they had higher capability to dissipate excess light as heat, which was supported by their lower chlorophyll but higher carotenoids content. Although there was a linear correction between leaf temperature and photosynthetic photon flux density (PPFD), no correlations were found between stomatal conductance (gs) and PPFD, gs and leaf temperature. Light-saturated photosynthetic CO2 assimilation rate (Asat) was significantly higher in HL leaves than those of IL and LL leaves. However, all leaves had similar light-saturated stomatal conductance. Although LL leaves had higher leaf total reduced nitrogen that those of IL and HL leaves, none of them seemed to suffer from nitrogen deficiency during the experimental period. To conclude, G. speciosum is able to survive under different growth irradiances without watering and adding fertilizers.

The combined effects of PAR and temperature including the chilling-light stress on the photosynthesis of a temperate brown alga, Sargassum patens (Fucales), based on field and laboratory measurements

The combined effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a temperate Japanese brown alga, Sargassum patens (Fucales), were determined by field and laboratory measurements. Underwater measurements of the natural population of this alga in Kagoshima, Japan, revealed that the effective quantum yield (Φ PSII ) declined with increasing incident PAR, with minimum Φ PSII occurring during noon to early afternoon. Φ PSII recovered in the evening, indicating dynamic photoinhibition. In laboratory experiments, Φ PSII was also negatively correlated with PAR, which decreased after 12 h of continuous exposure to 200 (low) and 1000 (high) μmol photons m−2 s−1 at 8, 20, and 28 °C. Maximum quantum yield (F v /F m ) at 8 °C with low PAR failed to recover after 12 h of dark acclimation, suggesting the influence of low temperature in chronic photoinhibition. Photosynthesis–irradiance (P–E) curves likewise revealed lower net photosynthetic rates and photoinhibition at 8 °C. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (8–40 °C) revealed that the maximum gross photosynthetic rate (GP max) occurred at 26.9 °C. F v /F m after 72 h of temperature exposures was stable at 8–32 °C; but it was deactivated at 36 and 40 °C. This species is well-adapted to the current range of temperature in the temperate region of Japan (7–28 °C). However, the combined effects of low temperature and PAR may influence algal photosynthetic efficiency and so may be limiting at the marginal region of northern distribution of this temperate species.

Acclimation responses to high light by Guazuma ulmifolia Lam. (Malvaceae) leaves at different stages of development.

The re-composition of deforested environments requires the prior acclimation of seedlings to full sun in nurseries. Seedlings can overcome excess light either through the acclimation of pre-existing fully expanded leaves or through the development of new leaves that are acclimated to the new light environment. Here, we compared the acclimation capacity of mature (MatL, fully expanded at the time of transfer) and newly expanded (NewL, expanded after the light shift) leaves of Guazuma ulmifolia Lam. (Malvaceae) seedlings to high light. The seedlings were initially grown under shade and then transferred to full sunlight. MatL and NewL were used for chlorophyll fluorescence and gas exchange analyses, pigment extraction and morpho-anatomical measurements. After the transfer of seedlings to full sun, the MatL persisted and acclimated to some extent to the new light condition, since they underwent alterations in some morpho-physiological traits and maintained a functional electron transport chain and positive net photosynthesis rate. However, long-term exposure to high light led to chronic photoinhibition in MatL, which could be related to the limited plasticity of leaf morpho-anatomical attributes. However, the NewL showed a high capacity to use the absorbed energy in photochemistry and dissipate excess energy harmlessly, attributes that were favoured by the high structural plasticity exhibited by these leaves. Both the maintenance of mature, photosynthetically active leaves and the production of new leaves with a high capacity to cope with excess energy were important for acclimation of G.ulmifolia seedlings.

Direct impact of the sustained decline in the photosystem II efficiency upon plant productivity at different developmental stages

The impact of chronic photoinhibition of photosystem II (PSII) on the productivity of plants remains unknown. The present study investigated the influences of persistent decline in the PSII yield on morphology and productivity of Arabidopsis plants that were exposed to lincomycin at two different developmental stages (seedling and rosette stage). The results indicated that, although retarded, the lincomycin treated plants were able to accomplish the entire growth period with only 50% of the maximum quantum yield of primary photochemistry (Fv/Fm) of the control plants. The decline in quantum yield limited the electron transport rate (ETR). The impact of lincomycin on NPQ was not significant in seedlings, but was pronounced in mature plants. The treated plants produced an above ground biomass of 50% compared to control plants. Moreover, a linear relationship was found between the above ground biomass and total rosette leaf area, and the slope was decreased due to photoinhibition. The starch accumulation was highly inhibited by lincomycin treatment. Lincomycin induced a significant decrease in seed yield with plants treated from the rosette state showing higher yield than those treated from the seedling stage. Our data suggest that the sustained decline of PSII efficiency decreases plant productivity by constraining the ETR, leaf development and starch production.

Physiological responses to drought and experimental water deficit and waterlogging of four clones of cacao (Theobroma cacao L.) selected for cultivation in Venezuela

Cocoa is sensitive to both water deficit (WD) and waterlogging (WL), especially during the juvenile stage. The aim of this study was to compare in four cocoa clones the water status, gas exchange and photochemical activity of PSII of adult trees subjected to natural drought in the field, and young individuals in the greenhouse subjected to either WD or WL. In the greenhouse, pots in which eight-month-old saplings were growing were placed in 8-L plastic buckets and either weekly watered (control, C), left without watering (WD), or filled with tap water up to 1cm above soil surface (WL). In the field, drought affected clones differently in their water relations and photosynthetic responses. In the greenhouse, after 35 days of treatment, the four clones showed decreases in water potential which were stronger under WD than WL. Photosynthetic rate (PN) decreased on average 86% due to WD and 60% due to WL, clone 415 being the most susceptible to both stresses. On average, PN reached 100% of control values ​​7 d after re-watering but only 74% after drainage. Stomatal closure with maintenance of leaf water was the general response to WD. Clone 439, with highest relative lenticel area, was the one in which PN recovered most from WL. Lenticel hypertrophy, particularly in clone 439, apparently was one of the mechanisms of acclimation to WL. Chronic photoinhibition occurred in no case. We conclude that all clones are adequate for growth in the field, although 415 is the one with lower PN during the rainy season. Saplings of all clones are suitable for cultivation in drought-prone areas for dry periods of at least a month, and clone 439 is the one best suited for soils under short-term WL .