The responses of photosynthetic gas exchange and chlorophyll a fluorescence to changes of irradiance and temperature in two species of Miscanthus

Photosynthetic CO₂ uptake and chlorophyll (Chl) a fluorescence of C₄ perennial grasses, Miscanthus floridulus (Labill) Warb and M. transmorrisonensis Hayata, from altitudes in central Taiwan of 390 and 2700 m, respectively, were studied at 10 and 25 °C to find if the species differ in their photosynthetic responses to a low temperature, and whether their photosystems 2 become more susceptible to the photoinhibition at low temperatures. For both species, the maximum photosynthetic rate (P_max) was reduced when the leaves were exposed to 10 °C. At irradiances higher than 400 µmol m^-2 s^-1, the values of F_v/F_m were reduced in both species at 10 °C but not at 25 °C, which indicated the photoinhibition at 10 °C. Reductions in P_max and the values of F_v/F_m at 10 °C were lesser in M. transmorrisonensis than in M. floridulus.     

Photosynthetic activity including the thermal‐ and chilling‐light sensitivities of a temperate Japanese brown alga Sargassum macrocarpum

The effects of irradiance, temperature, thermal‐ and chilling‐light sensitivities on the photosynthesis of a temperate alga, Sargassum macrocarpum (Fucales) were determined by a pulse amplitude modulation (PAM)‐chlorophyll fluorometer and dissolved oxygen sensors. Oxygenic photosynthesis–irradiance curves at 8, 20, and 28°C revealed that the maximum net photosynthetic rates (NP _max) and saturation irradiance were highest at 28°C, and lowest at 8°C. Gross photosynthesis and dark respiration determined over a range of temperatures (8–36°C) at 300 μmol photons m^?2 s^?1 revealed that the maximum gross photosynthetic rate (GP_max) occurred at 27.8°C, which is consistent with the highest seawater temperature in the southern distributional limit of this species in Japan. Additionally, the maximum quantum yields of photosystem II (F _v/F _m) during the 72‐h temperature exposures were stable at 8–28°C, but suddenly dropped to zero at higher temperatures, indicative of PSII deactivation. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m^?2 s^?1 at 8, 20, and 28°C revealed greater declines in their effective quantum yields (Φ _PSII) under high irradiance. While Φ _PSII under low irradiance were very similar with the initial F _v/F _m under 20 and 28°C, values rapidly decreased with exposure duration at 8°C. At this temperature, F _v/F _m did not recover to initial values even after 12 h of dark acclimation. Final F _v/F _m of alga at 28°C under high irradiance treatment also did not recover, suggesting its sensitivity to photoinhibition at both low and high temperatures. These photosynthetic characteristics reflect both the adaptation of the species to the general environmental conditions, and its ability to acclimate to seasonal changes in seawater temperature within their geographical range of distribution.     

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ¹³C analyses were done. Predawn values of F_v/F_m were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday F_v/F_m values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P _max) was 14.58 ± 0.22 μmol m^-2 s^-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g _s) was profoundly responsive in comparison to P _max which resulted in a high water use efficiency. This was further confirmed by δ¹³C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P _max and g _s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species

The effect of a wide range of temperatures (?15 and 60°C) in darkness or under strong irradiation [1,600 μmol(photon) m^?2 s^?1] on quantum yield of photosystem II photochemistry and xanthophyll cycle pigments was investigated in a tropical fruit crop (Musa sp.) and a temperate spring flowering plant (Allium ursinum L.). In darkness within the nonlethal thermal window of A. ursinum (from ?6.7 to 47.7°C; 54.5 K) and of Musa sp. (from ?2.2°C to 49.5°C; 51.7 K) maximal quantum yield of PSII photochemistry (F_v/F_m) was fairly unaffected by temperature over more than 40 K. At low temperature F_v/F_m started to drop with ice nucleation but significantly only with initial frost injuries (temperature at 10% frost damage; LT₁₀). The critical high temperature threshold for PSII (T_c) was 43.8°C in A. ursinum and 44.7°C in Musa sp. Under strong irradiation, exposure to temperatures exceeding the growth ones but being still nonlethal caused photoinhibition in both species. Severity of photoinhibition increased with increasing distance to the growth temperature range. ΔF/F_m′ revealed distinctly different optimum temperature ranges: 27–36°C for Musa sp. and 18–27°C for A. ursinum exceeding maximum growth temperature by 2–7 K. In both species only at temperatures > 30°C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40°C β-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45°C, in A. ursinum lutein remained unchanged, neoxanthin levels decreased in the supraoptimal temperature range. In darkness, F_v/F_m was highly temperature-insensitive in both species. Under strong irradiation, whenever growth temperature was exceeded, photoinhibition occurred with xanthophylls being changed only under supraoptimal temperature conditions as an antiradical defence mechanism.     

The analysis of determining factors and evaluation of tolerance to photoinhibition in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Photoinhibition is a significant constraint for improvement of radiation-use efficiency and yield potential in cereal crops. In this work, attached fully expanded leaves of seedlings were used to assay the factors determining photoinhibition and for evaluation of tolerance to photoinhibition in wheat (Triticum aestivum L.). Our results showed that even 1 h under PPFD of 600 µmol(photon) m^?2 s^?1 could significantly reduce maximal quantum yield of PSII photochemistry (F_v/F_m) and performance index (PI) compared to low light [300 µmol(photon) m^?2 s^?1]. The decrease of F_v/F_m and PI was more noticeable with the increase of light intensity; irradiance higher than 800 µmol(photon) m^?2 s^?1 resulted in photoinhibition. Compared to 25°C, lower (20°C) or higher temperature (≥ 35°C) aggravated photoinhibition, while slightly high temperature (28°) alleviated photoinhibition. At 25°C, irradiance of 1,000 µmol(photon) m^–2 s^–1 for 1 h was enough to cause photoinhibition and a significant decrease of F_v/F_m, PI, trapped energy flux, electron transport flux, and density of reaction center as well as increase of dissipated energy flux per cross section were observed. In addition, seedlings at 21–32 days after planting showed a relatively stable phenotype, while the younger or older seedlings indicated an increased susceptibility to photoinhibition, especially in senescing leaves. Finally, six wheat varieties with relative tolerance to photoinhibition were identified from 22 Chinese winter wheat varieties by exposing attached leaves of the 25-d old seedlings for 1 h to 1,000 µmol(photon) m^–2 s^–1 at 25°C. Therefore, our work established a possible method for development of new wheat varieties with enhanced tolerance to photoinhibition.     

Influence of nitrogen supply and growth irradiance on photoinhibition and recovery in Heuchera americana (Saxifragaceae)

Prior work demonstrated that Heuchera americana, an evergreen herb inhabiting the deciduous forest understory in the southeastern United States, has a 3-4-fold greater photosynthetic capacity under the low-temperature, strong-light, open canopies of winter compared to the high-temperature, weak-light, closed canopies of summer. Moreover, despite the reductions in soil nitrogen, the chilling temperatures, and the increased quantum flux associated with winter, chronic photoinhibition was not observed in this species at this time of the year. We were interested in the photosynthetic acclimation and photoinhibition characteristics of this species when grown under contrasting light and nitrogen regimes. Newly expanded shade-acclimated leaves of forest-grown plants exposed to strong light varying in intensity and duration at 25°C showed a reduction in F_v/F_m (the ratio of variable to maximum room temperature chlorophyll fluorescence measured after dark adaptation), which was correlated with a decline in ø_a (the intrinsic quantum yield of CO₂-saturated O₂ evolution on an absorbed light basis). Plants grown in the glasshouse under contrasting light (high and low light; HL and LL, respectively) and nitrogen supply (high and low nitrogen; HN and LN, respectively) regimes showed that photosynthetic acclimation to HL was impaired in LN regimes. The HL-LN plants also had the lowest values of F_v/F_m and of ø on both incident and absorbed light bases and had 50% less chlorophyll (per unit area) compared to plants from other growth regimes. Controlled exposure to bright light at low temperatures (2-3°C) for 3 h resulted in a sharp decrease in F_v/F_m (and rise in F_o, the minimum fluorescence yield) in all plants. Shade-grown plants from both N regimes were highly susceptible to chronic photoinhibition, as indicated by a greater reduction in F_v/F_m and incomplete recovery after 18 h in weak light at 25°C. The HL-HN plants were the least susceptible to chronic photoinhibition, having the smallest decrease in F_v/F_m with near full recovery within 6 h. The decline in Fv/Fm in HL-LN plants was comparable to that of shade-acclimated plants, but recovered fully within 6 h. Low-N plants from both light regimes displayed greater increases in F_o which did not return to pretreatment levels after 18 h of recovery. These studies indicate that HL-LN plants were sensitive to chronic photoinhibition and, at the same time, had a high capacity for dynamic photoinhibition. Experimental garden studies showed that H. americana grown in an open field in summer were photoinhibited and did not fully recover overnight or during extended periods of weak light. These results are discussed in relation to the photosynthetic acclimation of H. americana under natural conditions.     

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ¹³C analyses were done. Predawn values of F_v/F_m were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday F_v/F_m values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P _max) was 14.58 ± 0.22 µmol m^-2 s^-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g _s) was profoundly responsive in comparison to P _max which resulted in a high water use efficiency. This was further confirmed by δ¹³C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P _max and g _s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition.     

Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves

The responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000 /umol m^?2 s^?1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15 h exposure to 5°C at high light, the F_v/sb/F_mratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their F_v/F_m ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theF_v F_m ratio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in F_v/F_m may not simply be related to zeaxanthin levels or to amounts of D₁ protein alone but result from multifactoral influences.     

Cold Acclimation Ability and Photosynthesis among Species of the Tropical Coffea Genus

Abstract: Three Coffea species (C. arabica cv. Icatu, C. canephora cv. Apoatã and C. dewevrei) were tested in order to identify and study the mechanisms of tolerance to low, non‐freezing temperatures. Several photosynthesis‐related parameters were monitored during a 20‐day period of gradual temperature decrease, from 25/20 °C (day/night) down to 15/10 °C, during chilling treatments (15/4 °C), and upon rewarming (25/20 °C). Differences were found among species, both during low temperature exposure and during rewarming. In general, Coffea species showed cold‐induced photoinhibition of photosynthesis, which was attributable to biochemical (in vivo ribulose‐1,5‐bisphosphate carboxylase/oxygenase activity and carbohydrate synthesis) and biophysical (antennae functioning, photosystem II efficiency and linear electron transport) inactivation, rather than to stomatal constraints. The moderately low temperature of 15/10 °C was enough to cause a negative impact on net photosynthesis (A), mostly due to low (initial) rubisco activity in all species. However, C. arabica cv. Icatu showed a higher tolerance to chilling and recovered quickly and completely upon rewarming, as assessed from the impacts on the photosynthetic machinery (e.g. A_max, F_o, F_v/F_m, F_v′/F_m′, q_P, ?_e, rubisco activity) and on carbohydrate metabolism. Such lesser effects are likely to be related to the strong increases and higher contents of zeaxanthin, lutein and β‐carotene that presumably increased the ability to dissipate excitation energy and contributed to protect the photosynthetic apparatus. During cold exposure, a significant reduction of the α/β carotene ratio, which is considered an acclimation feature, was observed solely in C. arabica cv. Icatu. However, C. canephora cv. Apoatã and, especially, C. dewevrei showed to be highly cold‐sensitive. In these latter species, the photoinhibitory impairments to photosynthesis were stronger, probably due to the lower contents of protecting pigments during chilling conditions that lead to a higher vulnerability to excess excitation energy. Moreover, the mesophyll impairments (e.g. A_max, F_v/F_m, ?_e) became significant even at moderately low temperatures of 15/10 °C, and a lower ability to recover after chilling exposure was observed. The limitation of in vivo rubisco activity and A_max may have been due to substrate limitation, but disturbances in sugar metabolism could also play an important role in the expression of chilling sensitivity in C. canephora cv. Apoatã and C. dewevrei.     

The Effect of Photoautotrophy on Photosynthesis and Photoinhibition of Gardenia Plantlets during Micropropagation

We studied the relationships between the degree of photoautotrophy, photosynthetic capacity, and extent of photoinhibition of Gardenia jasminoides Ellis plantlets in vitro. Two successive micropropagation stages (shoot multiplication and root induction), and three culture conditions [tube cap closure, photosynthetic photon flux density (PPFD), and sucrose concentration] which may influence the development of photoautotrophy in vitro were assayed. The ratios of variable chlorophyll fluorescence to either maximal (F_v/F_m) or ground (F_v/F₀) values were low, irrespective of the culture stage or growing conditions. Incomplete development of the photosynthetic apparatus and permanent photoinhibition may be involved. However, F_v/F_m and F_v/F₀ increased from shoot multiplication to root induction owing to a decrease in F₀ and an increase in F_m. This suggests that photoinhibition decreases later during micropropagation, when the photoautotrophy of plantlets is more advanced. The low sucrose content and high PPFD increased the photoinhibition of plantlets, whereas growth in tubes with permeable caps showed the opposite effect. The only culture factor with a significant (positive) effect on maximum photosynthetic rate (P _max) was PPFD. At shoot multiplication net photosynthetic rate (P _N) was positively correlated with the half time of the increase from F₀ to F_m (t_1/2). Such association may be mainly due to a common response of both traits to higher PPFD in culture. Within each culture stage, no relationship was observed between P _N and the degree of photoautotrophy, which was positively correlated with F_v/F_m and F_v/F₀ during root induction. During shoot multiplication, these correlations were not significant, or were even negative. Hence during the last stage of micropropagation, plantlets with a higher degree of photoautotrophy are less photoinhibited, whereas they do not follow this pattern at the earlier stage.     

Photoinhibition in the Antarctic moss Grimmia antarctici Card when exposed to cycles of freezing and thawing

Freezing and thawing of the endemic moss species Grimmia antarctici Card, caused photoinhibition. When snow cover was removed from moss in the field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of variable to maximum chlorophyll a fluorescence (F_v/F_m), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures. A series of controlled laboratory studies found that the light conditions that prevail between freezing and thawing events influenced the recovery from photoinhibition observed during freezing and thawing, with low light conditions facilitating the greatest rates of recovery. After four cycles of freezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5°C and 15 μmol quanta m^?2 s^?1. These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down-regulation of photo-system II when photosynthetic carbon assimilation is limited by low temperatures.     

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade‐tolerant and mid‐successional, and Acer mono, shade‐tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F_v/F_m) of detached leaf discs exposed to about 2000 μmol m^?1 s^?1 photon flux density (PFD) for 2 h under controlled conditions (25 to 28 °C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light‐saturated photosynthetic rate (B. platyphylla, 9·87, 5·85 and 2·82; Q. mongolica, 8·05, 6·28 and 4·41; A. mono, 7·93, 6·11 and 5·08 μmol CO₂ m^?1 s^?1for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring ΔF/F′_m on the first day of transfer to natural daylight. Compared with ΔF/F′_m in AM, the lower ΔF/F′_m in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of ΔF/F′_m showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade‐grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.     

Temperature response and photoinhibition investigated by chlorophyll fluorescence measurements for four distinct species of dipterocarp trees

The effects of strong light in combination with elevated temperatures on the photosynthetic system were examined in 4 dipterocarp tree species with ecologically different habitats. The 4 dipterocarp tree species were: Shorea platyclados originated from upper dipterocarp forests, Shorea parvifolia– lowland and hill dipterocarp forests, Shorea assamica– lowland dipterocarp forests, and Dipterocarpus oblongifolius– riparian fringes. S. platyclados and D. oblongifolius have higher growth and survival rates in open sites than S. parvifolia and S. assamica. Tolerance of high temperature among the species was assessed by determining the critical temperatures (T_c) at which the minimal fluorescence (F_o) began to rise sharply. This was measured by exposing plants to an increasing temperature of about 1°C min^?1. The intrinsic thermotolerance of the thylakoid membrane appears to be the highest for D. oblongifolius (T_c=46.4°C), intermediate for S. platyclados (45.7°C), and lowest for S. parvifolia and S. assamica (45.2 and 45.3°C, respectively). The temperature‐dependent efficiency of PSII electron transport (ΔF/F′_m), photochemical quenching (q_P), and the efficiency of light capture of open PSII (F′_v/F′_m) were measured at the photosynthetic steady state at least 10 min after the light exposure (180 μmol m^?2 s^?1 PFD). Stable temperature responses of ΔF/F′_m and q_P were observed in S. platyclados and D. oblongifolius, while those in S. parvifolia and S. assamica were more temperature‐dependent and severely affected at 45°C. Little difference was observed in temperature‐dependent F′_v/F′_m among species. Photoinhibitory light exposure (1600 μmol m^?2 s^?1 PFD) for 2 h at 40°C had little effect on the recovery kinetics from photoinhibition of S. platyclados and D. oblongifolius compared with those at 35°C. In contrast, the recovery from photoinhibition was retarded in S. parvifolia and S. assamica. These findings suggest that even at 40°C, a temperature below T_c, an exposure to strong light exacerbated photoinhibition in S. parvifolia and S. assamica corresponding to the closure of PSII reaction centers, as indicated by the decrease in q_P at this temperature. Thus, S. platyclados and D. oblongifolius, which occur at uplands and riparian fringes with frequent disturbances, are suggested to have higher photosynthetic tolerance to elevated temperatures contributing to a circumvention of photoinhibition.     

Involvement of betacyanin in chilling-induced photoinhibition in leaves of <Emphasis Type="Italic">Suaeda salsa</Emphasis>

Seeds of Suaeda salsa were cultured in dark for 3 d and betacyanin accumulation in seedlings was promoted significantly. Then the seedlings with accumulated betacyanin (C+B) were transferred to 14/10 h light/dark and used for chilling treatment 15 d later. Photosystem 2 (PS2) photochemistry, D1 protein content, and xanthophyll cycle during the chilling-induced photoinhibition (exposed to 5 °C at a moderate photon flux density of 500 μmol m⁻² s⁻¹ for 3 h) and the subsequent restoration were compared between the C+B seedlings and the control (C) ones. The maximal efficiency of PS2 photochemistry (F_v/F_m), the efficiency of excitation energy capture by open PS2 centres (F_v′/F_m′), and the yield of PS2 electron transport (Φ_PS2) of the C+B and C leaves both decreased during photoinhibition. However, smaller decreases in F_v/F_m, F_v′/F_m′, and Φ_PS2 were observed in the C+B leaves than in C ones. At the same time, the deepoxidation state of xanthophyll cycle, indicated by (A+Z)/(V+A+Z) ratio, increased rapidly but the D1 protein content decreased considerably during the photoinhibition. The increase in rate of (A+Z)/(V+A+Z) was higher but the D1 protein turnover was slower in C+B than C leaves. After photoinhibition treatment, the plants were transferred to a dim irradiation (10 μmol m⁻² s⁻¹) at 25 °C for restoration. During restoration, the chlorophyll (Chl) fluorescence parameters, D1 protein content, and xanthophyll cycle components relaxed gradually, but the rate and level of restoration in the C+B leaves was greater than those in the C leaves. The addition of betacyanins to the thylakoid solution in vitro resulted in similar changes of F_v/F_m, D1 protein content, and (A+Z)/(V+A+Z) ratio during the chilling process. Therefore, betacyanin accumulation in S. salsa seedlings may result in higher resistance to photoinhibition, larger slowing down of D1 protein turnover, and enhancement of non-radiative energy dissipation associated with xanthophyll cycle, as well as in greater restoration after photoinhibition than in the control when subjected to chilling at moderate irradiance.     

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

Understanding 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.     

The photosynthetic performance of a cultivated Japanese green alga Caulerpa lentillifera in response to three different stressors,temperature, irradiance,and desiccation

The effects of temperature, irradiance, and desiccation on the photosynthesis of a cultivated Japanese green alga Caulerpa lentillifera (Caulerpaceae) were determined by a pulse amplitude modulation (PAM)-chlorophyll fluorometer and dissolved oxygen sensors. The photochemical efficiency in the photosystem II (F_v/F_m and ΔF/F_m') during the 72-h temperature exposures (8, 12, 16, 20, 24, 28, 32, 36, and 40°C) was generally stable at 16–32°C but quickly dropped at lower and higher temperatures. The photosynthesis–temperature curve at 200 μmol photons m^?2 s^?1 also revealed that the maximum gross photosynthesis (GP_max) occurred at 30.7°C (30.5–30.9, 95% highest density credible intervals). Photosynthesis–irradiance curves at 16, 24, and 32°C quickly saturated, then expressed photoinhibition, and revealed that the maximum net photosynthetic rates (NP_max) and saturation irradiance (E_k) were highest at 32°C and lowest at 16°C. Continuous 6-h exposure to irradiances of 200 (low) and 400 (high) μmol photons m^?2 s^?1 at 16, 24, and 32°C expressed greater declines in their ΔF/F_m' at 16°C, revealing chronic chilling-light stress. The response to continuous desiccation (~480 min) under 50% humidity at 24°C showed that ΔF/F_m' dropped to zero at 480-min aerial exposure, and the treatments of more than 60-min desiccation did not return to the initial level even after 24-h subsequent rehydration in seawater. Likewise, ΔF/F_m' fell when the absolute water content (AWC) of the frond dropped below AWC of 90% and mostly did not return to the initial level even after 24-h subsequent rehydration in seawater, signifying a low tolerance to desiccation.

     

Photosynthetic acclimation in shade-developed leaves of Euterpe edulis Mart (Arecaceae) after long-term exposure to high light

To analyze acclimation of Euterpe edulis seedlings to changes in light availability, we transferred three-year-old seedlings cultivated for six months under natural shade understory [≈ 1.3 mol(photon) m^?2 d^?1] to a forest gap [≈ 25.0 mol(photon) m^?2 d^?1]. After the transfer, changes in chlorophyll fluorescence and leaf gas-exchange parameters, as well as in the light-response curves of photosynthesis and photosynthetic induction parameters, were analyzed during the following 110 days. Simultaneously measured photosynthetic characteristics in the shaded seedlings grown in understory served as the control. Despite the fact that the understory seedlings were under suboptimal conditions to achieve their light-saturated net photosynthetic rate (P _Nmax), light-response curves and photosynthetic induction parameters indicated that the species had the low respiration rate and a fast opening of stomata in response to the intermittent occurrence of sunflecks, which exerted a feed-forward stimulation on P _Nmax. Sudden exposure to high light induced photoinhibition during the first week after the transfer of seedlings to gap, as it was shown by the abrupt decline of the maximal quantum yield of PSII photochemistry (F_v/F_m). The photoinhibition showed the time-dependent dynamics, as the F_v/F_m of the seedlings transferred to the forest gap recovered completely after 110 days. Furthermore, the net photosynthetic rate increased 3.5-fold in relation to priorexposure values. In summary, these data indicated that more than 21 days was required for the shade-acclimated seedlings to recover from photoinhibition and to relax induction photosynthetic limitations following the sudden exposure to high light. Moreover, the species responded very quickly to light availability; it highlights the importance of sunflecks to understory seedlings.     

Photosynthetic characteristics of female vegetative tissues of Porphyra katadai var. hemiphylla (Bangiales, Rhodophyta) in culture

In this study, chlorophyll fluorescence parameters (?F/F _m′, F _v/F _m) and oxygen evolution of female vegetative tissues of Porphyra katadai var. hemiphylla in unisexual culture (FV) and in mixed culture with male vegetative tissues (FV-M) were followed at 5–20 °C, 10 and 80 μmol photons m^?2 s^?1. The formation of reproductive tissues was closely correlated with decreasing photosynthetic activities. At the same temperature the tissues cultured under 80 μmol photons m^?2 s^?1 showed a greater extent of maturation than those under 10 μmol photons m^?2 s^?1, and their decrease in photosynthesis was also larger. Under the same light intensity the extent of maturation increased with increasing temperature, and both cultures showed higher values of ?F/F _m′ and F _v/F _m at 10 and 15 °C, while their oxygen evolution became negative at 15–20 °C during the later period. Under the same culture condition the maturation of FV-M culture was relatively faster than that of FV culture, while their photosynthetic activity, especially ?F/F _m′, was lower.     

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m^?1 s^?2; 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO₂ uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RT_AGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F _v/F _m) in both species. However, the depression in F _v/F _m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.     

Physiological responses to nitrogen and sulphur addition and raised temperature in <Emphasis Type="Italic">Sphagnum balticum</Emphasis>

Sphagnum, the main genus which forms boreal peat, is strongly affected by N and S deposition and raised temperature, but the physiological mechanisms behind the responses are largely unknown. We measured maximum photosynthetic rate (NP_max), maximum efficiency of photosystem II [variable fluorescence (F _v)/maximum fluorescence yield (F _m)] and concentrations of N, C, chlorophyll and carotenoids as responses to N and S addition and increased temperature in Sphagnum balticum (a widespread species in the northern peatlands) in a 12-year factorial experiment. NP_max did not differ between control (0.2 g N m⁻² year⁻¹) and high N (3.0 g N m⁻² year⁻¹), but was higher in the mid N treatment (1.5 g N m⁻² year⁻¹). N, C, carotenoids and chlorophyll concentration increased in shoot apices after N addition. F _v/F _m did not differ between N treatments. Increased temperature (+3.6°C) had a small negative effect on N concentration, but had no significant effect on NP_max or F _v/F _m. Addition of 2 g S m⁻² year⁻¹ showed a weak negative effect on NP_max and F _v/F _m. Our results suggest a unimodal response of NP_max to N addition and tissue N concentration in S. balticum, with an optimum N concentration for photosynthetic rate of ~13 mg N g⁻¹. In conclusion, high S deposition may reduce photosynthetic capacity in Sphagnum, but the negative effects may be relaxed under high N availability. We suggest that previously reported negative effects on Sphagnum productivity under high N deposition are not related to negative effects on the photosynthetic apparatus, but differences in optimum N concentration among Sphagnum species may affect their competitive ability under different N deposition regimes.