Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

The role of temperature in enhancing the insecticidal activity of Bacillus thuringiensis against Spodoptera littoralis larvae

The present study scrutinised how far temperature would affect the velocity of the insecticidal activity of Bacillus thuringiensis, as the rapidity of pest control achievements is of a great concern. Third instar Spodoptera littoralis larvae were treated with Bt at three concentration levels under five different temperatures (15°C, 20°C, 25°C, 30°C and 35°C). LT₅₀s were evaluated in each case. The LT₅₀ values showed various levels of reductions as temperature and/or Bt concentration increased, indicating that the velocity of mortality (1/LT₅₀) and/or the rapidity of Bt activity was almost temperature dependant. However, relatively high and low reduction percentages in the LT₅₀ values on the elevation of 5°C were obtained at lower and higher temperature ranges, respectively. The temperature coefficient, Q ₁₀ values, determined within narrow ranges (5°C) showed great reductions when temperature increased from 15°C to 20°C at all Bt concentrations. Raising temperature by 5°C above 20°C or 25°C almost caused similar Q ₁₀ values indicating constant increase in the response of Bt activity within 20–30°C temperature range. Q ₁₀ values over 30°C were comparatively very low. This proved that decrease in Q ₁₀ values due to the rise of temperature was dependant on the starting temperature.     

Seasonal changes in the physical state of crown water associated with freezing tolerance in winter wheat

The relationship between freezing tolerance (expressed as LT₅₀, the lethal freezing temperature for 50% of plants) and the amount and physical state (as determined by spin-lattice [T₁] and spin-spin [T₂] relaxation times of protons) of water in crown tissue was examined in contrasting winter wheat (Triticum aestivum L.) varieties grown under field conditions from 1992 to 1994. During acclimation, the LT₅₀ values decreased from around -7 to -17, -20 and -27°C in PI 173438, Chihokukomugi and Valuevskaya, respectively. Tissue water content decreased continuously through autumn to reach a plateau around 3 g H₂O (g dry weight)^-1 in early winter when LT₅₀ was still decteasing, and then gradually increased under snow cover. A significant negative correlation was found between mean minimum air temperatures and freezing tolerance prior to the establishment of continuous snow cover. In contrast, a positive association between mean minimum temperatures and crown tissue water content was significant only when air temperatures were above 0°C, as water content did not decrease further at sub-zero temperatures. Seasonal changes in T₁ were closely related to changes in freezing tolerance. T₁ decreased until January even though water content stopped decreasing. Further tests on 15 field-grown varieties confirmed a strong negative association between freezing tolerance and T₁. The results suggest that cold hardening is comprised of two stages, with the transition occurring at ca 0°C. Development of hardiness was related to (1) a reduction in water content in the first stage (at minimum temperatures > 0°C), and (2) a change in physical state of water without much reduction in water content in the second stage. Varietal differences in hardiness thus arise due to changes in both water content and physical state of water.     

Induced expression of the class II chitinase gene during cold acclimation and dehydration of bermudagrass (Cynodon sp.)

Bermudagrass cultivars vary greatly in their ability to survive freezing temperatures as a result of a differential ability to cold acclimate (CA) at temperatures slightly above 0°C. Little information exists on the genetic and physiological mechanisms associated with the cold acclimation process in bermudagrass. Experiments were conducted to study the changes in chitinase gene expression during cold acclimation of freeze-tolerant bermudagrass cultivars. A chitinase gene (CynCHT1) was isolated from ’Midiron’ bermudagrass. Because the hydrophilic protein putatively encoded by the gene lacked an N-terminal cysteine-rich domain and a hydrophobic C-terminal extension, it was classified a class II chitinase. The expression patterns of this and related chitinase genes in response to CA, drought, and ABA were investigated in freeze-tolerant ’MSU’ (LT₅₀=?11°C), Midiron (LT₅₀=?10°C) and ’Uganda’ (LT₅₀=?8°C) bermudagrasses. Northern-blot analysis indicated expression in the crown tissues induced by CA at 8°C/2°C day/night temperature cycles. Induction of gene expression was evident in tissues sampled at 2 and 28 days after initiating CA. Expression after 2-days de-acclimation at 28°C/24°C was similar to control levels. Significantly higher levels of CA-induced chitinase gene expression were observed in MSU and Midiron, compared to Uganda. Similar expression patterns were observed among the cultivars in responses to drought and ABA. These results suggest that chitinases have important roles in bermudagrass response to low temperature and dehydration stresses.     

Cold-induced changes of enzymes,proline, carbohydrates,and chlorophyll in wheat

Quantitative changes in total leaf soluble proteins, proline, carbohydrate content, chlorophyll fluorescence, guaiacol peroxidase (POD) and catalase (CAT) activities were determined in a less cold-hardy (LCH) spring cv. Kohdasht (LT₅₀ = −6°C), a semi cold-hardy (SCH) facultative cv. Azar 2 (LT₅₀ = −15°C), and a cold-hardy (CH) winter cv. Norstar (LT₅₀ = −26°C) of wheat (Triticum aestivum L.) exposed to 4°C for 9 weeks. Seedlings were grown in a controlled growth room for 14 days at 20°C and then transferred to 4°C (experimental day 0) for 63 days (cold treatment); otherwise they were maintained continuously at 20°C (control treatment). The samples were harvested 0, 2, 21, 28, 42, and 63 days after exposure to 4°C. The results showed significant low temperature (LT)-induced accumulation of total soluble proteins, proline, and carbohydrates and elevation in activities of CAT and POD in leaves of SCH and CH winter cultivars rather than in LCH spring cultivar. In contrast, the chlorophyll fluorescence (F _v/F _m) declined during LT treatment irrespective of cultivar. The results suggest that developmental traits such as vernalization requirement of wheat affects on cold-tolerance expression system of plants.     

High Temperatures and Net CO2 Uptake, Growth, and Stem Damage for the Hemiepiphytic Cactus Hylocereus undatus1

Hylocereus undatus, which is native to tropical forests experiencing moderate temperatures, would not be expected to tolerate the extremely high temperatures that can be tolerated by cacti native to deserts. Nevertheless, total daily net CO₂ uptake by this hemiepiphytic cactus, which is widely cultivated for its fruits, was optimal at day/night air temperatures of 30/20°C, temperatures that are higher than those optimal for daily net CO₂ uptake by cacti native to arid and semiarid areas. Exposure to 35/25°C for 30 weeks led to lower net CO₂ uptake than at 10 weeks; exposure to 40/30°C led to considerable necrosis visible on the stems at 6 weeks and nearly complete browning of the stems by 19 weeks. Dry mass gain over 31 weeks was greatest for plants at 30/20°C, with root growth being especially noteworthy and root dry mass gain representing an increasing percentage of plant dry mass gain as day/night air temperatures were increased. Viability of chlorenchyma cells, assayed by the uptake of the vital stain neutral red into the central vacuoles, was decreased 50 percent by a one‐hour treatment at 55°C compared with an average of 64°C for 18 species of cacti native to deserts. The lower high‐temperature tolerance for H. undatus reflected its low high‐temperature acclimation of only 1.4°C as growth temperatures were raised by 10°C compared with an average acclimation of 5.3°C for the other 18 species of cacti. Thus, this tropical hemiepiphytic cactus is not adapted to day/night air temperatures above ca 40/30°C, although its net CO₂ uptake is optimal at the relatively high day/night air temperatures of 30/20°C.     

Differential Effects of Day and Night Temperature on Development to Flowering in Rice

There are conflicting reports with regard to difference in effectsof day temperature (T_D) and night temperatures (T_N) on plantdevelopment. The objective of this study is to determine whetherthere are different effects ofT_DandT_Non development from sowingto flowering in rice (Oryza sativaL.). Plants of 24 rice cultivars were grown in naturally-lightedgrowth chambers at five diurnally constant (22, 24, 26, 28 and32 °C) and four diurnally fluctuating temperatures (26 /22,30 /22, 22 /26 and 22 /30 °C forT_D/T_Nwith 12hd^-1each) witha constant photoperiod of 12hd^-1. The treatments were selectedto enable the separation of effects ofT_DandT_Non developmentrate (DR). The response of DR to constant temperatures was typically nonlinear.This nonlinearity could not explain the difference in floweringdates between fluctuating temperatures with the same mean dailyvalue but oppositeT_D/T_Ndifferences. Differential effects ofT_DandT_NonDR to flowering were detected in all but one cultivar. In mostcases,T_Dexerted a greater influence thanT_N, in contrast withmany previous reports based on the assumption of a linearitybetween DR and temperature. The data were further analysed bya nonlinear model which separated effects ofT_DandT_N. The estimatedvalue for the optimumT_Nwas generally 25 –29 °C, about2 –4 °C lower than the estimated optimumT_Din mostcultivars. The effects ofT_DandT_Non DR were found to be interactivein some cultivars. These results form a new basis for modellingflowering dates in rice. Oryza sativa; rice; flowering; development; day and night temperature; thermoperiodicity     

Regulation of root respiration in two species of Plantago that differ in relative growth rate: the effect of short- and long-term changes in temperature

This study investigates the effect of short‐ and long‐term changesin temperature on the regulation of root respiratory O₂ uptakeby substrate supply, adenylate restriction and/or the capacityof the respiratory system. The species investigated were the lowland Plantagolanceolata L. and alpine Plantago euryphylla Briggs, Carolin& Pulley, which are inherently fast‐ and slow‐growing, respectively. Theplants were grown hydroponically in a controlled environment (constant23 °C). The effect of long‐term exposure to lowtemperature on regulation of respiration was also assessed in P.lanceolata using plants transferred to 15/10 °C(day/night) for 7 d. Exogenous glucose and uncoupler (CCCP)were used to assess the extent to which respiration rates were limitedby substrate supply and adenylates. The results suggest that adenylatesand/or substrate supply exert the greatest control overrespiration at moderate temperatures (e.g. 15–30 °C)in both species. At low temperatures (5–15 °C),CCCP and glucose had little effect on respiration, suggesting thatrespiration was limited by enzyme capacity alone. The Q₁₀ (proportionalincrease of respiration per 10 °C) of respirationwas increased following the addition of CCCP and/or exogenousglucose. The degree of stimulation by CCCP was considerably lowerin P. euryphylla than P. lanceolata. This suggeststhat respiration rates operate much closer to the maximum capacity in P.euryphylla than P. lanceolata. When P. lanceolata wastransferred to 15 °C for 7 d, respirationacclimated to the lower growth temperature (as demonstrated by an increasein respiration rates measured at 25 °C). In addition,the Q₁₀ was higher, and the stimulatory effectof exogenous glucose and CCCP lower, in the cold‐acclimated rootsin comparison with their warm‐grown counterparts. Acclimation of P.lanceolata to different day/night‐time temperatureregimes was also investigated. The low night‐time temperature wasfound to be the most important factor influencing acclimation. The Q₁₀ valueswere also higher in plants exposed to the lowest night‐time temperature.The results demonstrate that short‐ and long‐term changes in temperaturealter the importance of substrate supply, adenylates and capacityof respiratory enzymes in regulating respiratory flux.     

The neotropical shrub Lupinus elegans,from temperate forests,may not adapt to climate change

Considering that their distribution is limited to altitudinal gradients along mountains that are likely to become warmer and drier, climate change poses an increased threat to temperate forest species from tropical regions. We studied whether the understorey shrub Lupinus elegans, endemic to temperate forests of west‐central Mexico, will be able to withstand the projected temperature increase under seven climate change scenarios. Seeds were collected along an altitudinal gradient and grown in a shade‐house over 7 months before determining their temperature tolerance as electrolyte leakage. The plants from colder sites tolerated lower temperatures, i.e. the temperature at which half of the maximum electrolyte leakage occurred (LT₅₀), ranged from −6.4 ± 0.7 to −2.4 ± 0.3 °C. In contrast, no pattern was found for tolerance to high temperature (LT₅₀ average 42.8 ± 0.3 °C). The climate change scenarios considered here consistently estimated an increase in air temperature during the present century that was higher for the maximum air temperature than for the mean or minimum. In particular, the anomaly from the normal maximum air temperature at the study region ranged from 2.8 °C by 2030 to 5.8 °C by 2090. In this respect, the inability of L. elegans to adapt to increasingly higher temperatures found here, in addition to a possible inhibition of reproduction caused by warmer winters, may limit its future distribution.     

Response of in vitro pollen germination and pollen tube growth of groundnut (Arachis hypogaea L.) genotypes to temperature

Air temperatures of greater than 35 °C are frequently encountered in groundnut‐growing regions, especially in the semi‐arid tropics. Such extreme temperatures are likely to increase in frequency under future predicted climates. High air temperatures result in failure of peg and pod set due to lower pollen viability. The response of pollen germination and pollen tube growth to temperature was quantified in order to identify differences in pollen tolerance to temperature among 21 groundnut genotypes. Plants were grown from sowing to harvest in a poly‐tunnel under an optimum temperature of 28/22 °C (day/night). Pollen was collected at anther dehiscence and was exposed to temperatures from 10° to 47·5 °C at 2·5 °C intervals. The results showed that a modified bilinear model most accurately described the response to temperature of percentage pollen germination and maximum pollen tube length. Genotypes were found to range from most tolerant to most susceptible based on both pollen characters and membrane thermostability. Mean cardinal temperatures (T_min, T_opt and T_max) averaged over 21 genotypes were 14·1, 30·1 and 43·0 °C for percentage pollen germination and 14·6, 34·4 and 43·4 °C for maximum pollen tube length. The genotypes 55‐437, ICG 1236, TMV 2 and ICGS 11 can be grouped as tolerant to high temperature and genotypes Kadiri 3, ICGV 92116 and ICGV 92118 as susceptible genotypes, based on the cardinal temperatures. The principal component analysis identified maximum percentage pollen germination and pollen tube length of the genotypes, and T_max for the two processes as the most important pollen parameters in describing a genotypic tolerance to high temperature. The T_min and T_opt for pollen germination and tube growth, rate of pollen tube growth were less predictive in discriminating genotypes for high temperature tolerance. Genotypic differences in heat tolerance‐based on pollen response were poorly related (R² = 0·334, P = 0·006) to relative injury as determined by membrane thermostability.     

Genotypic variation in cold tolerance influences the yield of Miscanthus

When grown in Europe, Miscanthus genotypes often produce yields lower than their potential due to late emergence of shoots in the spring or to damage from late frosts when shoots emerge too early. Here, we investigate genotypic variation in the base temperature (T_b) for shoot emergence and in the lethal temperature for shoots (LT₅₀) in four Miscanthus genotypes. In all genotypes, lowering temperature increased the time to shoot emergence, with T_b ranging from 8.6°C in Sac‐5 to 6°C in Sin‐H9. Frost treatments below ?8°C resulted in a marked reduction in growth in all four genotypes. Sin‐H9 was the most frost tolerant with an LT₅₀ of ?9.3°C. There was little variation found in leaf osmotic potential, but leaf moisture content was significantly lower in Sin‐H9 than in the other genotypes. The lower thermal requirement for emergence and lower LT₅₀ seen in Sin‐H9 was incorporated into a model of Miscanthus production. The model showed an extended growing season that was predicted to increase yields by up to 25%.     

Growth,nodulation and nitrogen fixation in soybean as affected by air humidity and root temperature

Growth, nodulation and N₂ fixation inGlycine max L. Merr., cv. Biison as affected by the relative humidity of air (RH) during the dark period (95 or 50 – 65 %) and day/night root temperature (T_r) (28/28, 25/25, 18/18, 22/28, 22/18 °C) were studied. The growth parameters (plant fresh and dry mass, yield), nodulation (nodule number and fresh mass) and N₂ fixation abilities (total nitrogen content, nitrogenase activity) increased significantly with the increasing T_r. In addition, at the same T_r during the day all studied parameters were increased at the higher T_r during the dark period. Growth, nodulation and N₂ fixation were significantly enhanced at low RH. The findings indicate that all studied parameters could be regulated by environmental factors during the dark period.     

Differential gene expression in cucumber plants in response to brief daily cold treatments

The mechanisms of plant responses to short-term cold treatments applied daily in the period of active growth remain unknown. Cucumber (Cucumis sativus L.) plants were subjected to brief drops of temperature (2 h, 12°C) at the end of each night over a 6-day period (DROP treatment) and to prolonged (6 days) cooling at 12°C (permanent low-temperature treatment, PLT). The plants exposed to cold treatments and control plants grown at 20°C were compared in terms of cold resistance and changes in gene expression. Cold resistance of plants was determined on the basis of LT₅₀ temperature. The response of cucumber genetic machinery was assessed by means of a differential display method based on polymerase chain reaction (PCR). The changes in mRNA pool in cells of cucumber plants subjected to permanent and periodic chilling were assessed after comparing the populations of PCR fragments of cDNA. In both types of chilling protocols, the cold resistance started to increase from the 2nd day of low temperature treatment. At the end of the experiment (on the 6th day), the increment in cold resistance was three times larger for DROP compared to PLT treatment. Analysis of mRNA pool showed that the numbers of amplified fragments were nearly identical in both types of low-temperature treatment. The higher level of cold resistance under DROP conditions was assumed to depend on features of metabolism.     

Thermomorphogenic and Photomorphogenic Control of Stem Elongation in Fuchsia Is Not Mediated by Changes in Responsiveness to Gibberellins

Stem elongation in Fuchsia × hybrida was influenced by cultivation at different day and night temperatures or in different light qualities. Internode elongation of plants grown at a day (25°C) to night (15°C) temperature difference (DIF+10) in white light was almost twofold that of plants grown at the opposite temperature regime (DIF−10). Orange light resulted in a threefold stimulation of internode elongation compared with white light DIF−10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF−10 and DIF+10. Flower development was accelerated at DIF−10 compared with DIF+10 in both white and orange light. To examine whether the effects of DIF and light quality on shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins (GAs), the stem elongation responses of paclobutrazol-treated plants to applied gibberellins were determined. In the absence of applied gibberellins paclobutrazol (>0.32 μmol plant⁻¹) strongly retarded shoot elongation. This inhibition was nullified by the application of about 10–32 nmol of GA₁, GA₄, GA₉, GA₁₅, GA₁₉, GA₂₀, GA₂₄, or GA₄₄. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia and their effects on stem elongation. Received October 4, 1997; accepted December 17, 1997     

Behavioral responses of Colorado potato beetle larvae to combinations of temperature and insolation, under field conditions

In short-term field trials at combinations of ambient temperature (°C) and insolation (W·m⁻²), larval Colorado potato beetles (Leptinotarsa decemlineata [Say] [Coleoptera: Chrysomelidae]) were observed after their release on the adaxial surface of leaflets on potato plants (Solanum tuberosum L. Solanaceae). The larvae either began feeding or moved under the leaflet; mean interval from release to expression of these behaviors (2.9±0.05 min [n =358]) was independent of air temperature and insolation. Proportion of larvae moving under the leaflet increased logistically with both air temperature and insolation. A 1 W·m⁻² change in insolation (P) evoked the same effect on this proportion as a 0.0838 °C change in air temperature (T _a ), so the two quantities were combined as T^*=T _a +P·0.0838 °C/(W·m⁻²), which has units of °C. The proportion of larvae moving under the leaflet increased logistically with T^*. In 1-day field trials we monitored air temperature, insolation and proportion of larvae under the leaflet, and compared the latter to predictions from the logistic regression derived from the short-term trials. Consistently more larvae occurred under leaflets than predicted from the logistic regression; this bias diminished as T^* increased until at T^*≥40 °C, observed and predicted proportions were equal. This pattern of deviation from the predictions of the logistic regression is consistent with a thermoregulatory strategy in which larvae move away from hostile conditions, rather than seek optimal conditions.     

Temperature regulation in the echidna (Tachyglossus aculeatus)

The echidna (Tachyglossus aculeatus) maintained a body temperature of 30.7°C ± 1.03 s.d. (N = 23) at ambient temperatures (T_A) between 0 and 25°C. It may, however, also become hypothermic at low T_A. At T_A = 30°C or above the echidna became hyperthermic. The thermoneutral range was about 20–30°C. At low T_A the metabolic rate might be increased several fold. The thermal conductance was at a minimum at T_A = 20°C, and was not further reduced at lower T_A. At higher T_A the thermal conductance increased up to five-fold. The evaporation showed little change with increasing T_A. At the highest T_A we used (33°C) the evaporation on the average accounted for the dissipation of only about one-third of the metabolic heat produced. These findings suggest that the echidna, although it can maintain its body temperature at low ambient temperature, cannot rely upon evaporation as the major avenue for heat loss at high ambient temperature.     

A new method to determine the energy saving night temperature for vegetative growth of Phalaenopsis

Knowledge of the energy saving night temperature (i.e. a relatively cool night temperature without affecting photosynthetic activity and physiology) and a better understanding of low night temperature effects on the photosynthetic physiology of Phalaenopsis would improve their production in terms of greenhouse temperature control and energy use. Therefore, Phalaenopsis‘Hercules’ was subjected to day temperatures of 27.5°C and night temperatures of 27.0°C, 24.2°C, 21.2°C, 18.3°C, 15.3°C or 12.3°C in a growth chamber. A new tool for the determination of the energy saving night temperature range was developed based on temperature response curves of leaf net CO₂ exchange, chlorophyll fluorescence, organic acid content and carbohydrate concentrations. The newly developed method was validated during a complete vegetative cultivation in a greenhouse environment with eight Phalaenopsis hybrids (i.e. ‘Boston’, ‘Bristol’, ‘Chalk Dust', ‘Fire Fly’, ‘Lennestadt’, ‘Liverpool’, ‘Precious’, ‘Vivaldi’) and day/night temperature set points of 28/28°C, 29/23°C and 29/17°C. Temperature response curves revealed an overall energy saving night temperature range for nocturnal CO₂ uptake, carbohydrate metabolism, organic acid accumulation and photosystem II (PSII) photochemistry of 17.1°C to 19.9°C for Phalaenopsis‘Hercules’. At the lower end of this energy saving night temperature range, a high malate‐to‐citrate ratio switched towards a low ratio and this transition seemed to alleviate effects of night chilling induced photoinhibition. At night temperatures of 24°C or higher, the degradation of starch, glucose and fructose indicated an increased respiratory CO₂ production. During the greenhouse validation experiment, the differences between the eight Phalaenopsis hybrids with regard to their response to the warm day/cool night temperature regimes were remarkably large. In general, the day/night temperature of 29/17°C led to a significantly lower biomass accumulation and less leaves which were in addition shorter, narrower and smaller in size as compared to the day/night temperature regimes of 28/28°C and 29/23°C. During week 25 of the cultivation period, plants matured and flower initiation steeply increased for all hybrids and in each day/night temperature regime. Before week 25, early spiking was only sufficiently suppressed in the 29/23°C and 29/17°C temperature regimes for three hybrids (‘Boston’, ‘Bristol’ and ‘Lennestadt’) but not in the other five hybrids. Although a considerable biochemical flexibility was demonstrated for Phalaenopsis‘Hercules’, inhibition of flowering after exposure to a combination of warm days and cool nights appeared to be largely hybrid dependent.     

Freezing and high temperature thresholds of photosystem 2 compared to ice nucleation, frost and heat damage in evergreen subalpine plants

Freezing and high temperature thresholds of photosystem 2 (PS2), ice formation and frost and heat damage were measured in leaves of evergreen subalpine plants under conditions of naturally low (winter) to high (summer) PS2 efficiencies (F_V/F_M). The temperature‐dependent change in basic Chl fluorescence (F₀) (T‐F₀) technique that is usually used to assess the high temperature threshold of PS2 in a new approach was applied to test freezing temperature thresholds of PS2. T‐F₀ curves (+5 °C to ?10 °C at 2 K h^?1) revealed a significant, sudden increase in F₀ on extracellular ice formation (?4.0 or ?5.5 °C). The rise in F₀ was recorded 0.3–0.6 K below ice nucleation (10–20 min later) and was produced by freeze dehydration of cells. The rise in F₀ was not caused by frost damage, as during winter LT₅₀ was lower than ?27 °C and not by formation of ice on the leaf surface. Hence, F₀ measurements during freezing are a useful tool to distinguish between surface ice and extracellular ice inside the leaf tissue which cannot be differentiated by other ice‐detecting methods. PS2 efficiency significantly affected the shape of the high temperature T‐F₀ curves (20–65 °C at 1 K min^?1). Under F_V/F_M >0.6, two F₀ maxima were recorded. The fast rise phase to the first F₀ maximum corresponded with tissue heat damage (LT₅₀: 46.9–54.3 °C). The second F₀ maximum occurred at leaf temperatures between 55 and 60 °C. Under F_V/F_M <0.2 only, the second F₀ maximum was detectable. Lack of awareness of the missing F₀ maximum would lead to an overestimation of the PS2 high temperature threshold by >10 K; hence, under low F_V/F_M, it cannot be determined by the T‐F₀ technique.     

Influence of post-treatment temperature, solvents and emulsifiers on the toxicity of cypermethrin to larvae of Spodoptera litura (Fab.)

Larvae of the tobacco caterpillar, Spodoptera litura (Fab.) were topically bioassayed with cypermethrin emulsion formulation and proprietary EC in combination with different solvents and emulsifiers for their toxicity at 16 ± 2°C and 27 ± 1°C. Among different solvents, the descending order of efficacy (with relative toxicity in parenthesis) against 9 ± 1 day old larvae of S. litura (Fab.) at 16 ± 2°C than 27 ± 1°C was: xylene (7.41), aromex (5.54), cyclohexane (4.93), C-IX (3.93), benzene (2.40) and toluene (2.26). With emulsifier, the order was: Triton X-100 (7.41), SN₄S (4.60), Teepol (2.90), Tween-80 (1.49), Swascofix DP-50 (1.28), SN₄R (1.10) and S₅₂B (1.07). However, the proprietary cypermethrin EC was 2.31 times more toxic at 16 ± 2°C than at 27 ± 1°C.