The effects of temperature on the availability and acquisition of potato leafroll luteovirus by Myzus persicae

The concentration of potato leafroll luteovirus (PLRV) did not differ in potato plants with secondary infections grown at 15°C or 27°C. Detached leaves of plants grown at 15°C or 27°C were used as sources of PLRV for peach-potato aphids (Myzus persicae Sulz.) both at 15°C and 27°C. At comparable temperature during virus acquisition, aphids which fed on leaves of plants kept previously at 15°C contained more viral antigen detected by ELISA than aphids which fed on leaves of plants grown at 27°C. The aphids which acquired PLRV at 27°C contained evidently more viral antigen than those which acquired PLRV at 15°C. The greatest amount of PLRV was found in the aphids which acquired the virus at 27°C from the leaves of plants kept at 15°C. The ability of M. persicae to transmit PLRV to Physalis ftoridana Rydb. generally decreased with decrease in the amount of PLRV in vectors.     

Mutualistic association of Paecilomyces formosus LHL10 offers thermotolerance to Cucumis sativus

We investigated in this study the influence of an endophytic fungus, Paecilomyces formosus LHL10, on the thermotolerance of cucumber (Cucumis sativus) upon exposure to high (38°C) and low (8°C) temperature stresses. The results showed that endophyte-inoculated plants had significantly higher plant growth attributes under high-temperature stress. However, they were either low or insignificant in non-inoculated control and inoculated plants with 8°C treatments. Lower stress-promulgated water deficit and cellular membrane damage were observed in endophyte-treated plants after 38°C treatment than in control plants under 8°C stress. Total polyphenol, reduced glutathione, and lipid peroxidation activities were reduced in endophyte-associated plants after exposure to 38°C as compared with control and 8°C-treated plants. The concentration of saturated fatty acids (palmitic-C16:0; stearic-C18:0) was lower in endophyte-treated plants with or without low-temperature stress, but after 8°C treatment increased compared with controls. Unsaturated fatty acids (oleic-C18:1; linoleic-C18:2; linolenic-C18:3 acids) were similar at normal conditions; however, at 38°C, C18:2 and C18:3 were decreased, and C18:1 was increased in endophyte-treated plants compared with controls, while the inverse relationship was found at 8°C. Low levels of abscisic acid in P. formosus-associated plants after 38°C treatments revealed stress tolerance compared with control and 8°C-treated plants. In contrast, salicylic acid was pronounced in endophyte-treated plants after low-temperature stress as compared to other treatments. The results provide evidence that the response to P. formosus association was beneficial at normal growth temperature and had varying effects in response to temperature stress.     

Phytophthora cryptogea root rot of tomato in rock woo] nutrient culture. II. Effect of root zone temperature on infection, sporulation and symptom development

The effect of root-zone temperature on Phytophthora cryptogea root rot was studied in tomato cv. Counter grown under winter and summer conditions in rockwool culture. A nutrient temperature of 25°C resulted in increased root initiation and growth, higher in winter-grown than in summer-grown plants. Rhizosphere zoospore populations were greatly reduced at 25°C and above. Growth of P. cryptogea in vitro was optimal between 20°C and 25°C and completely suppressed at 30°C. Encystment was enhanced by increased temperatures above 20°C. Zoospore release in vitro occurred in cultures maintained at constant temperatures in the absence of the normal chilling stimulus. Optimal release was at 10°C; no zoospores were released at 30°C. Inoculated, winter-grown tomato plants maintained at 15°C developed acute aerial symptoms and died after 21 days. Comparable plants grown at a root-zone temperature of 25°C remained symptomless for the 3-months duration of the experiment. Summer-grown infected plants at the higher root temperature wilted but did not die. Enhanced temperature was ineffective as a curative treatment in summer-grown plants with established infection. Aerial symptoms of Phytophthora infection are seen as a function of the net amount of available healthy root. With high root zone temperatures this is determined by new root production and decreased inoculum and infection.     

Acclimation of photosynthesis, H2O2 content and antioxidants in maize (Zea mays) grown at sub-optimal temperatures

Maize plants were grown at 14, 18 and 20 °C until the fourth leaf had emerged. Leaves from plants grown at 14 and 18 °C had less chlorophyll than those grown at 20 °C. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was decreased at 14 °C compared with 20 °C, but the activation state was highest at 14 °C. Growth at 14 °C increased the abundance (but not the number) of Rubisco breakdown products. Phosphoenolpyruvate carboxylase (PEPC) activity was decreased at 14 °C compared with 20 °C but no chilling-dependent effects on the abundance of the PEPC protein were observed. Maximal extractable NADP-malate dehydrogenase activity increased at 14 °C compared with 20 °C whereas the glutathione pool was similar in leaves from plants grown at both temperatures. Foliar ascorbate and hydrogen peroxide were increased at 14 °C compared with 20 °C. The foliar hydrogen peroxide content was independent of irradiance at both growth temperatures. Plants grown at 14 °C had decreased rates of CO₂ fixation together with decreased quantum efficiencies of photosystem (PS) II in the light, although there was no photo-inhibition. Growth at 14 °C decreased the abundance of the D1 protein of PSII and the PSI psaB gene product but the psaA gene product was largely unaffected by growth at low temperatures. The relationships between the photosystems and the co-ordinate regulation of electron transport and CO₂ assimilation were maintained in plants grown at 14 °C.     

Symbiotic growth of indigenons white clover (Trifolium repens) with local Rhizobium leguminosarum biovar trifolii

To study a possible adaptation of the symbiosis between white clover (Trifolium repens L.) and Rhizobium leguminosarum biovar trifolii with regard to light and temperature at northern latitudes, local seed populations of white clover and isolates of R. leguminosarum biovar trifolii from 3 different latitudes in Norway, 58°48'N, 67°20'N and 69°22'N, were used. The commercial cultivar Undrom was used as a reference plant. The experiments were done at 18 and 9°C under controlled conditions in a phytotron during the natural growing season at 69° 39'N. Growth of the plants was evaluated by number and size of leaves, dry matter production and total N-content. At 18°C the white clover plants were harvested twice while at 9°C there was only one growth period. The results from first harvest at 18°C and total growth at 9°C, showed that white clover populations from northern Norway had a lower growth potential than the population from the south and cv. Undrom. This difference was not apparent in the second growth period at 18°C. Growth of the plants from seeds to first harvest was enhanced by mineral nitrogen compared to plants dependent on Rhizobium only. However, after a second growth period dry weight and total nitrogen content of the plants with nitrogen fixation were comparable to the plants receiving mineral nitrogen. Statistical analysis showed that the most important factor for the variation in dry matter production was the plant population. Within the populations at 9°C and at first harvest at 18°C, there were no significant differences in dry matter production with different Rhizobium inoculum. In the second growth period at 18°C, different inoculum gave significantly different amount of dry matter within a population. The results showed a significant interaction between plant population and Rhizobium inoculum, and the results indicated that plants from the north gave higher yield when nodulated by Rhizobium from the north than from the south.     

Differential responses of Arabidopsis ecotypes to cold, chilling and freezing temperatures

Arabidopsis plants show an increase in freezing tolerance in response to exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. In the present study, we evaluated the physiological and morphological responses of various Arabidopsis ecotypes to continuous growth under chilling (14°C) and cold (6°C) temperatures and evaluated their basal freezing tolerance levels. Seedlings of Arabidopsis plants were extremely sensitive to low growth temperatures: the hypocotyls and petioles were much longer and the angles of the second pair of true leaves were much greater in plants grown at 14°C than in those grown at 22°C, whereas just intermediate responses were observed under the cold temperature of 6°C. Flowering time was also markedly delayed at low growth temperatures and, interestingly, lower growth temperatures were accompanied by longer inflorescences. Other marked responses to low temperatures were changes in pigmentation, which appeared to be both ecotype specific and temperature dependent and resulted in various visual phenotypes such as chlorosis, necrosis or enhanced accumulation of anthocyanins. The observed decreases in chlorophyll contents and accumulation of anthocyanins were much more prominent in plants grown at 6°C than in those grown at 14°C. Among the various ecotypes tested, Mt‐0 plants markedly accumulated the highest levels of anthocyanins upon growth at 6°C. Freezing tolerance examination revealed that among 10 ecotypes tested, only C24 plants were significantly more sensitive to subzero temperatures. In conclusion, Arabidopsis ecotypes responded differentially to cold (6°C), chilling (14°C) and freezing temperatures, with specific ecotypes being more sensitive in particular traits to each low temperature.     

Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT₅₀) – the net photosynthetic rate at an ambient CO₂ of 33 Pa (P_n33), the dependencies of P_n to light and to CO₂ and the room temperature chlorophyll a fluorescence were recorded under optimal conditions. Herbaceous plants acquired freezing tolerances to temperatures between ?10 and ?15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non-senescing leaves P_n33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around ?10°C Brassica survived down to ?24°C, but P_n33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C P_n33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to ?36°C: P_n33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at ?10°C induced a tolerance of ?32 to n33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts. Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative P_n33.     

Development of Ramularia Leaf Spot on Sugar Beet as Influenced by Temperature and the Age of the Host Plant

A method of inoculating sugar beet plants (Beta vulgaris L.) with Ramularia beticola Faut. & Lamb, is described. Following inoculation, disease development in relation to temperature and plant age was studied for more than a month. The incubation period was 18 days at 10°C compared to 14 days at 17°C. At 25°C no symptoms appeared. Both temperature and plant age significantly influenced disease level and rate of disease development. Plants incubated at 17°C were more severely diseased 33 days after inoculation than plants incubated at 10°C. Young plants (3 weeks at inoculation) Were more susceptible than older plants (5 and 8 weeks at inoculatson) under growth chamber conditions. In the field, symptoms of Ramularia leaf spot appear relatively late in the season and young leaves are rarely attacked. The inconsistency of these observations is discussed.     

Effects of water potential and temperature on the development of eyespot lesions in wheat

Growth of Pseudocercosporella herpotrichoides on potato dextrose agar at water potentials from -0.5 to -6.9 MPa was optimal at 20°C. At 12 and 20°C, six isolates of P. herpotrichoides grew more rapidly at -0.5 to -2 MPa than at -6.9 MPa. Wheat plants inoculated with P. herpotrichoides and grown in columns of soil at either 15 or 20°C developed more severe eyespot lesions under a heavy watering regime than under medium or light watering regimes. P. herpotrichoides penetrated leaf sheaths of inoculated plants grown in compost more rapidly at 10°C night/15°C day temperatures than at 5/10°C; death of leaf sheaths was also more rapid and consequently there were fewer living infected leaf sheaths at 10/15°C than at 5/10°C. Irrigating for 5 wk before harvest increased the severity of eyespot lesions in a 1983 wheat crop.     

Effects of root temperature on leaf gas exchange and xylem sap abscisic acid concentrations in six Cucurbitaceae species

Roots of six Cucurbitaceae species were exposed to low (14 °C), middle (24 °C), and high (34 °C) temperatures while aerial parts of plants were maintained at ambient temperatures between 23 and 33 °C. The highest dry mass (DM), photon-saturated rate of net photosynthesis (P _Nsat), and stomatal conductance (g _s) were found at 14 °C in figleaf gourd and turban squash plants, at 24 °C in cucumber and melon plants, while bitter melon and wax gourd plants had lower DM, P _Nsat, and g _s at 14 °C than at 24 or 34 °C. Sub-or supra-optimum root temperatures did not induce photoinhibition but induced slight changes in the quantum efficiency of photosystem 2, PS2 (Φ_PS2) and photochemical quenching (q_p). Meanwhile, xylem sap abscisic acid (ABA) concentration followed a contrasting change pattern to that of g _s. Thus the change in P _Nsat was mainly due to the change in g _s and roots played an important role in the regulation of stomatal behaviour by delivering increased amount of ABA to shoots at sub-or supra-optimum root temperatures.     

Adaptation of cucurbit species to changes in substrate temperature: Root growth,antioxidants, and peroxidation

To investigate their response to changes in substrate temperatures, the roots from six species of cucurbit plants were exposed to 14°C, 24°C, or 34°C, while their aerial portions were maintained at natural ambient temperatures (23°C to 33°C). These species could be classified into three groups based on their stress response: Group A,Cucurbita ficifolia and C.maxima, heatsensitive but cold-tolerant; Group B,Cucumis sativus and C.melo, heat- and cold-sensitive; and Group C,Luffa cylindrica andBenincasa hispida, heat-tolerant but cold-sensitive. The highest growth rates and lowest contents of malondialdehyde (MDA) for plants in Groups A, B, and C were achieved at temperatures of 14°C, 24°C, and 24°C to 34°C, respectively. Superoxide dismutase (SOD) activity was lowest in the roots exposed to optimal growth temperatures while activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (G-POD) operated coordinately in a complicated manner to prevent the accumulation of reactive oxygen species (ROS) in the root cells. Moreover, all plants, regardless of species, responded to unfavorable temperatures by increasing their synthesis of ascorbate and glutathione as well as by reducing the redox ratio of those two important antioxidants.     

Airborne limonene confers limited thermotolerance to Quercus ilex

The purpose of the study was to test the possible and controversial thermotolerance role of monoterpene production and emission and the related responses of antioxidants. Quercus ilex seedlings were exposed to a ramp of temperatures of 5°C steps from 25 to 50°C growing with and without limonene fumigation (7.5 µl l⁻¹). Net photosynthetic rates, maximal photochemical efficiency of PSII (F_v/F_m), oxidation state of ascorbic acid, and lipid peroxidation estimated by malondialdehyde concentrations of limonene-fumigated (LF) plants did not significantly differ from control (C) plants. No consistent changes in emissions of the other monoterpenes, α-pinene, β-phellandrene, β-pinene or β-myrcene were found. However, slight differences were found in the concentration of antioxidants. The amounts of α-tocopherol did not change or even tended to decrease at high temperatures in LF plants whereas they tended to increase by approximately 60% at 45 and 50°C relative to 25°C in C plants. Ascorbic acid reached its maximum concentration only at 45°C in LF plants whereas it reached its maximum at 35°C in C plants. β-Carotene did not decrease at high temperatures in LF plants whereas it decreased by approximately 15% at 45–50°C in C plants. Brown pigment index (BPI), an optical indicator of tissue oxidative processes, was lower in LF plants than in C plants. The photochemical reflectance index (PRI), an optical indicator of photosynthetic light use efficiency, was higher for LF plants than for C plants at elevated temperatures. Visual leaf damage (browning) tended to be less in LF plants than in C plans although not significantly (26.5 ± 8.5 versus 16.2 ± 4.8%). These results show that limonene does not confer clear and strong thermotolerance but might have some minor role. These results are in agreement with previous indications of weaker thermotolerance effect of monoterpenes than of isoprene.     

Effects of low temperature on growth and non-structural carbohydrates of the imperial bromeliad <Emphasis Type="Italic">Alcantarea imperialis</Emphasis> cultured in vitro

The imperial bromeliad Alcantarea imperialis grows naturally on rocky outcrops (‘inselbergs’) in regions where daily temperatures vary from 5 to 40°C. As carbohydrate metabolism is altered in response to cold, it could lead to reprogramming of the metabolic machinery including the increase in levels of metabolites that function as osmolytes, compatible solutes, or energy sources in order to maintain plant homeostasis. The aim of this study was to evaluate the effects of different temperatures on plant growth and non-structural carbohydrates in plants of A. imperialis adapted to low temperature. Seedlings of A. imperialis were grown in vitro under a 12-h photoperiod with four different day/night temperature cycles: 5/5°C, 15/15°C, 15/30°C (dark/light) and 30/30°C. Plants were also cultivated at 26°C in ex vitro conditions for comparison. The results showed an inverse relationship between temperature and germination time and no differences in the percentage of germination. Plants maintained for 9 months at 15°C presented a reduced number of leaves and roots, and a dry mass four times lower than plants grown at 30°C. Sugar content was higher in plants grown at 15°C than at 30°C. However, the highest amount of total sugar was found in plants growing under warm day/cold night conditions. Myo-inositol, glucose, fructose and sucrose were found predominantly under high temperatures, while under low temperatures, sucrose was apparently replaced by trehalose, raffinose and stachyose. Starch content was highest in plants grown under high temperatures. The lowest starch content was detected under low temperatures, suggesting its conversion into soluble carbohydrates to protect the plants against cold. These results indicated that low temperature retarded growth of A. imperialis and increased sugar levels, mainly trehalose, thus suggesting that these sugar compounds could be involved in cold tolerance.     

Response of transgenic rape plants bearing the <Emphasis Type="Italic">Osmyb4</Emphasis> gene from rice encoding a trans-factor to low above-zero temperature

Accumulation of soluble sugars (sucrose, fructose, and glucose), proline, phenols (total phenols and flavonoids), and antocyanins during adaptation to low-temperature stress (4°C) of two lines of spring rape (Brassica napus L., cv. Westar) characterized by weak (Bn-1) and strong (Bn-3) expression of the Osmyb4 transgene was studied. Vegetatively propagated transgenic and wild-type plants were grown in the hydroponic culture at 24°C; at the stage of 5–6 leaves, plants were exposed to 4°C for 5 days and then returned to the optimum temperature of 24°C for recovery. Transgenic plants were established to manifest improved cold and frost tolerance, which was evident from more active biomass accumulation at 4°C as compared with wild-type plants and from sustaining their viability after 2-day-long exposure to −6°C. Determination of MDA content showed that one of the reasons of their improved cold tolerance was their capability of maintaining oxidative homeostasis under low-temperature stress. This suggestion is supported by intense accumulation of phenols and antocyanins, manifesting pronounced antioxidant effects, by transgenic plants during their cold adaptation. Thus, during 2–5 days of plant exposure to 4°C, in transgenic plants the total content of phenols increased by 2.6–3.7 times, flavonoids — by 3.7–4.7 times, and antocyanins — by 3.5–5.3 times as compared with control plants growing at 24°C. Transgenic Bn-3 plants with strong expression of the Osmyb4 gene accumulated phenols and antocyanins at 4°C more actively than Bn-1 plants characterized by weak expression of this gene. Transgenic rape plants subjected to cold stress accumulated more proline, manifesting stress-protection effects, and lesser accumulation of soluble sugars. Before the beginning of experiment, the content of soluble sugars was approximately similar in wild-type plants and transgenic lines; at 4°C their level in transgenic plants was substantially lower than in control plants. As distinct from the process of cold adaptation, during recovery, the content of all tested stress-protection compounds dropped sharply. The results obtained indicate that active expression of the Osmyb4 gene from rice in the rape plants was accompanied not only by accumulation of compatible osmolytes but also by biosynthesis of antioxidants of phenolic nature.     

In vitro production of Indian citrus ringspot virus (ICRSV) free Kinnow plants employing thermotherapy coupled with shoot tip grafting

Indian citrus ringspot virus (ICRSV) is known to cause serious disease problem in Kinnow (Citrus nobilis Lour × C. deliciosa Tenora) plants. This work reports the elimination of ICRSV by using thermotherapy coupled with shoot tip grafting in vitro. Nodal segments from infected mother plants (indexed by indirect ELISA and RT-PCR) were treated both in water bath and moist hot air at different temperatures viz. 40, 45 and 50°C for 30, 60 and 120 min and cultured on MS medium containing 2-iP (1 mg/l) and malt extract (800 mg/l). Shoot tips were excised from the nodal sprouts and grafted on to rough lemon (C. jambhiri) rootstock under aseptic conditions. Water bath treatment was found to be more effective as compared to moist hot air treatment as maximum number of ICRSV free plants (36.84%) were obtained by grafting the tips (0.7 mm) taken from the nodal segments treated at 50°C in water bath for 2 h. In an alternate treatment regime, 1-year-old infected plants were kept at various temperatures viz.36, 38 and 40°C in a thermotherapy chamber. Maximum of 60% ICRSV free plants were obtained by grafting the tips (0.7 mm) from the plants placed at 40°C followed by the plants placed at 38°C (59.09%) and the least was observed in case of the plants placed at 36°C (40.74%). Only those plants/plantlets were considered virus free, which showed negative reaction both in Indirect ELISA and RT-PCR.     

Effects of Temperature on Disease Severity in Plants of Subterranean Clover Infected Singly or in Mixed Infection with Bean yellow mosaic virus and Kabatiella caulivora

Many epidemics involve plants infected with more than one pathogen, but few experiments address climate change scenarios that influence mixed infections. This study addresses the interactive effects of co‐infection and temperature on disease development in plants of the annual pasture species subterranean clover (Trifolium subterraneum), which is widely sown in different world regions. Bean yellow mosaic virus (BYMV) and the fungus Kabatiella caulivora are two important pathogens causing considerable production losses in pastures containing this species. Both occur together in such pastures causing a severe necrotic disease when mixed infection occurs. Effects of temperature on symptom expression were investigated in subterranean clover plants infected singly or in mixed infection with these pathogens. Plants were maintained in controlled environment rooms at 18°C, 20°C or 22.5°C after sap inoculation with BYMV. K. caulivora conidia suspensions were inoculated to plants once systemic BYMV symptoms developed. Plants were assessed for three disease assessment parameters, dead petioles numbers, marginal leaflet necrosis and overall plant damage. In general, mixed infection caused most severe symptoms, K. caulivora least severe symptoms, and BYMV symptoms of intermediate severity. In single infections, effects of temperature on disease severity differed between pathogens: BYMV symptoms were most pronounced at 18°C, but K. caulivora induced more severe symptoms at 20°C and 22.5°C. In mixed infections, disease severity generally followed the pattern developed with BYMV alone as temperature increased. Also, synergistic increase in disease severity sometimes occurred at 18°C, but increases were only additive at 20°C and 22.5°C. These results reflected the greater BYMV multiplication detected in infected leaves at 18°C compared with 20°C or 22.5°C. Our findings indicate that in rainfed subterranean clover pastures, as global warming progresses disease severity from infection with BYMV and K. caulivora alone may decline or increase, respectively, and mixed infection with them may become less damaging.     

Influence of Drought,High Temperature,and Carbamide Cytokinin 4-PU-30 on Photosynthetic Activity of Bean Plants. 1. Changes in Chlorophyll Fluorescence Quenching

Fifteen-day-old bean plants (Phaseolus vulgaris L.) grown in a climatic chamber were exposed to water deficit (WD) and high temperature (HT) stresses applied separately or in combination. Changes in chlorophyll fluorescence quenching were investigated. Bean plants that endured mild (42 °C, 5 h for 2 d) WD separately or in combination with HT did not change their q_P and q_N quenching (measured at 25 °C) compared with those of the control. After 5 min testing at 45 °C, q_P in control and droughted plants strongly decreased, while q_P of plants that experienced combined WD+HT stress was insignificantly influenced, suggesting the acclimation effect of HT treatments. At more severe stresses (after 3 d-treatment), q_P measured at 25 °C was the lowest in WD+HT plants and q_N values were the highest. But when measured at 45 °C, q_P of WD+HT plants had practically the same values as at 25 °C. Under these conditions q_P of WD plants also showed an adaptation to HT. Twenty-four hours after recovery, the unfavourable effects of the stresses were strongly reduced when measured at 25 °C, but they were still present when measured at 45 °C. Positive effect of the carbamide cytokinin 4-PU-30 was well expressed only in droughted plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

An analysis of photosynthetic response and adaptation to temperature in higher plants: temperature acclimation in the desert evergreen Nerium oleander L*

Abstract. Factors underlying the process of photosynthetic acclimation to temperature were investigated for the shrub Nerium oleander L. Ramets of a single clone were grown under day/night temperature regimes of 20°C/15°C or 45°C/32°C. Plants grown at the lower temperature regime possessed rates of photosynthesis twice that of the high-temperature grown plants when CO₂ fixation was measured at 20°C. In contrast, the plants grown at the high-temperature regime had twice the rate of CO₂ fixation of the 20°C/l 5°C-grown plants at a measurement temperature of 45° C. It was determined that the ability to acclimate to changes in temperature regime was present in fully mature leaves. A reciprocal transfer of plants between the two growth regimes resulted in the appearance of the CO₂ fixation characteristics appropriate to the new growth temperature after 12–14d. The response of CO₂ fixation to light, temperature, and CO₂ partial pressure and the temperature responses of soluble and membrane-bound photosynthetic enzyme systems were analysed to determine which components might be responsible for the superior photosynthetic performance of the 20°C/I5°C-grown plants at 20°C, and the enhanced high-temperature stability of the 45°C/32°C plants. The measured photosynthetic capacity of the 20°C/15°C plants could not be attributed to gross morphological, stomatal, or other physical changes, or to a general increase in the concentration of components of the photosynthetic process. Only a single enzyme, Fru-P₂ phosphatase, was affected to an extent similar to that of photosynthesis. The enhanced thermal stability of the 45°C/32°C plants may be attributed primarily to an enhanced stability of the chloroplast membrane-bound enzymatic activities and the stability of the photosynthetic carbon metabolism enzymes which require lighl for activation.     

Photosynthetic performance at low temperature of Bouteloua gracilis Lag., a high-altitude C4 grass from the Rocky Mountains, USA

The mechanisms controlling the photosynthetic performance of C₄ plants at low temperature were investigated using ecotypes of Bouteloua gracilis Lag. from high (3000 m) and low (1500 m) elevation sites in the Rocky Mountains of Colorado. Plants were grown in controlled‐environment cabinets at a photon flux density of 700 μ mol m^?2 s^?1 and day/night temperatures of 26/16 °C or 14/7 °C. The thermal response of the net CO₂ assimilation rate (A) was evaluated using leaf gas‐exchange analysis and activity assays of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate,orthophosphate dikinase (PPDK). In both ecotypes, a reduction in measurement temperature caused the CO₂‐saturated rate of photosynthesis to decline to a greater degree than the initial slope of A versus the intercellular CO₂ response, thereby reducing the photosynthetic CO₂ saturation point. As a consequence, A in normal air was CO₂‐saturated at sub‐optimal temperatures. Ecotypic variation was low when grown at 26/16 °C, with the major difference between the ecotypes being that the low‐elevation plants had higher A; however, the ecotypes responded differently when grown at cool temperature. At temperatures below the thermal optimum, A in high‐elevation plants grown at 14/7 °C was enhanced relative to plants grown at 26/16 °C, while A in low‐elevation plants grown at 14/7 °C was reduced compared to 26/16 °C‐grown plants. Photoinhibition at low growth temperature was minor in both ecotypes as indicated by small reductions in dark‐adapted F_v/F_m. In both ecotypes, the activity of Rubisco was equivalent to A below 17 °C but well in excess of A above 25 °C. Activities of PEPCase and PPDK responded to temperature in a similar proportion relative to Rubisco, and showed no evidence for dissociation that would cause them to become principal limitations at low temperature. Because of the similar temperature response of Rubisco and A, we propose that Rubisco is a major limitation on C₄ photosynthesis in B. gracilis below 17 °C. Based on these results and for theoretical reasons associated with how C₄ plants use Rubisco, we further suggest that Rubisco capacity may be a widespread limitation upon C₄ photosynthesis at low temperature.     

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.