Acclimation of photosystem II to high temperature in a suspension culture of soybean (Glycine max) cells requires proteins that are associated with the thylakoid membrane

In a study of the responses of photosystem II (PSII) to high temperature in suspension-cultured cells of soybean (Glycine max L. Merr.), we found that high temperatures inactivated PSII via two distinct pathways. Inactivation of PSII by moderately high temperatures, such as 41°C, was reversed upon transfer of cells to 25°C. The recovery of PSII required light, but not the synthesis of proteins de novo. By contrast, temperatures higher than 45°C inactivated PSII irreversibly. An increase in the growth temperature from 25 to 35°C resulted in an upward shift of 3°C in the profile of the heat-induced inactivation of PSII, which indicated that the thermal stability of PSII had been enhanced. This acclimative response was reflected by the properties of isolated thylakoid membranes: PSII in thylakoid membranes from cells that had been grown at 35°C exhibited greater thermal stability than that from cells grown at 25°C. Disruption of the vesicular structure of thylakoid membranes with 0.05% Triton X-100 decreased the thermal stability of PSII to a similar level in both types of thylakoid membrane. Proteins released by Triton X-100 from thylakoid membranes from cells grown at 35°C were able to increase the thermal stability of Triton-treated thylakoid membranes. These observations suggest that proteins that are associated with thylakoid membranes might be involved in the enhancement of the thermal stability of PSII.     

Photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole-canopy CO₂ gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (P_n) on warm, usually sunny, days (canopy air temperature [T_c]> 20°C), but had relatively high P_n on cool days (T_c<10°C). Laboratory measurements of light and temperature responses of P_n showed that high temperature, not visible irradiance, was responsible for depressions in P_n on warm sunny days. The optimal leaf temperatures (T_l) for P_n in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive P_n at 0°C T_l, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal P_n, and we estimate that their low-temperature compensation points occurred at ?2°C T_l (C. quitensis) and ?3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of P_n on warm days and their unusually low high-temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low P_n at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature-enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and P_n/intercellular CO₂ concentration (c_i) response curve analyses. The breakpoint or critical temperature (T_cr) of minimal fluorescence (F_o) was ≈42°C in both species, which was well above the temperatures where reductions in P_n were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for P_n. The optimal T_l for photochemical quenching (q_p) and the quantum yield of photosystem II (PSII) electron transfer (φ_PSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower q_p and greater non-photochemical quenching (q_NP), but had little effect on F_o, maximal fluorescence (F_m) or the ratio of variable to maximal fluorescence (F_v/F_m) in both species. In addition, carboxylation efficiencies or initial slopes of their P_n/c_i response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, T_c at our field site averaged 4.3°C and was below the temperature optima for P_n in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of P_n.     

Estimating heat tolerance among plant species by two chlorophyll fluorescence parameters

The heat tolerance of 8 temperate- and 1 subtropical-origin C₃ species as well as 17 tropical-origin ones, including C₃, C₄, and CAM species, was estimated using both F₀-T curve and the ratio of chlorophyll fluorescence parameters, prior to and after high temperature treatment. When leaves were heated at the rate of ca. 1 °C min⁻¹ in darkness, the critical temperature (T_c) varied extensively among species. The T_c's of all 8 temperate-origin species ranged between 40–46 °C in winter (mean temperature 16–19 °C), and between 32–48 °C in summer (mean temperature ca. 30 °C). Those for 1 subtropical- and 12 tropical-origin C₃ species ranged between 25–44 °C and 35–48 °C, and for 1 CAM and 4 C₄ species were 41–47 and 45–46 °C, respectively. Acclimating three C₃ herbaceous plants at high temperature (33/28 °C, day/night) for 10 d in winter caused their T_c's rising to nearly the values measured in summer. When leaves were exposed to 45 °C for 20 min and then kept at room temperature in darkness for 1 h, a significant correlation between RF_v/m (the ratio of F_v/F_m before and after 45 °C treatment) and T_c was observed for all tested temperate-origin C₃ species as well as tropical-origin CAM and C₄ species. However, F₀ and F_v/F_m of the tropical-origin C₃ species were less sensitive to 45 °C treatment, regardless of a large variation of T_c; thus no significant correlation was found between their RF_v/m and T_c. Thus T_c might not be a suitable index of heat tolerance for plants with wide range of environmental adaptation. Nevertheless, T_c's of tropical origin C₃ species, varying and showing high plasticity to seasonal changes and temperature treatment, appeared suitable for the estimation of the degree of temperature acclimation in the same species.     

Thermal sensitivity of white muscle lactate dehydrogenase isolated from a lake trout, (<Emphasis Type="Italic">Salmo trutta</Emphasis>), inhabiting lake Plav,Montenegro

To shed light on thermoadaptive properties of Salmo trutta from lake Plav (Montenegro), we undertook kinetic studies of pyruvate reduction rates and thermal stability analyses of white muscle LDH. We compared these with the data obtained for trout of the same, confirmed by us, Danubian lineage living in rivers and streams of Serbia and Montenegro. We also tested the effect of acclimation in captivity at 4 and 14 °C. The lake trout was of a typical smoltified phenotype (the size, the elongated silver colored body). At physiological substrate concentration, the breaks in the Arrhenius plots (critical temperature - T_c) correlated with acclimation temperatures or habitat water temperatures. Q₁₀ values for temperatures above T_c were close to one, in all cases except 4 °C acclimated trout. At temperatures below T_c Q₁₀ was close to two, except in the case of 14 °C acclimated trout. Lake trout had a highest Q₁₀ values at temperatures below T_c. It was conspicuous that within the entire range of tested temperatures the differences in Q₁₀ resulted from the effect of environmental temperature. Higher Q₁₀ values were obtained with LDH isolated from trout acclimated to 4 °C compared with LDH acclimated to 14 °C. E_a values were much lower at a temperature below T_c compared with temperatures above Tc. Thermal stability of muscle LDH was lower after acclimation to 14 compared to 4 °C, while extremely high thermostability was obtained with the lake trout enzyme. Our data support the concept that T_c values have distinct physiological significance.     

Irreversible changes in barley leaf chlorophyll fluorescence detected by the fluorescence temperature curve in a linear heating/cooling regime

The chlorophyll fluorescence (F) temperature curves in a linear time-temperature heating/cooling regime were used to study heat-induced irreversible F changes in primary green leaves of spring barley (Hordeum vulgare L. cv. Akcent). The leaf segments were heated in a stirred water bath at heating rates of 0.0083, 0.0166, 0.0333, and 0.0500 °C s⁻¹ from room temperature up to maximal temperature T _m and then linearly cooled to 35 °C at the same rate. The F intensity was measured by a pulse-modulated technique. The results support the existence of the two critical temperatures of irreversible F changes postulated earlier, at 45–48 and 53–55 °C. The critical temperatures are slightly dependent on the heating rate. Two types of parameters were used to characterize the irreversibility of the F changes: the coefficient of irreversibility μ defined as the ratio of F intensity at 35 °C at the starting/ending parts of the cycle and the slopes of tangents of linear parts of the F temperature curve. The dependence of μ on T _m revealed a maximum, which moved from 54 to 61 °C with the increasing heating/cooling rate v from 0.0083 to 0.0500 °C s⁻¹, showing two basic phases of the irreversible changes. The Arrhenius and Eyring approaches were applied to calculate the activation energies of the initial increase in μ. The values varied between 30 and 50 kJ mol⁻¹ and decreased slightly with the increasing heating rate.     

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

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

Water stress preconditioning and cotton root pressure-flux relationships

Summary A model was developed to describe interactive effects of exposure time and treatment on thermostability of excisedIllicium parviflorum Michx. root cell membranes using electrolyte leakage (L_c) procedures. Roots were moved from 25°C to treatment temperatures between 35°C and 60°C for 30 to 300 min. A sigmoidal response described L_c increases with increasing temperature at selected time exposures and the lethal exposure time decreased exponentially as temperature increased. The lethal temperature (52.0±1.1°C) for a 15 min exposure using this technique was comparable to the critical temperature (52.2±1.2°C) when roots were exposed to gradually increasing temperatures (4°C per h). Total protein content of roots began to decrease as temperatures increased from 35 to 40°C and the temperature corresponding to 50% reduction in total proteins was 49.1±2.2°C.     

Degradation of proteins from thylakoid membranes in senescing wheat leaves at high temperature

This investigation determined whether thylakoid proteins would be degraded more rapidly or not in senescing wheat (Triticum aestivum L. em. Thell.) leaves concurrently exposed to high temperatures. Excised leaves were pulse-labelled with [³⁵S]-methionine for a 12 h period, and then incubated at 22,32 or 42°C for 0, 1, 2, or 3 d, before extracting a thylakoid enriched membrane sample. After electrophoretic separation, two prominent [³⁵S]-labelled protein bands were chosen for further analyses. Band A contained the D-1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII) associated with photosystem II (PSII). Total protein, [³⁵S]-labelled protein, band A protein, and band B protein within the thylakoid enriched membrane samples were measured. Unlabelled thylakoid enriched membrane samples, extracted from leaves given similar treatments, were used to measure uncoupled whole-chain and photosystem II (PSII) electron transport and chlorophyll fluorescence. Accentuated decline in whole-chain and PSII electron transport, increasing F_o values, and decreasing F_max values were a result of high temperature injury in leaves treated at 42°C. None of the thylakoid enriched membrane protein fractions were degraded more rapidly in high-temperature treated leaves. Degradation of the total [³⁵S]-labelled membrane proteins and band B was inhibited by the 42°C treatment. The results indicate that high temperature stress may disrupt some aspects of normal senescence.     

Effects of elevated temperature on photosynthesis in desert plant <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis> S

Most plants growing in temperate desert zone exhibit brief temperature-induced inhibition of photosynthesis at midday in the summer. Heat stress has been suggested to restrain the photosynthesis of desert plants like Alhagi sparsifolia S. It is therefore possible that high midday temperatures damage photosynthetic tissues, leading to the observed inhibition of photosynthesis. In this study, we investigated the mechanisms underlying heat-induced inhibition of photosynthesis in A. sparsifolia, a dominant species found at the transition zone between oasis and sandy desert on the southern fringe of the Taklamakan desert. The chlorophyll (Chl) a fluorescence induction kinetics and CO₂ response curves were used to analyze the thermodynamic characters of both photosystem II (PSII) and Rubisco after leaves were exposed to heat stress. When the leaves were heated to temperatures below 43°C, the initial fluorescence of the dark-adapted state (F_o), and the maximum photochemical efficiency of PSII (F_v/F_m), the number of active reaction centers per cross section (RCs) and the leaf vitality index (PI) increased or declined moderately. These responses were reversed, however, upon cooling. Moreover, the energy allocation in PSII remained stable. The gradual appearance of a K point in the fluorescence curve at 48°C indicated that higher temperatures strongly impaired PSII and caused irreversible damage. As the leaf temperature increased, the activity of Rubisco first increased to a maximum at 34°C and then decreased as the temperature rose higher. Under high-temperature stress, cell began to accumulate oxidative species, including ammoniacal nitrogen, hydrogen peroxide (H₂O₂), and superoxide (O₂ ^·−), suggesting that disruption of photosynthesis may result from oxidative damage to photosynthetic proteins and thylakoid membranes. Under heat stress, the biosynthesis of nonenzyme radical scavenging carotenoids (Cars) increased. We suggest that although elevated temperature affects the heat-sensitive components comprising of PSII and Rubisco, under moderately high temperature the decrease in photosynthesis is mostly due to inactivation of dark reactions.     

Effect of temperature on the duration of egg development, and moulting and growth in juveniles of Crangonyx pseudogracilis (Crustacea: Amphipoda) in the laboratory

SUMMARY. The interval between moults is an extension of egg development time, increasing from birth to sexual maturity which is probably reached at instar 6 or 7. The duration of each instar increased with the animal's age. Incubation time for eggs and the intermoult interval have the same curvilinear inverse relationship with water temperature in the range 3.5–25°C. Results are expressed as degree-days above predicted threshold temperatures of 3.8°C for eggs and 3.2°C for instar 1 after birth, but inverse power-law relationships were a better fit to the results, with exponents of - 1.355 for eggs, - 1.263 for instar 1 and - 1.37 to - 1.92 for instars 2–4. Temperature — dependence apparently altered in instars 5 and 6 at 15–25°C. From a multiple regression of geometric mean moult interval (M_i, days) against mean age (A) and temperature (T, °C), M_i= 56.4 T^?0.7 e^0.016A, with mean ages of 106 days at 15°C and 85 days at 25°C after six moults. The mean number of primary flagellar segments on the antennules increased from 4.0 in instar 1 to 6.0 in instar 2 and 8.0 in instar 3. Thereafter, segments were added less regularly to give a mean of 13.2 in instar 7. In a natural population, when the sexes became distinctive they had 11–13 flagellar segments. From birth at c. 0.05 mg wet wt, individual growth rates were highly variable; mean growth rates (G_s, % wet wt day^?1) were similar in animals fed on dried, leached elm leaves and living, green leaves of Callitriche; there was a power-law relationship with temperature in the range 3.5–25°C, (G_s= 0.27 T^0.59). Faster growth rates were obtained on living leaves of Elodea. Sexual maturity is reached at c. 0.4–0.5 mg wet wt. A brief comparison is made with Gammarus pulex; C. pseudogracilis may be better adapted to warm-water habitats.     

The effects of development at sub-optimal growth temperatures on photosynthetic capacity and susceptibility to chilling-dependent photoinhibition in Zea mays

When plants of Zea mays L. cv. LG11 that have been grown at optimal temperatures are transferred to chilling temperatures (0–12°C) photoinhibition of photosynthetic CO₂ assimilation can occur. This study examines how growth at sub-optimal temperatures alters both photosynthetic capacity and resistance to chilling-dependent photoinhibition. Plants of Z. mays cv. LG11 were grown in controlled environments at 14, 17, 20 and 25°C. As a measure of the capacity for photosynthesis under light limiting conditions, the maximum quantum yields of CO₂ assimilation (φ^a.c) and O₂ evolution (φ^a.o) were determined for the laminae of the second leaves at photon fluxes of 50–150 μmol m^-2s^-1. To determine photosynthetic capacity at photon fluxes approaching light saturation, rates of CO₂ uptake (A₁₅₀₀) and O₂ evolution (A₁₅₀₀) were determined in a photon flux of 1500 μmol m^-2s^-1. In leaves developed at 14°C, φ and φ were 26 and 43%, respectively, of the values for leaves grown at 25°C. Leaves grown at 17°C showed intermediate reductions in φ and φ, whilst leaves developed at 20°C showed no significant differences from those grown at 25°C. Similar patterns of decrease were observed for A₁₅₀₀ and A_1500.0 with decreasing growth temperature. Leaves developed at 25°C showed higher rates of CO₂ assimilation at all light levels and measurement temperatures in comparison to leaves developed at 17 and 14°C. A greater reduction in A₁₅₀₀ relative to A_1500.0 with decreasing growth temperature was attributed to increased stomatal limitation. Exposure of leaves to 800–1000 μmol m^-2 s^-1 when plant temperature was depressed to ca 6.5°C produced a photoinhibition of photosynthetic CO₂ assimilation in all leaves. However, in leaves developed at 17°C the decrease in A₁₅₀₀ following this chilling treatment was only 25% compared to 90% in leaves developed at 25°C. Recovery following chilling was completed earlier in leaves developed at 17°C. The results suggest that growth at sub-optimal temperatures induces increased tolerance to exposure to high light at chilling temperatures. This is offset by the large loss in photosynthetic capacity imposed by leaf development at sub-optimal temperatures.     

Impact of two different types of heat stress on chloroplast movement and fluorescence signal of tobacco leaves

Although the chloroplast movement can be strongly affected by ambient temperature, the information about chloroplast movement especially related to high temperatures is scarce. For detailed investigation of the effects of heat stress (HS) on tobacco leaves (Nicotiana tabacum L. cv. Samsun), we used two different HS treatments in dark with wide range of elevated temperatures (25–45°C). The leaf segments were either linearly heated in water bath at heating rate of 2°C min⁻¹ from room temperature up to maximal temperature (T _m) and then linearly cooled down to 25°C or incubated for 5 min in water bath at the same T _m followed by 5 min incubation at 25°C (T-jump). The changes in light-induced chloroplast movement caused by the HS pretreatment were detected after the particular heating regime at 25°C using a method of time-dependent collimated transmittance (CT) and compared with the chlorophyll O–J–I–P fluorescence rise (FLR) measurements. The inhibition of chloroplast movement started at about 40°C while the fluorescence parameters responded generally at higher T _m. This difference in sensitivity of CT and FLR was higher for the T-jump than for the linear HS indicating importance of applied heating regime. A possible influence of chloroplast movement on the FLR measurement and a physiological role of the HS-impaired chloroplast movement are discussed.     

Temperature-induced Alterations in the Expression of Susceptibility of Cassava to Colletotrichum gloeosporioides f. sp. manihotis

Cassava plants (Manihot esculenta Crantz) were incubated at 15, 25 or 35 °C after inoculation of green stems with Colletotrichum gloeosporioides (Penz.) f. sp. manihotis Chev. At 25 °C, the cultivar TMS 30211 was less susceptible than the cultivar TMS 30337. At 35 °C, compared to 25 °C, the lesion diameter was reduced in both cultivars, the cassava stem extracts showed an enhanced fungitoxicity, the grem tubes were more sensitive to the stem extracts and a layer of lignified, cells developed earlier around the infected tissues. At 15 °C, the lesions in both cultivars extended more rapidly than at the other temperatures, even though 15 °C, compared to 25 °C, caused a 50% reduction of mycelial growth rate on oat meal agar.     

Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub-optimal temperature

The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near-optimal (25 °C) or at suboptimal (15 °C) temperature by measuring chlorophyll (ChI) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 °C, the quantum yield of PSII electron transport (ψ_PSII) and the rate of O₂ evolution decreased with decreasing temperature (from 25 to 4 °C) at a photon flux density of 125 μmol m^?2 s^?1. In leaves grown at 25 °C, the decrease of ψ_PSII correlated with a decrease of photochemical ChI fluorescence quenching (q_p), whereas in leaves crown at 15 °C q_p was largely insensitive to the temperature decrease. Compared with leaves grown at 25 °C, leaves grown at 15 °C were also able to maintain a higher fraction of oxidized to reduced Q_A (greater q_p) at high photon flux densities (up to 2000 μmol m^?2 s^?1), particularly when the measurements were performed at high temperature (25 °C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 °C exhibited a substantial quenching of the dark level of fluorescence F₀ (q₀) whereas this type of quenching was virtually absent in leaves grown at 25 °C. Furthermore, leaves grown at 15 °C were able to recover faster from photo inhibition of photosynthesis after a photoinhibitory treatment (1200 μmol m^?2 s^?1 at 25, 15 or 6 °C for 8 h) than leaves grown at 25 °C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near-optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 °C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 °C.     

The influence of chilling on photosynthesis and activities of some enzymes of sucrose metabolism in Lycopersicon esculentum Mill

The effects of chilling stress on leaf photosynthesis and sucrose metabolism were investigated in tomato plants (Lycopersicon esculentum Mill. cultivar Marmande). Twenty-one-day-old seedlings were grown in a growth chamber at 25/23 °C (day/night) (control) and at 10/8 °C (day/night) (chilled) for 7 days. The most evident effect of chilling was the marked reduction of plant growth and of CO₂ assimilation as measured after 7 days, the latter being associated with a decrease in stomatal closure and an increase in Ci. The inhibition in photosynthetic rate was also related to an impairment of photochemistry of photosystem II (PSII), as seen from the slight, but significant change in the ratio of F_v/F_m. The capacity of chilled leaves to maintain higher q_P values with respect to the controls suggests that some protection mechanism prevented excess reduction of PSII acceptors. The results of the determination of starch and soluble sugar content could show that chilling impaired sucrose translocation. The activity of leaf invertase increased significantly in chilled plants, while that of other sucrose-metabolizing enzymes was not affected by growing temperature. Furthermore, the increase in invertase (neutral and acid) activity, which is typical of senescent tissue characterized by reduced growth, seems to confirm that tomato is a plant which is not a plant genetically adapted to low temperatures.     

Heat stress induces in leaves an increase of the minimum level of chlorophyll fluorescence, Fo: A time-resolved analysis

A time-resolved study of the effects of heat stress (23 to 50°C) on F_o level of chlorophyll fluorescence of leaves having different antenna content has been performed in order to elucidate the causes of heat induced increase of F_o in vivo. The multi-exponential deconvolution of the decays after a picosecond flash at F_o have shown that the best fit in both wild-type and the mutant chlorina F2 of barley leaves is obtained with three components in the temperature range utilized (100, 400 and 1200 ps at 23°C). In intermittent light greened pea leaves, a fourth long lifetime component (4 ns at 23°C) is needed. The comparison of the three types of leaves at 23°C shows that the content of the LHCII b complex does not affect the lifetimes of the two main components (100 and 400 ps) and affects their preexponential factors. This result suggests that in the PS II unit the exciton transfer from LHC IIb to the rest of the antenna is irreversible. The effects of heat stress on individual lifetime components, T_i, included several changes. Utilizing for PS II unit an extended ‘Reversible Radical Pair’ model, having three compartments, to interpret the variations of T_i and A_i induced by temperature increases, it can be inferred that heat determines: (i) an irreversible disconnection of a monor antenna complex which is not the LHC IIb complex, this effect is induced by temperatures higher than 40°C; (ii) a decrease of the quantum efficiency of Photosystem II photochemistry which is due to several effects: a decrease of the rate of charge separation, an increase of P⁺I^- recombination rate constant and a decrease of the stabilization of charges. These effects on Photosystem II photochemistry start to occur above 30°C and are partially reversible.     

Temperature‐dependent development of Neoseiulus barkeri (Acari: Phytoseiidae) on Tetranychus urticae (Acari: Tetranychidae) at seven constant temperatures

Abstract The effect of seven constant temperatures of 15, 20, 25, 27, 30, 35 and 37°C on developmental time of Neoseiulus barkeri Hughes were determined in laboratory conditions under 65%± 5% RH and a photoperiod of 12 : 12 (L : D) h on nymphal stages of Tetranychus urticae Koch. Total developmental time of females (from egg to adult emergence) at the above‐mentioned temperatures was 26.59, 14.43, 6.32, 5.64, 4.59, 3.98 and 4.67 days, respectively. Developmental rate of the N. barkeri increased as temperature increased from 15 to 35°C, but declined at 37°C. A linear and two nonlinear models were fitted to developmental rate of immature stages of N. barkeri to predict the developmental rate as a function of temperature, as well as to estimate the thermal constant (K) and critical temperatures (i.e., T_min, T_opt and T_max). The estimated values of the T_min and K for total developmental time using the linear model were 12.07°C and 86.20 degree‐days (DD), respectively. The T_min and T_max estimated by the Sharpe‐Schoolfield‐Ikemoto (SSI) model were 11.90°C and 37.41°C, respectively. The estimated T_opt for overall immature stage development of N. barkeri by the Lactin and SSI models were 33.89°C and 24.51°C, respectively. Based on the biological criteria of model evaluation, the linear and SSI models were found to be the best models for describing the developmental rate of overall immature stages of N. barkeri and estimating the temperature thresholds.     

Properties of a resistance-breaking strain of potato virus X

During indexing of a potato germplasm collection from Bolivia, a strain of potato virus X (PVX), X_HB, which failed to cause local lesions in inoculated leaves of Gomphrena globosa was found in 7% of the clones. X_HB was transmitted by inoculation of sap to 56 species from 11 families out of 64 species from 12 families tested. It was best propagated in Nicotiana glutinosa or N. debneyi; Montia perfolia and Petunia hybrida were useful as local lesion hosts. Inoculated leaves of G. globosa plants kept at 10°, 14°, 18°, 22°, or 26 °C after inoculation were always infected symptomlessly. X_HB caused a mild mosaic, systemic chlorotic blotching or symptomless infection in 16 wild potato species and eight Andean potato cultivars, systemic necrotic symptoms in clone A6 and cultivar Mi Peru, and bright yellow leaf markings in cultivar Renacimiento. It caused necrotic local lesions in inoculated leaves of British potato cultivars with the PVX hypersensitivity gene Nb but then invaded the plants systemically without causing further necrosis; with gene Nx systemic invasion occurred but no necrotic symptoms developed. These reactions resemble those of PVX strain group four. X_HB differed from other known strains of PVX in readily infecting PVX-immune clones 44/1016/10, G. 4298.69 and USDA 41956, cultivars Saphir and Saco, and Solanum acaule PI 230554. X_HB had slightly flexuous filamentous particles with a normal length of 516 nm. It was transmitted readily by plant contact and it partially protected G. globosa leaves from infection with X_CP, a group two strain of PVX. Sap from infected N. glutinosa was infective after dilution to 10^--⁶ but not 10^--⁷ after 10 min at 75° but not 80 °C and after 1 yr at 20 °C. X_HB was readily purified from infected N. debneyi leaves by precipitation with polyethylene glycol followed by differential centrifugation. Microprecipitin tests showed that X_HB and X_CP are closely related serologically.