Wheat cultivars selected for high Fv/Fm under heat stress maintain high photosynthesis,total chlorophyll,stomatal conductance,transpiration and dry matter

The chlorophyll fluorescence parameter F_v/F_m reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three‐tiered approach of phenotyping by F_v/F_m to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on F_v/F_m value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North‐Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to F_v/F_m. The high F_v/F_m group maintained significantly higher total chlorophyll and net photosynthetic rate (P_N) than the low group, accompanied by higher stomatal conductance (g_s), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in P_N between the groups was not caused by differences in PSII capacity or g_s as the variation in F_v/F_m and intracellular CO₂ (C_i) was non‐significant under the given heat stress. This study validated that our three‐tiered approach of phenotyping by F_v/F_m performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.     

Photosynthetic characteristics and UV stress tolerance of Antarctic seaweeds along the depth gradient

The photosynthetic characteristics through P-E curves and the effect of UV radiation on photosynthesis (measured as rapid adjustment of photochemistry, F _v/F _m) and DNA damage (as formation of CPDs) were studied in field specimens of green, red and brown algae collected from the eulittoral and sublittoral zone of Fildes Peninsula (King George Island, Antarctic). The content of phenolic compounds (phlorotannins) and the antioxidant activity were also studied in seven brown algae from 0 to 40 m depth. The results indicated that photosynthetic efficiency (α) was high and did not vary between different species and depths, while irradiances for saturation (E _k) averaged 55 μmol m^?2 s^?1 in subtidal and 120 μmol m^?2 s^?1 in eulittoral species. The studied species exhibited notable short-term UV tolerance along the vertical zonation. In intertidal and shallow water species, decreases in F _v/F _m by UV radiation were between 0 and 18 %, while in sublittoral algae, decreases in F _v/F _m varied between 3 and 35 % relative to PAR treatment. In all species, recovery was high averaging 84–100 %. The formation of CPDs increased (15–150 %) under UV exposure, with the highest DNA damage found in some subtidal species. Phlorotannin content varied between 29 mg g^?1 DW in Ascoseira mirabilis from 8 m depth and 156 mg g^?1 DW in Desmarestia menziesii from 17 m depth. In general, phlorotannin concentrations were constitutively high in deeper sublittoral brown algae, which were correlated with higher antioxidant activities of algal extracts and low decreases in photosynthesis. UV radiation caused a strong decrease in phlorotannin content in the deep-water Himantothallus grandifolius, whereas in D. menziesii and Desmarestia anceps, induction of the synthesis of phlorotannins by UV radiation was observed. The antioxidant activity was in general less affected by UV radiation.     

Rehydration of Sugar Beet Plants after Water Stress: Effect of Cytokinins

The possibility to improve the recovery of sugar beet plants after water stress by application of synthetic cytokinins N⁶-benzyladenine (BA) or N⁶-(m-hydroxybenzyl)adenosine (HBA) was tested. Relative water content (RWC), net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), chlorophyll (Chl) a and Chl b contents, and photosystem 2 efficiency characterized by variable to maximal fluorescence ratio (F_v/F_m) were measured in control plants, in water-stressed plants, and after rehydration (4, 8, 24, and 48 h). Water stress markedly decreased parameters of gas exchange, but they started to recover soon after irrigation. Application of BA or HBA to the substrate or sprayed on leaves only slightly stimulated recovery of P_N, E, and g_s in rehydrated plants, especially during the first phases of recovery. Chl contents decreased only under severe water stress and F_v/F_m ratio was not significantly affected by water stress applied. Positive effects of BA or HBA application on Chl content and F_v/F_m ratio were mostly not observed.     

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.     

Nitrogen-induced variations in leaf gas exchange of spring triticale under field conditions

Nitrogen (N) is the key factor limiting photosynthetic processes and crop yield. Little is known about the response of leaf gas exchange of spring triticale (Triticosecale Wittm.) to N supply. The effect of N fertilizers on different gas exchange variables, i.e., photosynthetic rate (A), transpiration rate (E), stomatal conductance (g _s), instantaneous water use efficiency (WUE) and maximum quantum yield of photosystem II (PSII) (F _v/F _m), chlorophyll index (SPAD, soil–plant analysis development), and the relationship of these variables with yield were studied in spring triticale grown under field conditions. Six treatments of N—0, 90, 180, 90 + 30, 90 + 30 + 30 kg ha^?1 (applied as ammonium nitrate, AN) and one treatment of N 90 + 30 + 30 kg ha^?1 (applied as urea ammonium nitrate solution, UAN) were compared. The analysis of variance showed that throughout the triticale growing season, N fertilization had significant effects on A, WUE, g _s and SPAD. On average, N fertilizer application increased A values by 14–70%. E and F _v/F _m values were not influenced by N fertilization levels. The effect of growth stage and year on gas exchange variables and F _v/F _m and SPAD was found to be significant. At different growth stages, A values varied and maximum ones were reached at BBCH 31–33 (decimal code system of growth stages) and BBCH 59. With aging, values of A decreased independently of N fertilization level. The gas exchange variables were equally affected by both fertilizer forms. The interplay among grain yield, leaf gas exchange variables, F _v/F _m and SPAD of spring triticale was estimated. The statistical analysis showed that grain yield positively and significantly correlated with A and SPAD values throughout the growing season.     

Physiological responses of phytoplankton communities in the Irish Sea to simulated upwelling

Understanding the dynamics of upwelling systems, especially the interactions between nutrients and light, has benefited from the application of models of varying complexity. Validation of such models using unialgal cultures or field observations has often proven difficult, but short-term incubations of contained natural assemblages and use of instantaneous physiological indicators offer an alternative approach. In May and June 1996, phytoplankton communities deep in the euphotic zone were sampled from nearly identical physical environments. Replicate samples (20 l volume) were incubated on deck at 50% surface irradiance with either no nutrient additions (Controls) or additions of 20 μM nitrate (Enrichments). Over 24 h, variable fluorescence (F _v:F _m), nitrate reductase activity (NR), nutrients, chlorophyll a and particulate C and N were monitored. Initial chlorophyll a (～3 μg l^?1), phosphate (～0.2 μM), nitrate (～1.5 μM) and silicate (～3 μM) were similar in both months. Changes in NR and F _v:F _m indicated clear physiological responses to changes in irradiance and added nitrate that differed between months. In May, Controls and Enrichments responded in the same way. F _v:F _m stayed constant (0.5), chlorophyll a increased slightly, and NR activity increased markedly in all samples. In contrast, in June, treatments responded quite differently. F _v:F _m was near the theoretical maximum (0.7–0.8) initially and remained constant in Enrichments, but fell sharply in Controls. Declines in controls were also seen for chlorophyll a, and NR activity. Thus, the addition of 20 μM nitrate had a significant effect even though ambient levels of nitrate (>1 μM) should not have been limiting. Small (<20 μm) flagellates predominated in the May samples, but in June large and chain-forming centric diatoms constituted a significant proportion of the phytoplankton community. We conclude that the response of a phytoplankton community to environmental changes can depend on factors that are poorly represented by bulk measurements of chlorophyll, nutrients and particulate elements.     

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.     

Ozone sensitivity of four Pakchoi cultivars with different leaf colors: physiological and biochemical mechanisms

Ozone (O₃) is important air pollutant inducing severe losses of horticultural production. Cultivars of the same species, but with different leaf colors, may differ in their ozone sensitivity. However, it has not been clarified yet if different leaf coloration influences such a sensitivity. In this study, two purple-leafed and two green-leafed cultivars of Pakchoi were selected for ozone fumigation (240 ± 20 nmol mol^–1, 09:00–16:00 h). Elevated O₃ decreased chlorophyll content, increased anthocyanin (Ant) content, damaged cell membrane integrity, enhanced antioxidative enzyme activities, depressed photosynthetic rate (P _N) and stomatal conductance (g _s), inhibited maximal quantum yield (F_v/F_m) and effective quantum yield [Y_II] of PSII photochemistry, and caused visible injury. Purple-leafed cultivars with higher Ant contents were more tolerant than green-leafed cultivars as indicated by lower relative enhancement in malondialdehyde content and lower relative losses in P _N, g _s, F_v/F_m, and Y_II. The higher ability to synthesize Ant in the purple-leafed cultivars contributed to their higher photoprotective ability.     

Different responses of photosystem I and photosystem II in three tropical oilseed crops exposed to chilling stress and subsequent recovery

Key message

Different responses of photosystem I and II to chilling.

Abstract

Tropical crops are sensitive to chilling stress, but the underlying physiological mechanisms are unclear. We investigated the maximum quantum yield of PSII (F _v/F _m), the maximum photo-oxidizable P700 (P _m), the energy distribution in PSII, and the redox state of P700 in leaves of seedlings of three promising oilseed crops originating from tropical regions, Plukenetia volubilis, Jatropha curcas and Ricinus communis, during chilling treatment and subsequent recovery under a photon flux density of 450 μmol m^?2 s^?1. Our results showed that F _v/F _m decreased progressively and significantly to about 44.7, 62.2 and 77.0 % of the control after chilling treatment for 3 days in P. volubilis, J. curcas and R. communis, respectively, mainly due to the decrease in F _m (maximum fluorescence of PSII). After recovery under 18 °C for 6 days, F _v/F _m recovered to 81.4 and 94.9 % of the control in J. curcas and R. communis, but only to 26.3 % in P. volubilis. Under chilling stress and subsequent recovery, P _m remained stable in J. curcas and R. communis, whereas it decreased slightly in P. volubilis. These results indicated that PSII was more sensitive to chilling stress than PSI under moderate light for all three species, and that P. volubilis was the most susceptible. Cyclic electron flow around PSI and effective quantum yield of photosystem II [Y _(CEF)/Y _(II)] ratio were stimulated much more in J. curcas and R. communis compared with that in P. volubilis under chilling conditions, resulting in more severe injury as indicated by higher accumulation of hydrogen peroxide and malondialdehyde. There was a significantly negative relationship between F _v/F _m and Y _(CEF)/Y _(II), suggesting that stimulation of Y _(CEF)/Y _(II) plays a pivotal role in protecting PSI and PSII from photoinhibition caused by chilling stress.     

Interactions of Elevated CO2 Concentration and Drought Stress on Photosynthesis in Eucalyptus Cladocalyx F. Muell

Response of net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (c _i), and photosynthetic efficiency (F_v/F_m) of photosystem 2 (PS2) was assessed in Eucalyptus cladocalyx grown for long duration at 800 (C₈₀₀) or 380 (C₃₈₀) µmol mol^-1 CO₂ concentration under sufficient water supply or under water stress. The well-watered plants at C₈₀₀ showed a 2.2 fold enhancement of P _N without any change in g _s. Under both C₈₀₀ and C₃₈₀, water stress decreased P _N and g _s significantly without any substantial reduction of c _i, suggesting that both stomatal and non-stomatal factors regulated P _N. However, the photosynthetic efficiency of PS2 was not altered.     

Photosynthetic capacity of the capsule wall and its contribution to carbon fixation and seed yield in castor (<Emphasis Type="Italic">Ricinus communis</Emphasis> L.)

Defoliation occurs in castor due to several reasons, but the crop has propensity to compensate for the seed yield. Photosynthetic efficiency in terms of functional (gas exchange and chlorophyll fluorescence) and structural characteristics (photosynthetic pigment profiles and anatomical properties) of castor capsule walls under light- and dark-adapted conditions was compared with that of leaves. Capsule wall showed high intrinsic efficiency of photosystem II (F _v/F _m, 0.82) which was comparable to leaves (F _v/F _m, 0.80). With increasing photon flux densities (PFD), actual quantum yields and photochemical quenching coefficients of the capsule walls were similar to that in leaves, while electron transport rates reached a maximum corresponding to about 118 % of the leaves. However, maximum net photosynthetic rate of the capsule walls (2.60 µmol CO₂ m^?2 s^?1) was less than one-fourth of the leaves (15.67 µmol CO₂ m^?2 s^?1) at the CO₂ concentration of 400 µmol mol^?1, and the difference was attributed to about 80 % lower stomatal density and the 75 % lower total chlorophyll content of capsule walls than the leaves. Furthermore, seed weight in dark-adapted capsules was 2.70–12.42 % less as compared to the capsules developed under light. The results indicate that castor capsule walls are photosynthetically active (about 15–30 % of the leaves) and contribute significantly to carbon fixation and seed yield accounting for 10 % photoassimilates towards seed weight.     

Photoacclimation characteristics of Sargassum thunbergii germlings under different light intensities

The photosynthesis and growth responses of Sargassum thunbergii germlings to different light intensities (10, 60, and 300 μmol photons m^?2 s^?1) were investigated. Maximum photochemical efficiency (F _v/F _m), rapid light curves (RLCs), and photochemical and non-photochemical quenching (qP and NPQ) were estimated by a pulse amplitude-modulated fluorometer. The photosynthesis of S. thunbergii germlings exhibited different properties to optimize light capture and utilization. The excitation pressure (1???qP) was rapidly increased to approximately 0.27 showing that germlings responded to high light by chronic photoinhibition with an accumulation of closed reaction centers, which ultimately resulted in a slow growth. This was accompanied by a reduced F _v/F _m with time and a development of high capacity for NPQ. Although F _v/F _m in moderate-light germlings did not fully recover overnight, germlings demonstrated a less severe chronic photoinhibition considering the reduced degree of excitation pressure accumulation of approximately 0.15. The relative stability of photosynthetic capacity (rETR_max, E _k, and α) could endow germlings with the highest relative growth rate (RGR) of approximately 9.3 % day^?1 in moderate light. By contrast, low-light germlings demonstrated high F _v/F _m and F _o, corresponding high α collectively suggested greater efficiency of light absorption and energy transformation. Sustained increases in electron transport capacity (rETR_max and E _k) occurred in low-light germlings, which resulted in a stable RGR of over 8.2 % day^?1. Consequently, S. thunbergii germlings are considered to prefer low light regimes and have a relative capacity of moderate and high light tolerance. However, the light acclimation to oversaturating conditions is at the cost of slow growth to maintain survival.     

Effect of day/night temperature difference on chlorophyll content,photosynthesis and fluorescence parameters of tomato at fruit stage

In order to investigate the effect of day/night temperature difference (DIF) on photosynthetic characteristics of tomato plants (Solanum lycopersicum, cv. Jinguan 5) at fruit stage, an experiment was carried out in climate chambers. Five day/night temperature regimes (16/34, 19/31, 25/25, 31/19, and 34/16°C) with respective DIFs of -18, -12, 0, +12, and +18 were used and measured at mean daily temperature of 25°C. The results showed that chlorophyll (Chl) a, Chl b, net photosynthetic rate (P_N), stomatal conductance (g_s), maximum quantum yield of PSII photochemistry (F_v/F_m), effective quantum yield of PSII photochemistry (?_PSII), and photochemical quenching (qp) significantly increased under positive DIF, while they decreased with negative DIF. In contrast, the Chl a/b ratio and nonphotochemical quenching (NPQ) decreased under positive DIF, while increased with negative DIF. Chl a, Chl b, P_N, g_s, F_v/F_m, ?_PSII, and qp were larger under +12 DIF than those at +18 DIF, while Chl a/b and NPQ showed an opposite trend.     

Effects of 24-epibrassinolide and the synthetic brassinosteroid mimic on chili pepper under drought

Drought is major stress that severely reduces plant growth and productivity. To improve drought tolerance, an exogenous brassinosteroids (BRs) has been used effectively in the field condition. However, the application of BRs is expensive due to the scarcity of natural BRs and the multistep synthesis of BRs. In an attempt to reduce the cost, 7,8-dihydro-8α-20-hydroxyecdysone (DHECD) has been proposed to function as an imitation of 24-epibrassinolide (EBR). In this study, chili pepper plants (Capsicum annuum L. var. frutescens (L.) Kuntze) were sprayed with DHECD, EBR at 1 µM or distilled water (control). Plants were subjected to severe water stress (25% pot water capacity) for 5 days and their physiological effects and yield were investigated. The result showed that the applications of DHECD and EBR before the beginning of water stress could improve leaf water status determined by relative water content in plants grown under drought condition. The electrolyte leakage, lipid peroxidation level, and H₂O₂ production were significantly declined, while the accumulations of proline and total soluble sugar were increased in the treated plants. Moreover, the net photosynthesis (P_N) was elevated due to the increases of stomatal conductance (g_s) and intercellular CO₂ concentration (C_i) after BR pretreatments under drought. In addition, applications of DHECD and EBR maintained all chlorophyll fluorescence parameters; F_v/F_m, F_v′/F_m′, ΦPSII, qP, and ETR, to remain the photosynthesis. As a result, shoot biomass, fruit yield and capsaicin level were considerably enhanced in the treated plants. DHECD showed better performance to maintain membrane integrity; however, EBR had more effect on the osmotic maintenance. The result also showed that pretreatment with BRs had little or no effect on well-watered plants. The study concluded that DHECD and EBR alleviated the impact of drought on physiological responses and consequently minimized yield loss.     

Salinity improves chilling resistance in Suaeda salsa

Suaeda salsa L., a C3 euhalophytic herb, is native to saline soils, demonstrates high resistance to salinity stress. The effect of chilling stress on S. salsa under high salinity, particularly the change in unsaturated fatty acid content within membrane lipids, has not been investigated. After a 12 h chilling treatment (4 °C) performed under low irradiance (100 μmol m^?2 s^?1), the chlorophyll contents, maximal photochemical efficiency of photosystem II (F _v/F _m) and actual PSII efficiency (ΦPSII) were determined. These measurements were significantly decreased in S. salsa leaves in the absence of salt treatment yet there were no significant changes with a 200 mM NaCl treatment. Chlorophyll contents, F _v/F _m and ΦPSII in S. salsa under 200 mM NaCl were higher than those without salt treatment. The unsaturated fatty acid content and the double bond index (DBI) of major membrane lipids of monogalactosyldiacylglycerols, digalactosyldiacylglycerols (DGDG), sulphoquinovosyldiacylglycerols and phosphatidylglycerols (PG) significantly increased following the chilling treatment (4 °C) (with 12 h of low irradiance and 200 mM of NaCl). The DBI of DGDG and PG was decreased in the absence of the salt treatment. These results suggest that in the euhalophyte S. salsa, a 200 mM NaCl treatment increases chilling tolerance under conditions of low irradiance (100 μmol m^?2 s^?1).     

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

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

Crassulacean acid metabolism in the epiphytic fern <Emphasis Type="Italic">Patycerium bifurcatum</Emphasis>

The epiphytic fern Platycerium bifurcatum grows in different habitats characterized by drought and high irradiance stress. The plant shows diurnal malate oscillations, indicative for CAM expression only in cover leaves, but not in sporotrophophyll. In P. bifurcatum cover leaves exposed to high irradiance and desiccation, the decrease in both CO₂ assimilation (P _N) and stomatal conductance (g _s) was accompanied with occurrence of diurnal malate oscillations. Exogenously applied abscisic acid (ABA) induced the decrease in P _N and g _s, but no clear change in malate oscillations. The measurements of the maximum quantum efficiency of photosystem 2 (F_v/F_m) under high irradiance showed distinct photoinhibition, but no clear changes in F_v/F_m due to desiccation and ABA-treatment were found.     

The Photosynthesis and Chlorophyll a Fluorescence in Seedlings of Kandelia Candel (L.) Druce Grown under Different Nitrogen and NaCl Controls

Kandelia candel (L.) Druce is the dominant mangrove species on the west coast of northern Taiwan. We have measured the net photosynthetic rate (P _N) and chlorophyll (Chl) a fluorescence of seedlings grown at combinations of two nitrogen (0.01 and 0.1 mM) and two NaCl (250 and 430 mM NaCl) controls. With the same nitrogen level, seedlings grown at higher salinity (HS) had a significantly lower P _N and stomatal conductance (g _s) than those at lower salinity (LS). An increase in nitrogen availability significantly elevated P _N and g _s of the LS-grown seedlings. Compared to dark adapted leaves, the maximum quantum yield of photosystem 2 (PS2) (F_v/F_m) of leaves exposed to PFDs of 1200 and 1600 μmol m^-2 s^-1 for 2 h was significantly reduced. The degree of F_v/F_m reduction differed among leaves of the four types of treated plants. Chl fluorescence quenching analysis revealed differences among the examined plants in coefficients of non-photochemical and photochemical quenching. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of short-term heat exposure on physiological traits of indica rice at grain-filling stage

Heat stress impacts the quantity and quality of rice grains, particularly during grain-filling stage needed for grain development. In this study, the effect of short heat stress (42 °C, 30 min) on indica rice plants at the grain-filling stage (dough grain stage) was found by determining their physiological and growth traits F_v/F_m, ?F/F_m′, chlorophyll content, leaf water potential (LWP), membrane stability, relative leaf area (RLA), relative plant height (RPH), total grain weight per panicle (TGW) and 1000 GW. Thai economic rice cvs. KDML105 and Pathumthani 1 (PTT1) were compared to the heat-tolerant rice cultivars N22 and Dular and to the heat-sensitive rice cultivar IR64. The results showed that short heat stress exhibited effects on physiology and growth greater than the control (35 °C, 30 min) by reducing of F_v/F_m, ?F/F_m′, chlorophyll content, LWP, membrane stability and RLA. This result impacted the TGW and 1000 GW. A higher reduction of physiological traits was shown in IR64, followed by KDML105. In contrast, N22 and Dular were minimally affected by heat stress and were able to adapt and recover based on their grain weight that exhibited less of an effect. PTT1 was also impacted by heat stress similarly to Dular. Thus, short heat stress affected the physiological parameters of five rice cultivars at the dough grain stage. In addition, the five indica rice cultivars were classified into three groups: (1) the heat-tolerant group (N22, Dular and PTT1), (2) the moderately heat-tolerant group (KDML105), and (3) the heat-sensitive group (IR64) by PC-ORD program at 50% of similarity of the 13 physiological traits.     

Water relations, nutrient content and developmental responses of Euonymus plants irrigated with water of different degrees of salinity and quality

For 20 weeks, the physiological responses of Euonymus japonica plants to different irrigation sources were studied. Four irrigation treatments were applied at 100 % water holding capacity: control (electrical conductivity (EC) <0.9 dS m^?1); irrigation water normally used in the area (irrigator’s water) IW (EC: 1.7 dS m^?1); NaCl solution, NaCl (EC: 4 dS m^?1); and wastewater, WW (EC: 4 dS m^?1). This was followed by a recovery period of 13 weeks, when all the plants were rewatered with the same amount and quality of irrigation water as the control plants. Despite the differences in the chemical properties of the water used, the plants irrigated with NaCl and WW showed similar alterations in growth and size compared with the control even at the end of the recovery period. Leaf number was affected even when the EC of the irrigation water was of 1.7 dS m^?1 (IW), indicating the salt sensitivity of this parameter. Stomatal conductance (g_s) and photosynthesis (P_n), as well as stem water potential (Ψ_stem), were most affected in plants irrigated with the most saline waters (NaCl and WW). At the end of the experiment the above parameters recovered, while IW plants showed similar values to the control. The higher Na⁺ and Cl⁺ uptake by NaCl and WW plants led them to show osmotic adjustment throughout the experiment. The highest amount of boron found in WW plants did not affect root growth. Wastewater can be used as a water management strategy for ornamental plant production, as long as the water quality is not too saline, since the negative effect of salt on the aesthetic value of plants need to be taken into consideration.