Solute patterns and diurnal variation of photosynthesis and chlorophyll fluorescence in Korean coastal sand dune plants

Four plant species, Elymus mollis Trin., Carex kobomugi Ohwi, Glehnia littoralis F. Schmidt ex Miq., and Vitex rotundifolia L.f., are dominant perennial species in coastal sand dunes of Korea. We examined a physiological adaptation of these species by measurements of diurnal variation in photosynthesis and chlorophyll (Chl) fluorescence and solute patterns in leaves during one season (June), which is favorable for plant growth of all four species. All four species adopted different strategies in order to utilize radiation and to maintain water status under a fluctuating microclimate. Although the lowest water contents among four plant species was found, E. mollis with a high Chl and K⁺ content showed better photosynthetic performance, with high stomatal conductance (g _s), net photosynthetic rate (P _N), instantaneous carboxylation efficiency (CE), and water-use efficiency. Midday depression of P _N in E. mollis and G. littoralis, without a reduction of gs, was associated with a reduction in CE and maximum photochemical efficiency of PSII, indicating nonstomatal limitation. Photosynthesis depression in both C. kobomugi and V. rotundifolia, with relatively low g _s values, could be attributed to both stomatal and nonstomatal limitations. The high storage capacity for inorganic ions in E. molli, C. kobomugi, and G. littoralis may play an efficient role in regulating photosynthesis and maintaining leaf water status through stomatal control, and can also play an important role in osmotic adjustment.     

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.     

Gas exchanges of an endangered species <Emphasis Type="Italic">Syringa pinnatifolia</Emphasis> and a widespread congener <Emphasis Type="Italic">S. oblata</Emphasis>

Net photosynthetic rate (P_N), transpiration rate (E), water use efficiency (WUE), stomatal conductance (g_s), and stomatal limitation (L_s) were investigated in two Syringa species. The saturation irradiance (SI) was 400 µmol m^-2s^-1 for S. pinnatifolia and 1 700 µmol m^-2s^-1 for S. oblata. Compared with S. oblata, S. pinnatifolia had extremely low g _s . Unlike S. oblata, the maximal photosynthetic rate (P_max) in S. pinnatifoliaoccurred around 08:00 and then fell down, indicating this species was sensitive to higher temperature and high photosynthetic photon flux density. However, such phenomenon was interrupted by the leaf development rhythms before summer. A relatively lower P_N together with a lower leaf area and shoot growth showed the capacity for carbon assimilation was poorer in S. pinnatifolia.     

The effects of lead on photosynthetic performance of waxberry seedlings (<Emphasis Type="Italic">Myrica rubra</Emphasis>)

The photosynthesis was investigated 30 d after Pb treatment in Myrica rubra seedlings. The Pb treatment resulted in significantly increased Pb concentrations in shoots. Low Pb concentration exposure (≤2 mM) reduced the net photosynthetic rate (P_N), transpiration rate (E), and stomatal conductance (gs) without affecting the intercellular CO₂ concentration (C_i), chlorophyll (Chl) content, and Chl fluorescence parameters. At 10 d after severe Pb treatment (≥4 mM), P_N was inhibited and accompanied by Chl damage, while at 30 d, the inhibition of P_N was followed by an increase of C_i and a decrease of gs, E, Chl content, and Chl fluorescence parameters. M. rubra showed a promising prospect for use in the soil phytoremediation, when Pb concentration is low, but the remediation efficiency of M. rubra is limited if Pb exceeds 2 mM.     

A morphophysiological analysis of the effects of drought and shade on <Emphasis Type="Italic">Catalpa bungei</Emphasis> plantlets

The interactive effects of shade and drought on the morphological and physiological traits of Catalpa bungei plantlets were assessed. Seedling growth, biomass, biomass allocation, leaf morphology, chlorophyll (Chl) content and gas-exchange parameters were measured in plants raised for 3 months under three light levels [80% (HI), 50% (MI), 30% (LI)] and two water levels [moisture (M) and drought (D)]. The results showed that shade greatly decreased growth, biomass, leaf area (LA) and Chl a/b; increased specific leaf area (SLA) and Chl content; and reduced photosynthetic rate (P _n). Drought reduced the growth, biomass, LA, SLA, Chl a/b, P _n, stomatal conductance (G _s), transpiration rate (T _r) and intercellular carbon dioxide concentration (C _i) and increased the Chl content. Stomatal closure was an early physiological response to water stress. Light, water and their interaction significantly affected plant traits and their bivariate relationships. The phenotypic plasticity index of light (0.47) was much higher than that of water (0.21), indicating that light was the main driver of the variations observed. Under drought stress, growth, biomass, leaf and stem biomass allocation significantly decreased in the HI and MI environments, whereas no significant difference was observed in growth or biomass parameters under the LI condition. Furthermore, no significant difference was observed in P _n, G _s, or T _r under the LI condition under water stress. Our results showed that shade did not alter the negative effects caused by drought stress in MI but did alleviate the negative effects of the LI condition. In summary, the effect of drought on C. bungei plantlets depends on the irradiance conditions.     

Stomatal regulation in Douglas fir following a fungal-mediated chronic reduction in leaf area

Pathogens can cause chronic premature needle abscission in coniferous species. To assess the potential impacts on tree productivity, stomatal regulation was investigated in Douglas fir with chronic stomatal occlusion and defoliation from varying levels of the Swiss needle cast (SNC) fungus, Phaeocryptopus gaeumannii. Levels of SNC disease and subsequent defoliation were manipulated by choosing six sites with varying levels of disease and by foliar applications of fungicides on six trees per site. Diurnal measurements of leaf water potential (Ψ_leaf), stomatal conductance (g _s) and vapor pressure deficit (D) were made on six fungicide treated and six control trees per site. In addition, leaf specific hydraulic conductance was calculated on a single branch (K _{L_B}) from three trees per treatment per site. Stomatal conductance at D=1 kPa (g _sref) was negatively correlated with fungal colonization (number of fruiting bodies present in needle stomata) and positively correlated with K _{L_B}. Despite reduced needle retention in diseased trees, K _L declined due to a reduction in sapwood area and permeability (i.e., increasing presence of latewood in functional sapwood). In general, stomatal sensitivity to D for all foliage was consistent with stomatal regulation based on a simple hydraulic model [g _s=K _L(Ψ_soil?Ψ_leaf)/ D], which assumes strict stomatal regulation of Ψ_leaf. However, when fungal presence reduced maximum g _s below the potential maximum supported by hydraulic architecture, stomatal sensitivity was lower than expected based on the theoretical relationship: dg _s/dlnD=0.6·g _sref. The results indicate that losses in productivity associated with physical blockage of stomata and defoliation are compounded by additional losses in K _L and a reduction in g _s in remaining functional stomata.     

Assessment of photosynthetic potential of indoor plants under cold stress

Photosynthetic parameters including net photosynthetic rate (P_N), transpiration rate (E), water-use efficiency (WUE), and stomatal conductance (g_s) were studied in indoor C₃ plants Philodendron domesticum (Pd), Dracaena fragans (Df), Peperomia obtussifolia (Po), Chlorophytum comosum (Cc), and in a CAM plant, Sansevieria trifasciata (St), exposed to various low temperatures (0, 5, 10, 15, 20, and 25°C). All studied plants survived up to 0°C, but only St and Cc endured, while other plants wilted, when the temperature increased back to room temperature (25°C). The P_N declined rapidly with the decrease of temperature in all studied plants. St showed the maximum P_N of 11.9 μmol m^?2 s^?1 at 25°C followed by Cc, Po, Pd, and Df. E also followed a trend almost similar to that of P_N. St showed minimum E (0.1 mmol m^?2 s^?1) as compared to other studied C₃ plants at 25°C. The E decreased up to ≈4-fold at 5 and 0°C. Furthermore, a considerable decline in WUE was observed under cold stress in all C₃ plants, while St showed maximum WUE. Similarly, the g_s also declined gradually with the decrease in the temperature in all plants. Among C₃ plants, Pd and Po showed the maximum g_s of 0.07 mol m^?2 s^?1 at 25°C followed by Df and Cc. However, St showed the minimum g_s that further decreased up to ～4-fold at 0°C. In addition, the content of photosynthetic pigments [chlorophyll a, b, (a+b), and carotenoids] was varying in all studied plants at 0°C. Our findings clearly indicated the best photosynthetic potential of St compared to other studied plants. This species might be recommended for improving air quality in high-altitude closed environments.     

The Cooling Trend of Canopy Temperature During the Maturation,Succession, and Recovery of Ecosystems

The maximum exergy dissipation theory provides a theoretical basis for using surface temperature to measure the status and development of ecosystems, which could provide an early warning of rapid evaluation of ecosystem degradation. In the present study, we used the radiation balance of ecosystems to demonstrate this hypothesis theoretically. Further, we used empirical data to verify whether ecosystems gain more radiation, while lowering their surface temperatures, as they develop naturally. We analyzed 12 chronosequences from the FLUXNET database using meteorological data and heat fluxes. We included age, disturbance, and successional chronosequences across six climate zones. Net radiation (R _n) and the ratio of net radiation to global radiation (R _n/R _g) were used to measure the energy gain of the ecosystems. The maximum daily air temperature above the canopy (T _max) and thermal response number (TRN) were used to analyze the surface temperature trends with ecosystem natural development. The general trends of T _max, TRN, R _n, and R _n /R _g demonstrated that ecosystems become cooler and more stable, yet gain more energy, throughout their natural development. Among the four indicators, TRN showed the most consistent trends and highest sensitivity to ecosystem growth, succession, and recovery. Moreover, TRN was not significantly influenced by precipitation or wind. We propose that TRN can be used to rapidly evaluate or warn of ecosystem disturbance, senescence, and degradation without prior knowledge of species composition, nutrient status, and complex ecosystem processes.     

Effects of low irradiation on photosynthesis and antioxidant enzyme activities in cucumber during ripening stage

In order to investigate the effects of low irradiation (LI) on cucumber (Cucumis sativus L. cv. Jinyou 35) during a ripening stage, our experiment was carried out in a climate chamber. Two levels of PAR were set for plants: normal irradiation [NI, 600 μmol(photon) m^?2 s^?1] and low irradiation [LI, 100 μmol(photon) m^?2 s^?1], respectively. The experiments lasted for 9 d; then both groups of plants were transferred under NI to recover for 16 d. The plants showed severe chlorosis after the LI treatment. Chlorophyll (Chl) a, initial slope, photosynthetic rate at saturating irradiation (P_max), light saturation point, maximal photochemical efficiency of PSII (F_v/F_m), electron transport rate of PSII (ETR), soluble protein content, and catalase (CAT) activity in cucumber leaves decreased under LI stress, while Chl b, carotenoids, light compensation point, nonphotochemical quenching (q_N), superoxide dismutase (SOD), and malondialdehyde (MDA) exhibited an increasing trend under LI. After 16 d of recovery, values of P_max, F_v/F_m, ETR, q_N, SOD, CAT, MDA, and soluble protein were close to those of the control after one, three, and five days of the LI treatment, while those kept under LI for 7 and 9 d could not return to the control level. Therefore, 7 d of LI stress was a meteorological disaster index for LI in cucumber at the fruit stage.     

Low concentrations of glycine inhibit photorespiration and enhance the net rate of photosynthesis in <Emphasis Type="Italic">Caragana korshinskii</Emphasis>

The inhibition of photorespiration can be used to improve plant carbon fixation. In order to compare the effects of three photorespiration inhibitors [glycine, NaHSO₃, and isonicotinyl hydrazide (INH)], photosynthetic parameters of leaves sprayed respectively with these chemicals were examined and their inhibiting efficiency was evaluated in Caragana korshinskii. Our results showed that 5 mM glycine could reduce the photorespiratory rate (P_R) effectively, while the net photosynthetic rate (P_N), stomatal conductance (g_s), and intercellular CO₂ concentration (Ci) significantly increased. The ratio of electron flow for ribulose-1,5-bisphosphate (RuBP) carboxylation to RuBP oxygenation was elevated markedly. NaHSO₃ and INH could also suppress the PR in some cases, whereas PN was not improved. The glyoxylate content increased considerably after application of low concentrations of glycine. These results suggested that low concentrations of glycine could suppress photorespiration by feed-back inhibition of glyoxylate and enhance photosynthesis by regulating g_s, C_i, and the distribution of electron flow in C. korshinskii.     

Crown exposure to light and tree allometry of 11 tree species in a snowy cool-temperate forest in Japan

Crown exposure to light (CE) and tree allometry were investigated for 11 species in a snowy cool-temperate secondary forest dominated by Fagus crenata and Betula ermanii in Japan. The 11 species differentiated horizontal and vertical light gradients for regeneration. CE was highly variable across species in small trees, but variation in CE decreased with increasing height. The 11 species were classified into three patterns of height-dependent change in CE in comparison to community-level trends, and rank reversal of CE with increasing height was not apparent. Allometric relationships between trunk diameter (D) and height (H) and between D and trunk length (L) differed little between trees of high and low CE within species. In contrast, slopes of the allometric relationships between D and H differed across species; species with larger maximum height (H _max) were taller at a given D, as was noted in previous studies of warm-temperate and tropical forest trees. Differences in trunk angle among the species of different H _max were the main factor generating the differences in allometric relationships between D and H in this forest. Trunk angle increased with increasing height in the species of large H _max but decreased in those of small H _max. Hence, allometric relationships between D and L were not related to H _max. Since the species of small H _max grow laterally and are easily covered in snow during winter while those of large H _max grow vertically above snow cover, differences in trunk angle may reflect species mechanical properties.     

Temperature Responses of Photosynthesis and Respiration of Maize (<Emphasis Type="Italic">Zea mays</Emphasis>) Plants to Experimental Warming

Understanding the key processes and mechanisms of photosynthetic and respiratory acclimation of maize (Zea mays L.) plants in response to experimental warming may further shed lights on the changes in the carbon exchange and Net Primary Production (NPP) of agricultural ecosystem in a warmer climate regime. In the current study, we examined the temperature responses and sensitivity of foliar photosynthesis and respiration for exploring the mechanisms of thermal acclimation associated with physiological and biochemical processes in the North China Plain (NCP) with a field manipulative warming experiment. We found that thermal acclimation of A_n as evidenced by the upward shift of A_n-T was determined by the maximum velocity of Rubisco carboxylation (V_cmax), the maximum rate of electron transport (J_max), and the stomatal- regulated CO₂ diffusion process (g_s), while the balance between respiration and photosynthesis (R_d/A_g), and/or regeneration of RuBP and the Rubisco carboxylation (J_max/V_cmax) barely affected the thermal acclimation of A_n. We also found that the temperature response and sensitivity of R_d was closely associated with the changes in foliar N concentration induced by warming. These results suggest that the leaf-level thermal acclimation of photosynthesis and respiration may mitigate or even offset the negative impacts on maize from future climate warming, which should be considered to improve the accuracy of process-based ecosystem models under future climate warming.     

Photosynthetic behavior of Spanish Arbequina and Italian Maurino olive (<Emphasis Type="Italic">Olea europaea</Emphasis> L.) cultivars under super-intensive grove conditions

The study was carried out in a four-year-old super-high density olive grove in Central Italy to compare leaf gas exchanges of Spanish Arbequina and Italian Maurino olive cultivars. Overall, from mid July to mid November, Maurino had a slightly higher maximum light-saturated net photosynthetic rate (P _Nmax) than Arbequina. The lowest and the highest P _Nmax values were recorded at the end of July and in mid November, respectively. Current-season leaves showed similar or slightly higher P _Nmax values than one-year-old leaves. During the day Maurino always had slightly higher values or values similar to Arbequina, with the highest P _Nmax being in the morning. Maurino had similar or higher dark respiration rate (R _D) values compared to Arbequina. During the day, in both cultivars the R _D was lower at 9:00 than in the afternoon. The pattern of the photosynthetic irradiance-response curve was similar in the two genotypes, but the apparent quantum yield (Y _Q) was higher in Maurino. In both cultivars intercellular CO₂ concentration (C _i) tended to increase when P _Nmax decreased. The increase in C _i corresponded to a decrease in stomatal conductance (g _s). The transpiration rate (E) increased from mid July to the beginning of August, then decreased in September and increased again in November. Particularly in the morning, the current-season leaves showed similar or slightly higher E values than the one-year-old leaves. During the day, in both cultivars and at both leaf ages, E was higher in the afternoon. No effects on leaf gas exchanges due to the presence or absence of fruit on the shoot were found. Overall, there was satisfactory physiological adaptation for Arbequina to the conditions of Central Italy and for Maurino to the superintensive grove conditions.     

Altered torquoselectivity of fluorine in the iron-tricarbonyl-mediated thermal ring opening of 3-fluorocyclobutene: a density-functional exploration

There are eight possible pathways for the iron-tricarbonyl-assisted thermal electrocyclic ring opening of fluorocyclobutene due to variations in the orientation of the binding of Fe(CO)₃ relative to the fluorine substituent (R₁ or R₂), stereoselectivity (conrotatory or disrotatory, i.e., C or D), and the torquoselectivity of fluorine (inward-facing or outward-facing, i.e., in or out). A density functional study revealed that the energetically favored pathway is R₁ Din. Not only is the D mode favored energetically, but the in configuration was observed to be the lowest-lying mode of R₁, despite the general tendency of fluorine substituents to prefer an out configuration. Data on the activation hardness and aromaticity indices such as BAC and HOMA lead to the same conclusion. However, the R₂ mode surprisingly shows no particular preference for either the Cout or the Dout pathway (i.e., the R₂ mode shows less stereoselectivity than R₁). This behavior occurs due to the influences of both the fluorine substituent and metal coordination. Also, the geminal bond orbitals σ(C–F) and σ*(C–F) appear to participate in ring opening, given the excellent correlation of ?BE with the activation barrier of the transition state.     

Polyhydroxylated steroid compounds from the Far Eastern starfish <Emphasis Type="Italic">Distolasterias nipon</Emphasis>

Two new steroid glycosides: distolasteroside D₆, (24S)-24-O-(β-D-xylopyranosyl)-5α-cholestane-3β,6α,8,15β,16β,24-hexaol, and distolasteroside D₇, (22E,24R)-24-O-(β-D-xylopyranosyl)-5α-cholest-22-ene-3β,6α,8,15β,24-pentaol were isolated along with the previously known distolasterosides D₁, D₂, and D₃, echinasteroside C, and (25S)-5α-cholestane-3β4β,6α,7α,8,15α,16β,26-octaol from the Far Eastern starfish Distolasterias nipon. The structures of new compounds were elucidated by NMR spectroscopy and MALDI TOF mass spectrometry. Like neurotrophins, distolasterosides D₁, D₂, and D₃ were shown to induce neuroblast differentiation in a mouse neuroblastoma C1300 cell culture.     

Brassinosteroids improve photosystem II efficiency,gas exchange,antioxidant enzymes and growth of cowpea plants exposed to water deficit

Water deficit is considered the main abiotic stress that limits agricultural production worldwide. Brassinosteroids (BRs) are natural substances that play roles in plant tolerance against abiotic stresses, including water deficit. This research aims to determine whether BRs can mitigate the negative effects caused by water deficiency, revealing how BRs act and their possible contribution to increased tolerance of cowpea plants to water deficit. The experiment was a factorial design with the factors completely randomised, with two water conditions (control and water deficit) and three levels of brassinosteroids (0, 50 and 100 nM 24-epibrassinolide; EBR is an active BRs). Plants sprayed with 100 nM EBR under the water deficit presented significant increases in Φ_PSII, q_P and ETR compared with plants subjected to the water deficit without EBR. With respect to gas exchange, P _N, E and g _s exhibited significant reductions after water deficit, but application of 100 nM EBR caused increases in these variables of 96, 24 and 33%, respectively, compared to the water deficit + 0 nM EBR treatment. To antioxidant enzymes, EBR resulted in increases in SOD, CAT, APX and POX, indicating that EBR acts on the antioxidant system, reducing cell damage. The water deficit caused significant reductions in Chl a, Chl b and total Chl, while plants sprayed with 100 nM EBR showed significant increases of 26, 58 and 33% in Chl a, Chl b and total Chl, respectively. This study revealed that EBR improves photosystem II efficiency, inducing increases in Φ_PSII, q_P and ETR. This substance also mitigated the negative effects on gas exchange and growth induced by the water deficit. Increases in SOD, CAT, APX and POX of plants treated with EBR indicate that this steroid clearly increased the tolerance to the water deficit, reducing reactive oxygen species, cell damage, and maintaining the photosynthetic pigments. Additionally, 100 nM EBR resulted in a better dose–response of cowpea plants exposed to the water deficit.     

Parameters of photosynthesis light curve in <Emphasis Type="Italic">Salix dasyclados</Emphasis> and their changes during the growth season

A dependence of the photosynthesis rate on light is characterized by a number of parameters that are often used for comparison between plant species or for finding photosynthesis interconnections with other physiological processes. In order to properly assessed these parameters, we measured the maximum apparent photosynthesis rate (P _max), dark respiration rate (R _d), light compensation point (LCP), quantum yield corresponding to photosynthetic efficiency (QY), and the light saturation constant (K _s), taking into consideration the leaf plastochron index during vegetation of one of the willow species (Salix dasyclados Wimn.). The P _max value was the highest in the beginning of the growth season when the leaf reached 65% of its full area; after that P _max slowly declined. The most important cardinal value for R _d is its plateau reached by the end of leaf growth, i.e., later than the photosynthesis rate maximum. This plateau value also decreased during vegetation. The LCP value changed in the same way as R _d but reached its plateau simultaneously with the photosynthesis rate maximum. QY also reached its maximum at the same time with the photosynthesis rate; during vegetation it changed more than twofold. The K _s value also changed almost twofold during the season, reaching its maximum together or slightly later than the photosynthesis maximum and then remained constant. Thus, we have found significant changes in the parameters of the photosynthesis light curve during growth season. This shows that they can be used only after a thorough study of leaf development in each particular plant species. Usually performed measuring gas exchange parameters in fully developed leaves does not yield their maximum values and thus does not have any physiological sense.     

Predictive capability of a leaf optical meter for determining leaf pigment status during senescence

We conducted an experiment to assess the predictive capability of a leaf optical meter for determining leaf pigment status of Acer mono Maxim., A. ginnala Maxim., Quercus mongolica Fisch., and Cornus alba displaying a range of visually different leaf colors during senescence. Concentrations of chlorophyll (Chl) a, Chl b, and total Chl [i.e., Chl (a+b)] decreased while the concentration of carotenoids (Car) remained relatively static for all species as leaf development continued from maturity to senescence. C. alba exhibited the lowest average concentration of Chl (a+b), Chl a, and Car, but the highest relative anthocyanin concentration, while Q. mongolica exhibited the highest Chl (a+b), Chl b, and the lowest relative anthocyanin concentration. A. mono exhibited the highest Chl a and Car concentrations. The relationships between leaf pigments and the values measured by the optical meter generally followed an exponential function. The strongest relationships between leaf pigments and optical measurements were for A. mono, A. ginnala, and Q. mongolica (R ² ranged from 0.64 to 0.95), and the weakest relationships were for C. alba (R ² ranged from 0.13 to 0.67). Moreover, optical measurements were more strongly related to Chl a than to Chl b or Chl (a+b). Optical measurements were not related to Car or relative anthocyanin concentrations. We predicted that weak relationships between leaf pigments and optical measurements would occur under very low Chl concentrations or under very high anthocyanin concentrations; however, these factors could not explain the weak relationship between Chl and optical measurements observed in C. alba. Overall, our results indicated that an optical meter can accurately estimate leaf pigment concentrations during leaf senescence — a time when pigment concentrations are dynamically changing — but that the accuracy of the estimate varies across species. Future research should investigate how species-specific leaf traits may influence the accuracy of pigment estimates derived from optical meters.     

Influence of Daily Short-Term Temperature Drops on Respiration to Photosynthesis Ratio in Chilling-Sensitive Plants

Cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum L.), and sweet pepper (Capsicum annuum L.) plants were subjected daily over 13 days to short-term (2 h) temperature drops to 12, 8, 4, and 1°C (DROP treatments) at the end of night periods, and effects of these chilling treatments on the ratio of dark respiration in leaves (R_d) to gross photosynthesis (A_g) were examined. The results showed that DROP treatments affected the R_d/A_g ratio in leaves: this ratio increased significantly in cucumber and tomato plants and was slightly affected in pepper plants. When the temperature drops to 12°C were applied, the increase in R_d/A_g ratio in cucumber and tomato plants was entirely due to the rise in R_d. In the case of temperature drops to 8°C and below, the increase in R_d/A_g was determined by both elevation of R_d and the concurrent decrease in Ag. In cucumber plants, the extent of Ag and R_d changes increased with the DROP severity, i.e., with lowering the temperature of DROP treatment. The inhibition of photosynthesis by DROP treatment in cucumber plants was accompanied by the diminished efficiency of light energy use for photosynthesis and by the increase in the light compensation point. The elevation in R_d/A_g ratio in cucumber plants was accompanied by the decline in growth characteristics, such as accumulation of aboveground biomass, plant height, and leaf area. It was concluded that the R/A ratio is an important indicator characterizing the adaptive potential of chilling-sensitive plant species and their response to daily short-term temperature drops.     

Chemical and hydraulic signals regulate stomatal behavior and photosynthetic activity in maize during progressive drought

The objectives of this study were to investigate stomatal regulation in maize seedlings during progressive soil drying and to determine the impact of stomatal movement on photosynthetic activity. In well-watered and drought-stressed plants, leaf water potential (Ψ _leaf), relative water content (RWC), stomatal conductance (g _s), photosynthesis, chlorophyll fluorescence, leaf instantaneous water use efficiency (iWUE_leaf), and abscisic acid (ABA) and zeatin-riboside (ZR) accumulation were measured. Results showed that g _s decreased significantly with progressive drought and stomatal limitations were responsible for inhibiting photosynthesis in the initial stages of short-term drought. However, after 5 days of withholding water, non-stomatal limitations, such as damage to the PSII reaction center, became the main limiting factor. Stomatal behavior was correlated with changes in both hydraulic and chemical signals; however, changes in ABA and ZR occurred prior to any change in leaf water status. ABA in leaf and root tissue increased progressively during soil drying, and further analysis found that leaf ABA was negatively correlated with g _s (R ² = 0.907, p < 0.05). In contrast, leaf and root ZR decreased gradually. ZR in leaf tissue was positively correlated with g _s (R ² = 0.859, p < 0.05). These results indicate that ABA could induce stomatal closure, and ZR works antagonistically against ABA in stomatal behavior. In addition, the ABA/ZR ratio also had a strong correlation with g _s, suggesting that the combined chemical signal (the interaction between ABA and cytokinin) plays a role in coordinating stomatal behavior. In addition, Ψ _leaf and RWC decreased significantly after only 3 days of drought stress, also affecting stomatal behavior.