Effects of different irradiances on the photosynthetic process during ex-vitro acclimation of Anoectochilus plantlets

Six months old in vitro-grown Anoectochilus formosanus plantlets were transferred to ex-vitro acclimation under low irradiance, LI [60 μmol(photon) m⁻² s⁻¹], intermediate irradiance, II [180 μmol(photon) m⁻² s⁻¹], and high irradiance, HI [300 μmol(photon) m⁻² s⁻¹] for 30 d. Imposition of II led to a significant increase of chlorophyll (Chl) b content, rates of net photosynthesis (P _N) and transpiration (E), stomatal conductance (g _s), electron transfer rate (ETR), quantum yield of electron transport from water through photosystem 2 (Φ_PS2), and activity of ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO, EC 4.1.1.39). This indicates that Anoectochilus was better acclimated at II compared to LI treatment. On the other hand, HI acclimation led to a significant reduction of Chl a and b, P _N, E, g _s, photochemical quenching, dark-adapted quantum efficiency of open PS2 centres (F_v/F_m), probability of an absorbed photon reaching an open PS2 reaction centre (F_v′/F_m′), ETR, Φ_PS2, and energy efficiency of CO₂ fixation (Φ_CO2/Φ_PS2). This indicates that HI treatment considerably exceeded the photo-protective capacity and Anoectochilus suffered HI induced damage to the photosynthetic apparatus. Imposition of HI significantly increased the contents of antheraxanthin and zeaxanthin (ZEA), non-photochemical quenching, and conversion of violaxanthin to ZEA. Thus Anoectochilus modifies its system to dissipate excess excitation energy and to protect the photosynthetic machinery.     

Photosystem 2 photochemistry and pigment composition of a yellow mutant of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) under different irradiances

A yellow leaf colouration mutant (named ycm) generated from rice T-DNA insertion lines was identified with less grana lamellae and low thylakoid membrane protein contents. At weak irradiance [50 μmol(photon) m⁻² s⁻¹], chlorophyll (Chl) contents of ycm were ≈20 % of those of WT and Chl a/b ratios were 3-fold that of wild type (WT). The leaf of ycm showed lower values in the actual photosystem 2 (PS2) efficiency (Φ_PS2), photochemical quenching (q_P), and the efficiency of excitation capture by open PS2 centres 1 (F_v′/F_m′) than those of WT, except no difference in the maximal efficiency of PS2 photochemistry (F_v/F_m). With progress in irradiance [100 and 200 μmol(photon) m⁻² s⁻¹], there was a change in the photosynthetic pigment stoichiometry. In ycm, the increase of total Chl contents and the decrease in Chl a/b ratio were observed. Φ_PS2, q_P, and F_v′/F_m′ of ycm increased gradually along with the increase of irradiance but still much less than in WT. The increase of xanthophyll ratio [(Z+A)/(V+A+Z)] associated with non-photochemical quenching (q_N) was found in ycm which suggested that ycm dissipated excess energy through the turnover of xanthophylls. No significant differences in pigment composition were observed in WT under various irradiances, except Chl a/b ratio that gradually decreased. Hence the ycm mutant developed much more tardily than WT, which was caused by low photon energy utilization independent of irradiance.     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Enhancement of susceptivity to photoinhibition and photooxidation in rice chlorophyll <Emphasis Type="Italic">b</Emphasis>-less mutants

Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 μmol m⁻² s⁻¹). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0–3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (F_v/F_m), quantum efficiency of PS2 electron transport (Φ_PS2), photochemical quenching (q_P), as well as rate of photochemistry (P_rate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (D_rate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78–83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55–0.73) and high 1–q_P (0.57–0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 μmol m⁻² s⁻¹) and high, HI (700 μmol m⁻² s⁻¹) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size.     

Diurnal and seasonal variation in photosynthesis of peach palms grown under subtropical conditions

The Amazonian peach palm (Bactris gasipaes Kunth) has been grown for heart-of-palm production under subtropical conditions. As we did not see any substantial study on its photosynthesis under Amazonian or subtropical conditions, we carried out an investigation on the diurnal and seasonal variations in photosynthesis of peach palms until the first heart-of-palm harvest, considering their relationship with key environmental factors. Spineless peach palms were grown in 80-L plastic pots, under irrigation. Gas exchange and chlorophyll fluorescence emission measurements were taken in late winter, mid spring, mid summer and early autumn, from 7:00 to 18:00 h, with an additional chlorophyll fluorescence measurement at 6:00 h. The highest net CO₂ assimilation (P _N), observed in mid summer, reached about 15 μmol m⁻² s⁻¹, which was about 20% higher than the maximum values found in autumn and spring, and 60% higher than that in winter The same pattern of diurnal course for P _N was observed in all seasons, showing higher values from 8:00 to 9:00 h and declining gradually from 11:00 h toward late afternoon. The diurnal course of stomatal conductance (g _s) followed the same pattern of P _N, with the highest value of 0.6 mol m⁻² s⁻¹ being observed in February and the lowest one (0.23 mol m⁻² s⁻¹) in September. The maximal quantum yield of photosystem II (F_v/F_m) was above 0.75 in the early morning in all the months. The reversible decrease was observed around midday in September and October, suggesting the occurrence of dynamic photoinhibition. A significant negative correlation between the leaf-air vapour pressure difference (VPD_leaf-air) and P _N and a positive correlation between P _N and g _s were observed. The photosynthesis of peach palm was likely modulated mainly by the stomatal control that was quite sensible to atmospheric environmental conditions. Under subtropical conditions, air temperature (T _air) and VPD_leaf-air impose more significant effects over P _N of peach palm than an excessive photosynthetic photon flux density (PPFD). The occurrence of dynamic photoinhibition indicates that under irrigation, peach palms appeared to be acclimated to the full-sunlight conditions under which they have been grown.     

Photosynthetic and leaf respiration activity of <Emphasis Type="Italic">Malcolmia littorea</Emphasis> (L.) R. Br. in response to air temperature

Plant traits of Malcolmia littorea growing at the Botanic Garden of Rome and transplanted from the wild population developing along the Latium coast (Italy) were analyzed. The highest photosynthetic rates [P _N, 22.5 ± 0.5 μmol(CO₂) m⁻² s⁻¹], associated to the highest chlorophyll content (Chl, 60 ± 5 SPAD units), and respiration rates [R, 11.1 ± 0.2 μmol(CO₂) m⁻² s⁻¹] were reached in spring, when mean air temperature (T _m) was in the range 17°C to 23°C. P _N, Chl, and R decreased by 86, 38, and 59% in summer when mean maximum air temperature (T _max) was 30.3 ± 2.6°C. Leaf water potential decreased by 34% in summer compared to the spring value, and it was associated to a relative water content (RWC) of 74 ± 4%, and to a water-use efficiency (WUE) of 2.15 ± 0.81 μmol(CO₂) mmol⁻¹(H₂O). Moreover, also low air temperatures determined a significant P _N and R decreases (by 52 and 40% compared to the maximum, respectively). Responsiveness of gross photosynthetic rate (P _g) to R was higher than that to P _N as underlined by the slope of the regression line between the two variables. The results underlined a low tolerance to both high- and low air temperatures of M. littorea. The selected key traits (R, WUE, Chl) by the discriminant analysis might be used to monitor the M. littorea wild population in the long time. The ex situ cultivated plants could be propagated and used to increase the individuals number of the wild population.     

Chill-induced inhibition of photosynthesis was alleviated by 24-epibrassinolide pretreatment in cucumber during chilling and subsequent recovery

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8°C and 100 μmol m⁻² s⁻¹ PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18°C and 600 μmol m⁻² s⁻¹ PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P _N) and stomatal conductance (g _s), and increased rate of O₂ ^·− formation and H₂O₂ and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F_v/F_m). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ_PSII) and photochemical quenching (q_P), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m⁻² s⁻¹) further aggravated the decrease in P _N, g _s, Φ_PSII and q_P, and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F_v/F_m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g _s, Φ_PSII, q_P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period.     

Photosynthesis of lichen symbiotic alga Trebouxia erici as affected by irradiance and osmotic stress

The relation between oxygen evolution rate (OER) and quantum yield of photochemical reactions in photosystem 2 (Φ_PS2) was examined in lichen symbiotic alga Trebouxia erici Ahmadjian (strain UTEX 911) exposed to different irradiances and osmotic stress (2 M sucrose for 60 h). Linear relationship was found between OER and Φ_PS2 in control cell suspension within irradiance range of 0 – 500 μmol m⁻² s⁻¹. Under osmotic stress, OER and Φ_PS2 were significantly reduced. Relation between OER and Φ_PS2 was curvilinear due to strong osmotically-induced inhibition of OER at high irradiance. The highest used irradiance (500 μmol m⁻² s⁻¹) was photoinhibitory for osmotically-stressed T. erici because non-photochemical quenching (NPQ) increased substantially. Energy-dependent quenching represented major part of NPQ increase. Osmotic stress led also to the reduction of capacity of photochemical processes in PS 2 (F_V/F_M) and increase in F₀/F_M. These changes indicated negative effects of osmoticum on structure and function of photosynthetic apparatus.     

Stomatal and non-stomatal limitations to photosynthesis in field-grown grapevine cultivars

Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera × V. labrusca) cultivars Campbell Early and Kyoho grown in the field were compared with respect to gas exchanges and actual quantum yield of photosystem 2 (Φ_PS2) in late May. Net photosynthetic rate (P_N) of the two cultivars rapidly increased in the morning, saturated at photosynthetic photon flux density (PPFD) from 1200 to 1500 μmol m⁻² s⁻¹ between 10:00 and 12:00 and slowly decreased after midday. Maximum P_N was 13.7 and 12.5 μmol m⁻² s⁻¹ in Campbell Early and Kyoho, respectively. The stomatal conductance (g_s) and transpiration rate changed in parallel with P_N, indicating that P_N was greatly affected by g_s. However, the decrease in P_N after midday under saturating PPFD was also associated with the observed depression of Φ_PS2 at high PPFD. The substantial increase in the leaf to air vapour pressure deficit after midday might also contribute to decline of g_s and P_N.     

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F _P >10 μmol m⁻² s⁻¹) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F _P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO₂ fluxes were ca. −1.75 and −0.60 μmol m⁻² s⁻¹, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m⁻² s⁻¹, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO₂ release of ca. 0.60 and 0.20 μmol m⁻² s⁻¹, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F _P, the model applied here failed to describe daytime carbon exchange only as a function of F _P and could not be used over most of the year to fill gaps in the data.     

Photosynthesis performance in sweet almond [<Emphasis Type="Italic">Prunus dulcis</Emphasis> (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth’s surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-B_c), 4.42 (UV-B₁), 7.32 (UV-B₂) and 9.36 (UV-B₃) kJ m⁻² d⁻¹. Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F₀) and F₀/F_m, and a decrease in variable fluorescence (F_v, F_v/F_m, F_v/F₀ and F₀/F_m) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (F_m). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P _N) at UV-B₂ and UV-B₃ levels and this was accompanied by a reduction in stomatal conductance (g _s) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m⁻² d⁻¹. The most sensitive indicators for UV-B stress were F_v/F₀, Chl a content and P _N. Significant P<0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.     

Chlorophyll fluorescence performance of sweet almond [Prunus dulcis (Miller) D. Webb] in response to salinity stress induced by NaCl

One-year old sweet almond (Prunus dulcis) seedlings were submitted to four levels of salt stress induced by NaCl, namely 0.3, 0.5, 0.7, and 1.0 S m⁻¹. Effects of salt stress on a range of chlorophyll (Chl) fluorescence parameters (Chl FPs) and Chl contents were investigated in order to establish an eco-physiological characterization of P. dulcis to salinity. Salt stress promoted an increase in F₀, F_s, and F₀/F_m and a decrease in F_m, F′_m, F_v/F_m, q_P, ΔF/F′_m, F_v/F₀, and UQF_(rel), in almost all Chl fluorescence yields (FY) and FPs due to its adverse effect on activity of photosystem 2. No significant changes were observed for quenchings q_N, NPQ, and q_N(rel). The contents of Chl a and b and their ratio were also significantly reduced at increased salt stress. In general, adverse salinity effects became significant when the electric conductivity of the nutrient solution (EC_n) exceeded 0.3 S m⁻¹. The most sensitive salt stress indicators were F_v/F₀ and Chl a content, and they are thus best used for early salt detection in P. dulcis. Monitoring of a simple Chl FY, such as F₀, also gave a good indication of induced salt stress due to the significant correlations observed between the different Chl FYs and FPs. Even essential Chl FYs, like F₀, F_m, F′_m, and F_s, and mutually independent Chl FPs, like F_v/F₀ and q_P, were strongly correlated with each other.     

Effects of water stress on photosynthetic activity,dry mass partitioning and some associated metabolic changes in four provenances of neem (<Emphasis Type="Italic">Azadirachta indica</Emphasis> A. Juss)

Chlorophyll (Chl) content, dry mass, relative water content (RWC), leaf mass per area (LMA), proline (Pro) content, malondialdehyde (MDA) content, superoxide dismutase (SOD) and peroxidase (POD) activity, P _N-PAR response curves and gas exchange were studied to determine the effects of water stress on photosynthetic activity, dry mass partitioning and metabolic changes in four provenances of neem (Azadirachta indica A. Juss). The results indicated that provenance differences existed in the adaptation response to water stress that included changes to growth strategies coupled with ecophysiological and metabolic adjustments. As water stress increased, stomatal conductance (g _s), net photosynthetic rate (P _N), transpiration rate (E), and leaf RWC decreased while LMA increased in all provenances. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots, and enzyme activities of SOD and POD were highest in neem originating from Kalyani (KA) provenance and lowest in neem originating from New Dehli (ND) provenance. In contrast, water stress increased MDA content least in KA and most in ND. Furthermore, neem originating from ND also had the greatest decrease in Chl a/b ratio while the ratio was least affected in neem originating from KA. These findings suggest neem originating from KA may have more drought resistance than neem originating from ND. The data from P _N-PAR response curves are less clear. While these curves showed that drought stress increased compensation irradiance (I _c) and dark respiration (R _D) and decreased saturation irradiance (I _s) and maximum net photosynthetic rate (P _max), the extent of decline in P _max was provenance dependent. P _max under non-waterlimiting conditions was higher in neem originating from Jodhpur (MA) (about 14 μmol m⁻² s⁻¹) than in the other three provenances (all about 10 μmol m⁻² s⁻¹), but mild water stress had minimal effect on P _max of these three provenances whereas P _max of MA provenance declined to 10 μmol m⁻² s⁻¹, i.e. a similar value. However, under severe water stress P _max of MA and KA provenances had declined to 40% of non-stressed values (about 6 and 4 μmol m⁻² s⁻¹, respectively) whereas the decline in P _max of neem originating from Kulapachta (KU) and ND provenances was about 50% of nonstressed values (about 5 μmol m⁻² s⁻¹). These data suggest the P _N responses of KU and ND provenances are most tolerant, and KA and MA least tolerant to increasing water stress, but also suggest MA provenance could be the most desired under both non-water-limiting and water-limiting conditions due to highest P _max in all conditions.     

Chilling stress and chilling tolerance of sweet potato as sensed by chlorophyll fluorescence

We studied changes in the chlorophyll (Chl) fluorescence components in chilling-stressed sweet potato (Ipomoea batatas L. Lam) cv. Tainung 57 (TN57, chilling-tolerant) and cv. Tainung 66 (TN66, chilling-susceptible). Plants under 12-h photoperiod and 400 μmol m⁻² s⁻¹ irradiance at 24/20 °C (day/night) were treated by a 5-d chilling period at 7/7 °C. Compared to TN66, TN57 exhibited a significantly greater basic Chl fluorescence (F₀), maximum fluorescence (F_m), maximum fluorescence yield during actinic irradiation (F_m′ ), and the quantum efficiency of electron transport through photosystem 2, PS2 (Φ_PS2). Chilling stress resulted in decrease in the potential efficiency of PS2 (F_v/F_m), Φ_PS2, non-photochemical fluorescence quenching (NPQ), non-photochemical quenching (q_N), and the occurrence of chilling injury in TN66. Chilling increased the likelihood of photoinhibition, characterized by a decline in the Chl fluorescence of both cultivars, and photoinhibition during low temperature stress generally occurred more rapidly in TN66.     

Photosystem 2 activity of <Emphasis Type="Italic">Citrus volkameriana</Emphasis> (L.) leaves as affected by Mn nutrition and irradiance

Citrus volkameriana (L.) plants were grown for 43 d in nutrient solutions containing 0, 2, 14, 98, or 686 μM Mn (Mn₀, Mn₂, Mn₁₄, Mn₉₈, and Mn₆₈₆, respectively). To adequately investigate the combined effects of Mn nutrition and irradiance on photosystem 2 (PS2) activity, irradiance response curves for electron transport rate (ETR), nonphotochemical quenching (q_N), photochemical quenching (q_P), and real photochemical efficiency of PS2 (Φ_PS2) were recorded under 10 different irradiances (66, 96, 136, 226, 336, 536, 811, 1 211, 1 911, and 3 111 μmol m⁻² s⁻¹, I₆₆ to I₃₁₁₁, respectively) generated with the PAM-2000 fluorometer. Leaf chlorophyll content was significantly lower under Mn excess (Mn₆₈₆) compared to Mn₀; its highest values were recorded in the treatments Mn₂-Mn₉₈. However, ETR and Φ_PS2 values were significantly lower under Mn₀ compared to the other Mn treatments, when plants were exposed to irradiances ≥96 μmol m⁻² s⁻¹. Furthermore, Mn₀ plants had significantly higher values of q_N and lower values of q_P at irradiances ≤226 and ≥336 μmol m⁻² s⁻¹, respectively, than those grown under Mn₂-Mn₆₈₆. Irrespective of Mn treatment, the values of Φ_PS2 and q_N decreased, while those of q_P increased progressively by increasing irradiance from I₁₃₆ to I₃₁₁₁. Finally, Mn₂-Mn₉₈ plants were less sensitive to photoinhibition of photosynthesis (≥811 μmol m⁻² s⁻¹) than the Mn₆₈₆ (≥536 μmol m⁻² s⁻¹) and Mn₀ (≥336 μmol m⁻² s⁻¹) ones.     

Inhibition of photosynthetic processes in foliose lichens induced by temperature and osmotic stress

Negative effects of osmotically-induced dehydration of two foliose lichen species, Lasallia pustulata and Umbilicaria hirsuta, was studied at physiological (22 °C), low (5 °C) and freezing temperature (−10 °C), using chlorophyll (Chl) fluorescence. In both species, exposure to increasing sucrose concentrations led to a pronounced decrease in potential (F_V/F_M), and actual (Φ₂) quantum yields of photochemical processes in photosystem 2. L. pustulata was more sensitive to osmotic stress, because comparable osmotic dehydration inhibited F_V/F_M and Φ₂ more than in U. hirsuta. Critical concentration of sucrose that fully inhibited photochemical processes of photosynthesis was 2.5 M, which represented water potential (Ψ_w) of −18.8 MPa. Decrease in background Chl fluorescence (F₀) and increase in non-photochemical quenching (qN) revealed two phases of osmotic stress in lichens: phase I with no change (Ψ_w 0 to −6.6 MPa) and phase II (Ψ_w −11.3 to −18.8 MPa) typical by substantial change in Chl fluorescence parameters. Effects of thallus anatomy on species-specific response to osmotic dehydration is discussed and attributed to the results obtained by optical microscopy and Chl fluorescence imaging technique.     

Growth and physiological changes in saplings of Minquartia guianensis and Swietenia macrophylla during acclimation to full sunlight

Low light availability under a forest canopy often limits plant growth; however, sudden increase in light intensity may induce photoinhibition of photosynthesis. The aim of this study was to evaluate the ecophysiological changes that occur in potted plants of Minquartia guianensis and Swietenia macrophylla during the acclimation process to full sunlight. We used six full-sun independent acclimation periods (30, 60, 90, 120, 150, and 180 days) and a control kept in the shade. Shading was obtained by placing plants under the canopy of a small forest. The F_v/F_m ratio, net photosynthetic rate (P _N), the maximum carboxylation velocity of Rubisco (V _cmax), maximum electron transport rate (J _max), specific leaf area (SLA), and growth were assessed at the end of each of the six acclimation periods. Plant exposure to full sunlight caused a sudden decrease in the F_v/F_m ratio (photoinhibition) particularly in Minquartia. Photooxidation (necrotic patches) of the leaf tissue was observed in upper leaves of Minquartia. The higher P _N values were observed in Swietenia under full sun, about 12 μmol(CO₂) m⁻² s⁻¹. V _cmax25 values were higher after 90 days of acclimation, about 14 μmol(CO₂) m⁻² s⁻¹ for Minquartia, and 35 μmol(CO₂) m⁻² s⁻¹ for Swietenia. At the end of a 180-d acclimation period J _max25 was 35 μmol(electron) m⁻² s⁻¹ for Minquartia and 60 μmol(electron) m⁻² s⁻¹ for Swietenia. SLA was higher in Swietenia than in Minquartia. In Minquartia, monthly rate of leaf production per plant (MRLP) was positive (0.22 leaf month⁻¹) after four months in the open. Whereas, in Swietenia MRLP was positive (0.56 leaf month⁻¹) after an acclimation period of two months. After six months in the open, height growth rates were 3.5 and 28 mm month⁻¹ for Minquartia and Swietenia, respectively. The greater acclimation capacity of Swietenia was associated to an enhanced photosynthetic plasticity under full sun. In Minquartia, transition to full-sun conditions and lack of physiological adjustment resulted in severe photoinhibition and loss of leaves.     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence responses of <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> seedlings subjected to progressive saline stress in the Tarim desert highway ecological shelterbelt

In order to assess the long-term impacts of saline groundwater irrigation to Haloxylon ammodendron, one of the main shrubs in the Tarim desert highway ecological shelterbelt, we irrigated the H. ammodendron seedlings with progressive saline groundwater (3–30 g L⁻¹, simulation environment in the Tarim desert highway ecological shelterbelt) and investigated the diurnal variations of chlorophyll a (Chl a) fluorescence parameters, such as maximal quantum yield of photosystem II (PSII) photochemistry (F_v/F_m), quantum yield of photochemical energy conversion in PSII (Y_II), the apparent rate of electron transport at the PSII level (ETR), photochemical quenching coefficient (q_P), non-photochemical quenching (NPQ), quantum yield of nonregulated non-photochemical energy loss in PSII (Y_NO) and quantum yield of regulated non-photochemical energy loss in PSII (Y_II), at approximately 2-h intervals. F_v/F_m with 5 g L⁻¹ (S2) was lower than that with 2 g L⁻¹ (S1) but a little higher than 20 g L⁻¹ (S5), respectively. Under the low light [photosyntheticallyactive radiation (PAR) ≤ 250 μmol m⁻² s⁻¹, at 08:00, 10:00 and 20:00 h of the local time], S1 kept the lowest Y_II and the highest Y_NPQ; while under the high light (PAR ≥ 1500 μmol m⁻² s⁻¹), the Y_II performed S1>S2>S5, and the reverse Y_NPQ; under mild light (250 μmol m^t-2 s⁻¹ ≤ PAR ≤ 1500 μmol m⁻² s⁻¹), S1 remained the highest Y_II, no matter the light and the salinity, the similar Y_NO almost occurred basically. The results showed that the sand-binding plant H. ammodendron could regulate its energy-utilizing strategies. The S2 might be the most suitable salinity of the irrigation water for H. ammodendron in the Tarim desert highway ecological shelterbelt in the northwest of China.     

Photosynthetic parameters of young Brazil nut (<Emphasis Type="Italic">Bertholletia excelsa</Emphasis> H. B.) plants subjected to fertilization in a degraded area in Central Amazonia

In an experimental site for reforestation of degraded area, three-year-old plants of Bertholletia excelsa Humb. & Bonpl. were subjected to different fertilization treatments: T0 = unfertilized control, T1 = green fertilization (branches and leaves) and T2 = chemical fertilization. Higher net photosynthetic rates (P _N) were observed in T1 [13.2±1.0 μmol(CO₂) m⁻² s⁻¹] compared to T2 [8.0±1.8 μmol(CO₂) m⁻² s⁻¹] and T0 [4.8±1.3 μmol(CO₂) m⁻² s⁻¹]. Stomatal conductance (g _s), transpiration rate (E) and water use efficiency (WUE) of individuals of T1 and T2 did not differ significantly, however, they were by 88, 55 and 63%, respectively, higher in T1 than in the control. The mean values of variable fluorescence (F_v), performance index (P.I.) and total chlorophyll [Chl (a+b)] were higher in T1. Our results indicate that green fertilization improves photosynthetic structure and function in plants of B. excelsa in young phase.     

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F _v/F _m), maximum relative electron transport rate (rETR_max), photosynthetic efficiency (α), and the photoadaptive index (E _k). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0–5 cm from ice–water interface) expressed low F _v/F _m (0.331–0.426) and low α (0.098–0.130 (μmol photons m⁻²s⁻¹)⁻¹) in December. F _v/F _m and α increased in March and May (0.468–0.588 and 0.141–0.438 (μmol photons m⁻²s⁻¹)⁻¹, respectively) indicating increased photosynthetic activity. In addition, increases in rETR_max (3.3–16.4 a.u.) and E _k (20–88 μmol photons m⁻² s⁻¹) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).