<Emphasis Type="Italic">CDPK</Emphasis> gene expression in somatic embryos of <Emphasis Type="Italic">Panax ginseng</Emphasis> expressing <Emphasis Type="Italic">rolC</Emphasis>

A somatic embryogenesis protocol for plant regeneration of northern red oak (Quercus rubra) was established from immature cotyledon explants. Embryogenic callus cultures were induced on Murashige and Skoog medium (MS) containing 3% sucrose, 0.24% Phytagel™, and various concentrations of 2,4-dichlorophenoxyacetic acid (2,4-d) after 4 weeks of culture in darkness. A higher response (66%) of embryogenic callus was induced on 0.45 μM 2,4-d. Higher numbers of globular- (31), heart- (17), torpedo- (12), and cotyledon-stage (8) embryos per explant were obtained by culturing embryogenic callus on MS with 3% sucrose, 0.24% Phytagel™, and devoid of growth regulators after 8 weeks culture in darkness. Continuous sub-culturing of embryogenic callus on medium containing 2,4-d yielded only compact callus. Desiccation of embryos for 3 days in darkness at 25 ± 2°C followed by cold storage at 4°C in darkness for 8 weeks favored embryo germination and development of plantlets. Cotyledon-stage embryos subjected to desiccation and chilling treatment cultured on MS with 3% sucrose, 0.24 Phytagel™, 0.44 μM 6-benzylaminopurine (BA), and 0.29 μM gibberellic acid germinated at a higher frequency (61%) than with 0.44 μM BA alone and control cultures. Germinated plantlets developed a shoot and root, were acclimatized successfully, and maintained in a growth room for plantlet development.     

Ethylene biosynthesis in a chilling-sensitive<Emphasis Type="Italic">Arabidopsis</Emphasis> mutant,<Emphasis Type="Italic">chs4-2</Emphasis>

We investigated chilling-induced changes in ethylene levels in Arabidopsis to find plants with distinct patterns of ethylene production in the cold-related biosynthetic pathway. The sensitive mutants identified here includedchs1-2,chs4-2, andchs6-2. Among these, plants of thechs4-2 mutant produced more ethylene than did the wild type after both were transferred from 4°C or 10°C to 22°C. This mutant also showed less freezing tolerance and more electrolyte leakage than the wild-type plants. Our results suggest a relationship between ethylene biosynthesis and chilling sensitivity in the mutant To determine which of the enzymes involved in ethylene biosynthesis were induced by chilling, we tested the activities of ACC synthase and ACC oxidase in both mutant and wild-type plants, and found greater activity by ACC synthase as well as a higher ACC content in the mutants after all the plants were transferred from 10°C to 22°C. However, ACC oxidase activity did not differ between mutant and wild-type plants in response to chilling treatment Therefore, we conclude thatchs4-2 mutants produce more ethylene than do other mutants or the wild type during their recovery from chilling conditions. Furthermore, we believe that ACC synthase is the key enzyme involved in this response.     

Chilling-induced changes of vacuolar proton pumps in hypocotyls of <Emphasis Type="Italic">Vigna unguiculata</Emphasis>

Vigna unguiculata (cowpea) is a legume adapted to high temperatures and is sensitive to low temperatures. Temperature is one of the limiting factors of growth and yield for many crops but its effect on cowpea metabolism is not known. We investigated the effect of chilling on activity of vacuolar proton pumps (V-ATPase and V-PPase) and their protein content in tonoplast vesicles of cowpea hypocotyls. Seedlings grown for 7 days at 10 or 4°C were used for experiments. Chilling treatment at 10 or 4°C markedly suppressed growth of cowpea seedlings. Following chilling at 10 and 4°C, activity of both proton pumps and the relative amount of V-PPase and subunit A of V-ATPase were significantly increased. Both substrate hydrolysis and H⁺ transport activities of V-PPase remained at relatively high levels during chilling treatment. For V-ATPase, treatment at 10°C for 6 days increased the ATP hydrolysis activity. However, the H⁺ transport activity of the enzyme was increased when treated for 4 days but was markedly decreased when treated for 6 days. Our results provide evidence for different regulation for these vacuolar proton pumps, indicating that V-PPase is the more stable proton pump throughout chilling stress.     

Benzyladenine Promotes Flowering in <Emphasis Type="Italic">Doritaenopsis</Emphasis> and <Emphasis Type="Italic">Phalaenopsis</Emphasis> Orchids

Two experiments were performed to determine how application of the cytokinin benzyladenine (BA) influenced flowering in Doritaenopsis and Phalaenopsis orchid clones. In the first experiment, two vegetative orchid clones growing in 15-cm pots were transferred from a 28°C greenhouse that inhibited flowering to a 23°C greenhouse for flower induction (day 0). A foliar spray (0.2 L m⁻²) containing BA at 100, 200, or 400 mg L⁻¹ or 25, 50, or 100 mg L⁻¹ each of BA and gibberellins A₄ + A₇ (BA+GA) was applied on days 0, 7, and 14. Plants treated with BA alone at 200 or 400 mg L⁻¹ had a visible inflorescence 3–9 days earlier and had a mean of 0.7–3.5 more inflorescences and 3–8 more flowers per plant than nontreated plants. The application of BA+GA had no effect on inflorescence number and total flower number at the rates tested. In the second experiment, three orchid clones received a single foliar spray of BA at 200 mg L⁻¹ at six time points relative to time of transfer from 29°C to 23°C (−1, 0, +1, +2, +4, or +6 weeks). A separate group of plants received a BA application at week 0 but was maintained at 29°C. Inflorescence number was greatest in all three orchid clones when plants were treated with BA 1 week after the temperature transfer. Plants that were sprayed with BA and maintained at 29°C did not initiate inflorescences. The promotion of flowering by the application of BA suggests that cytokinins at least partially regulate inflorescence initiation of Doritaenopsis and Phalaenopsis, but its promotion is conditional and BA application cannot completely substitute for an inductive low temperature.     

Activities of photosystem II and antioxidant enzymes in chickpea (<Emphasis Type="Italic">Cicer arietinum</Emphasis> L<Emphasis Type="Italic">.</Emphasis>) cultivars exposed to chilling temperatures

This study was carried out to determine the effect of chilling on both cold-acclimated and non-acclimated chickpea (Cicer arietinum L.) cultivars (Gökçe and Can?tez 87). Chickpea seedlings grown in soil culture for 12 days were subjected to chilling temperatures (2 and 4°C for 12 days) after maintaining in cold-acclimation (10°C, 7 days) or non-acclimation (25°C, 7 days) periods. The lowest values of growth parameters were obtained with cold-acclimated plants, whereas non-acclimated plants exhibited the lowest water content values, especially at 2°C. There was no effect of cold-acclimation period on chlorophyll fluorescence parameters. Plants subjected to chilling temperatures after cold-acclimation were more tolerant with respect to chlorophyll fluorescence parameters, and Gökçe had better photosystem II (PSII) photochemical activity. In the chilling treatments, total chlorophyll (a + b) content reduced, especially at 2°C, while anthocyanin and flavonoid contents increased to a greater extent in Gökçe and carotenoid content of the cultivars did not change. Malondialdehyde (MDA) content was higher for Can?tez 87, mostly at 2°C, while proline accumulation was greater for Gökçe. The cold-acclimation period led to a remarkable increase in antioxidant enzyme activities of both cultivars. The superoxide dismutase (SOD) activity was much higher in Gökçe for both chilling temperatures and the ascorbate peroxidase (APX) activity increased only in the cold-acclimated 4°C treatments. Similarly, with APX activity, the glutathione reductase (GR) and peroxidase (POD) activities of cultivars were higher in cold-acclimated plants at both the chilling temperatures, mostly in Gökçe. The results of this study indicate that cold-acclimation increased the cultivars ability to withstand the chilling temperatures. The lower MDA content and higher antioxidant and photochemical activities in Gökçe indicated an enhanced chilling tolerance capacity of this cultivar to protect the plant from oxidative damage.     

Seed size and seed germination in the Mediterranean fire-prone shrub <Emphasis Type="Italic">Cistus </Emphasis><Emphasis Type="Italic">ladanifer</Emphasis>

The germination response of different sized seeds from individuals of a Mediterranean fire-prone shrub (Cistus ladanifer) was investigated in relation to pre-germination heating. A control (no heating), a low temperature during a short exposure time (50°C during 5 min), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, low intensity pasture fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Mean seed weight only showed a marginally significant positive correlation with the proportion of germinated seeds whatever the pre-germination treatment. These results suggest that seed dormancy is unrelated to seed size and that under the experimental conditions used in this study, the effect of seed size on seed germination is low. Nevertheless, larger seeds could be favoured in natural conditions, especially under the high competition scenario which arise after wildfires. Control seeds showed a negative correlation between seed size and germination velocity suggesting that lighter seeds could take advantage from early germination in recruitment events in the absence of wildfires. Nevertheless, even the lower pre-germination heating treatment turns this correlation in not significant, suggesting a strong selection pressure (unrelated to seed size) for early germination after fire events. In our study, different sized seeds of C. ladanifer seem to perform better under different germination scenarios suggesting that seed size variation could be maintained by the alternation of recruitments without wildfires and recruitments after wildfire events.     

A comparative study on the protective role of trehalose and LEA proteins against abiotic stresses in transgenic Chinese cabbage (<Emphasis Type="Italic">Brassica campestris</Emphasis>) overexpressing<Emphasis Type="Italic">CaLEA</Emphasis> or<Emphasis Type="Italic">otsA</Emphasis>

Trehalose and LEA proteins, representative low MW chemicals that are synthesized under dehydration, are known to protect plants from drought stress. To compare their effectiveness on enhancing tolerance against various abiotic stresses, we generated transgenic Chinese cabbage plants overexpressingE. ctdi trehalose-6-phosphate synthase gene (otsA) or hot pepper (Capsicum annuum) LEA protein gene(CaLEA). Both transgenic plants exhibited altered phenotype including stunted growth and aberrant root development When subjected to drought, salt or heat stress, these plants showed remarkably improved tolerance against those stresses compared with nontransformants. After dehydration treatment, leaf turgidity and fresh weight was better maintained in both transgenic plants. GaUEA-plants performed somewhat better under dehydrated condition. When treated with 250 mM NaCI, both otsA-plants and CaLEA-plants remained equally healthier than nontransformants in maintaining leaf turgidity and delaying necrosis. Furthermore, leaf Chi content and Fv/Fm was maintained considerably higher in both transgenic plants than nontransformants. After heat-treatment at 45°C, both transgenic plants appeared much less damaged in external shape and PSII function, but LEA proteins were more protective. Our results indicate that although both trehalose and LEA proteins are effective in protecting plants against various abiotic stresses, LEA proteins seem to be more promising in generating stress-tolerant transgenic plants.     

Germination of mature seeds of <Emphasis Type="Italic">Calanthe tricarinata</Emphasis> Lindl., an endangered terrestrial orchid,by asymbiotic culture <Emphasis Type="Italic">in vitro</Emphasis>

The effects of culture conditions on the asymbiotic germination of mature seeds of Calanthe tricarinata Lindl., an endangered terrestrial cool-climate orchid, were examined. Specifically, conditions such as illumination, temperature, and the addition of plant growth regulators to the medium were studied. Mature seeds were harvested from plants that had been collected in Toyama Prefecture, Japan, and maintained at the Botanic Gardens of Toyama. Solidified “New Dogashima” medium was used as the basal medium, and it was supplemented with 6-benzyladenopurine (BA) or α-naphthalene acetic acid (NAA). White light at 40 μmol m⁻² s⁻¹, with a 16-h photoperiod, inhibited the germination of seeds by 53–80%, as compared to dark controls in genotypes examined. The optimal temperature for the germination of seeds in darkness was 20°C and the germination frequency reached 60%, whereas it was only 28% at 25°C. While both NAA and BA stimulated germination, BA was more effective than NAA. After storage for 18 mo at 5°C, seeds incubated on medium that contained 0.2 mg l⁻¹ BA germinated at a frequency of 36%, which was twice that of seeds grown without any plant growth regulators. The frequency of subsequent germination decreased during storage of seeds at 5°C for approximately 2 yr, dropping from 61% to 13%. The protocorms obtained in this study were developed to plantlets readily after transferring to fresh 1/2 MS medium without any plant growth regulators. They were successfully acclimatized in green house after two to three subcultures in vitro. The significant role of a reproducible protocol for the germination of mature seeds is discussed in terms of the ex situ conservation of endangered orchid species.     

Effects of low night temperature on pigments,chl <Emphasis Type="Italic">a</Emphasis> fluorescence and energy allocation in two bitter gourd (<Emphasis Type="Italic">Momordica charantia</Emphasis> L.) genotypes

Using two different inbred lines of Momordica charantia (bitter gourd), Y-106-5 and Z-1-4, the cell membrane stability, leaf water potential, pigment contents and the chlorophyll a fluorescence were investigated with different low night temperature (LNT) treatments over a 7 day time period and the sequent a 7 day recovery. Under LNT treatments, electrolyte leakage increased in both inbred lines and it increased more significantly in Y-106-5 plants than that in Z-1-4. The content of Chl b and total Chl decreased, while the Chl a/b ratio increased in stressed plants of the two lines. Almost all LNT treatments induced little change in Chl a content in Z-1-4 whereas obvious decreases in 5 and 8°C treated Y-106-5 plants were observed. Chilling changed the water status of plants and induced decreases of leaf water potential (LWP) in 5 and 8°C treated plants. LNT treatments also resulted in changes in the chlorophyll fluorescence parameters in bitter gourd leaves. The potential PSII activity (F _v/F _o) was reduced obviously by LNT stress and showed more sensitive to LNT than the maximum quantum efficiency of PSII primary photochemistry (F _v/F _m). The efficiency of open PSII centers exhibited a slight decrease whereas the photochemical quenching efficient (q _P) was affected more seriously by LNT stress in both two inbred lines. The allocation of energy was rearranged by LNT stress. The light fraction used for PSII photochemistry (P) was reduced, while that used for heat dissipation (D) and the third fraction of absorbed light defines excess energy (E) increased due to the chilling stress. The impacts of LNT stress on bitter gourd generally increased with the number of LNT chilling and the severe night chilling. Plants were little affected by 12°C night chilling and the most acute damage was found in 5°C night chilling treatments. A 7 day recovery mitigated the adverse effects of LNT for both lines and almost all LNT treated plants restored to control levels except 5°C night chilling treated Y-106-5 plants. The two lines have a variance in tolerance to LNT stress and display obvious differences of phenotypes under extreme conditions.     

Effect of temperature stress on the endogenous cytokinin content in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh plants

The levels of three endogenous cytokinin equivalents: zeatin (Z), iso-pentenyladenine (iP) and dihydrozeatin (dZ) in two Arabidopsis thaliana (L.) Heynh genotypes — wild type (wt) and ethylene-insensitive mutant (eti5), were compared using enzyme immunoassay (ELISA). Cytokinin content was measured after exposure to low (4 °C for 24 h in darkness) or high temperature (38 °C for 24 h in darkness). Measurements were performed immediately and 24, 48 and 120 h after treatments. It was found that at normal growth conditions eti5 plants contained more endogenous cytokinins compared to the wild type. At both temperature treatments mutant plants had decreased total cytokinin levels. Wild-type plants treated with high temperature (HT) exhibited reduced total cytokinins (with the exception of rates at 48 h), while low temperature (LT) treatment resulted in elevated total amount of the studied equivalents (except at 24 h). The obtained results suggested that HT had greater effect on cytokinin levels than LT since it caused more profound changes in the total content. We assume that this was due to the natural chilling tolerance of Arabidopsis plants.     

Effects of seed hydropriming in presence of exogenous proline on chilling injury limitation in <Emphasis Type="Italic">Vigna radiata</Emphasis> L. seedlings

Three-day-old seedlings (t ₀ stage) of Vigna radiata (L.) Wilczek obtained from seeds hydroprimed (H) and hydroprimed with proline (HPro) were examined. H and HPro slightly improved mung bean seed germination and seedlings growth at 5°C. The best growth was observed in the seedlings obtain from HPro5 (5 mM) seeds in comparison with the seedlings obtained from the control-non-primed seeds and H seeds. Exposure of mung bean seedlings grown from non-primed seeds to chilling for 4 days induced chilling injury: membrane lipid peroxidation, decrease in endogenous proline level and inhibition of growth of roots and hypocotyls. The seedlings obtain from HPro seeds grew better during the time of chilling and after rewarming at 25°C. The possible role of HPro in chilling injury limitation is discussed.     

Effects of SAG12-ipt expression on cytokinin production,growth and senescence of creeping bentgrass (<Emphasis Type="Italic">Agrostis stolonifera</Emphasis> L.) under heat stress

The study was conducted to determine the effects of expression of a transgene encoding adenine isopentenyl transferase (ipt), which controls cytokinin synthesis, on growth and leaf senescence of creeping bentgrass (Agrostis stolonifera L.), subjected to heat stress. Creeping bentgrass (cv. Penncross) was transformed with ipt ligated to a senescence-activated promoter (SAG12). Eight SAG12-ipt transgenic lines exhibiting desirable turf quality and a transgenic control line (transformed with the empty vector) were evaluated for morphological and physiological changes under normal growth temperature (20°C) and after 14 days of heat stress (35°C) in growth chambers. Six of the SAG12-ipt lines developed more tillers than the control line during establishment under normal growth temperature of 20°C. Following 14 days of heat stress, four of the SAG12-ipt lines had increased 65–83% of roots and for all six SAG12-ipt lines root elongation continued, whereas root production ceased and total root length decreased for the control line. Root isopentenyl adenine (iPA) content increased 2.5–3.5 times in five of the SAG12-ipt lines, whereas in the control line iPA decreased 20% after 14 days at 35°C. Total zeatin riboside (ZR) content was maintained at the original level or increased in five of the SAG12-ipt lines, whereas in the control line ZR decreased under heat stress. Our results suggest expression of SAG12-ipt in creeping bentgrass stimulated tiller formation and root production, and delayed leaf senescence under heat stress, suggesting a role for cytokinins in regulating cool-season grass tolerance to heat stress.     

Desiccation of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> at high temperature

Haberlea rhodopensis plants, growing under low irradiance in their natural habitat, were desiccated to air-dry state at a similar light intensity (about 30 μmol m⁻² s⁻¹) under optimal (23/20°C, day/night) or high (38/30°C) temperature. Dehydration of plants at high temperature increased the rate of water loss threefold and had a more detrimental effect than either drought or high temperature alone. Water deficit decreased the photochemical activity of PSII and PSI and the rate of photosynthetic oxygen evolution, and these effects were stronger when desiccation was carried out at 38°C. Some reduction in the amount of the main PSI and PSII proteins was observed especially in severely desiccated Haberlea leaves. The results clearly showed that desiccation of the homoiochlorophyllous poikilohydric plant Haberlea rhodopensis at high temperature had more damaging effects than desiccation at optimal temperature and in addition recovery was slower. Increased thermal energy dissipation together with higher proline and carotenoid content in the course of desiccation at 38°C compared to desiccation at 23°C probably helped in overcoming the stress.     

Effect of temperature on the life history of <Emphasis Type="Italic">Eretmocerus</Emphasis> sp. nr. <Emphasis Type="Italic">furuhashii</Emphasis>, a parasitoid of <Emphasis Type="Italic">Bemisia tabaci</Emphasis>

Eretmocerus sp. nr. furuhashii (Hymenoptera: Aphelinidae) is an indigenous parasitoid of Bemisia tabaci (Gennadius)(Hemiptera: Aleyrodidae) from southern China; the effects of constant temperatures on the life history of E. sp. nr. furuhashii were examined in the laboratory. The developmental period ranged from 39.2 days at 20°C to 12.40 days at 32°C. A total of 263.4 degree-days were required to complete development with a lower developmental threshold temperature of 11.1°C. Of the eggs produced, 59.3% completed development at 20°C with completion increasing to 71.5% at 26°C. Adult female longevity was 10.8 days at 20°C and 5.2 days at 32°C while the mean daily offspring reproduced per female was highest at 29°C with 5.9 offspring. Adult oviposition peaked three days after emergence at 26, 29 and 32°C, and four days post-emergence at 20°C and 23°C. The total numbers of offspring produced per female ranged from 25.7 individuals at 32°C to 41.1 individuals at 20°C. The sex ratio had a female bias and ranged from 0.72 at 17°C to 0.51 at 35°C. The intrinsic rate of increase was 0.1727 at 29°C followed with 0.1606 at 32°C. Results indicated that E. sp. nr. furuhashii reaches its maximum biological potential at temperatures ranging from 26°C to 32°C.     

Characterization and genetic analysis of a low-temperature-sensitive mutant, <Emphasis Type="Italic">sy</Emphasis>-<Emphasis Type="Italic">2</Emphasis>, in <Emphasis Type="Italic">Capsicum chinense</Emphasis>

A temperature-sensitive mutant of Capsicum chinense, sy-2, shows a normal developmental phenotype when grown above 24°C. However, when grown at 20°C, sy-2 exhibits developmental defects, such as chlorophyll deficiency and shrunken leaves. To understand the underlying mechanism of this temperature-dependent response, phenotypic characterization and genetic analysis were performed. The results revealed abnormal chloroplast structures and cell collapse in leaves of the sy-2 plants grown at 20°C. Moreover, an excessive accumulation of reactive oxygen species (ROS) resulting in cell death was detected in the chlorophyll-deficient sectors of the leaves. However, the expression profile of the ROS scavenging genes did not alter in sy-2 plants grown at 20°C. A further analysis of fatty acid content in the leaves showed the impaired pathway of linoleic acid (18:2) to linolenic acid (18:3). Additionally, the Cafad7 gene was downregulated in sy-2 plants. This change may lead to dramatic physiological disorder and alteration of leaf morphology in sy-2 plants by losing low-temperature tolerance. Genetic analysis of an F₂ population from a cross between C. chinense ‘sy-2’ and wild-type C. chinense ‘No. 3341’ showed that the sy-2 phenotype is controlled by a single recessive gene. Molecular mapping revealed that the sy-2 gene is located at a genomic region of the pepper linkage group 1, corresponding to the 300 kb region of the Ch1_scaffold 00106 in tomato chromosome 1. Candidate genes in this region will reveal the identity of sy-2 and the underlying mechanism of the temperature-dependent plant response.     

Tissue culture of <Emphasis Type="Italic">Sinningia speciosa</Emphasis> and analysis of the <Emphasis Type="Italic">in vitro</Emphasis>-generated <Emphasis Type="Italic">tricussate whorled phyllotaxis</Emphasis> (<Emphasis Type="Italic">twp</Emphasis>) variant

Two protocols were developed for the efficient regeneration of Sinningia speciosa from leaf explants via two developmental pathways. The first method involved formation of callus and buds, followed by subsequent root growth, in Murashige and Skoog medium (MS) containing 2.0 mg l⁻¹ 6-benzylaminopurine (BA) and 0.2 mg l⁻¹ α-naphthalene acetic acid (NAA), with a regeneration efficiency of 99.0%. The second method involved producing callus and roots, followed by subsequent formation of buds, in MS medium supplemented with 1.0–5.0 mg l⁻¹ NAA, and resulted in a regeneration efficiency of 90.4%. Our experiments indicate that the root-first pathway resulted in a lower plant regeneration efficiency. Through five continual generations using the buds-first method, a total of 215 regenerated plants were obtained in the last generation, and eight exhibited a phenotype we named tricussate whorled phyllotaxis (twp). Six of the regenerated twp variant plants maintained their tricussate whorled phyllotaxis phenotype, showing no other abnormalities, while one reverted to a wild type before flowering and another formed two rounds of sepals. Physiological analysis revealed that the twp plants responded differently than wild type to exogenous NAA and 2,3,5-triiodobenzoic acid (TIBA), while high-performance liquid chromatography (HPLC) analysis showed that the levels of endogenous indole-3-acetic acid (IAA) and gibberellin (GA) were lower in twp than wild-type plants. These results suggest that the formation of the twp mutant may be related to phytohormones and that the twp variant could be an important material for investigating the molecular mechanism of plant phyllotaxis patterning.     

Temperature responses of photosynthesis and respiration in <Emphasis Type="Italic">Populus</Emphasis><Emphasis Type="Italic">balsamifera</Emphasis> L.: acclimation versus adaptation

To examine the role of acclimation versus adaptation on the temperature responses of CO₂ assimilation, we measured dark respiration (R _n) and the CO₂ response of net photosynthesis (A) in Populus balsamifera collected from warm and cool habitats and grown at warm and cool temperatures. R _n and the rate of photosynthetic electron transport (J) are significantly higher in plants grown at 19 versus 27°C; R _n is not affected by the native thermal habitat. By contrast, both the maximum capacity of rubisco (V _cmax) and A are relatively insensitive to growth temperature, but both parameters are slightly higher in plants from cool habitats. A is limited by rubisco capacity from 17–37°C regardless of growth temperature, and there is little evidence for an electron-transport limitation. Stomatal conductance (g _s) is higher in warm-grown plants, but declines with increasing measurement temperature from 17 to 37°C, regardless of growth temperature. The mesophyll conductance (g _m) is relatively temperature insensitive below 25°C, but g _m declines at 37°C in cool-grown plants. Plants acclimated to cool temperatures have increased R _n/A, but this response does not differ between warm- and cool-adapted populations. Primary carbon metabolism clearly acclimates to growth temperature in P. balsamifera, but the ecotypic differences in A suggest that global warming scenarios might affect populations at the northern and southern edges of the boreal forest in different ways.     

High Temperature Stimulates <Emphasis Type="Italic">DWARF4 (DWF4)</Emphasis> Expression to Increase Hypocotyl Elongation in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Hypocotyl growth occurs as a result of an interaction between environmental factors and endogenous phytohormones. In Arabidopsis, high temperature promotes auxin synthesis to increase hypocotyl growth. We previously showed that exogenously provided auxin stimulates expression of the brassinosteroid (BR) biosynthetic gene DWARF4. To determine whether temperature-induced hypocotyl elongation depends on BR biosynthesis, we examined the morphological responses to high temperature and the expression pattern of DWF4pro:GUS in different genetic backgrounds, which are as follows: Ws-2 wild-type, iaa19/msg2, bri1-5, and dwf7-1. In contrast to the wild-type, growth of the three genotypes at 29°C did not significantly increase hypocotyl length; whereas, with the exception of iaa19/msg2, the roots were elongated. These results confirm that BR biosynthesis and signaling pathways are required for hypocotyl growth at high temperature. Furthermore, a GUS histochemical assay revealed that a temperature of 29°C greatly increased DWF4pro:GUS expression in the shoot and root tips compared to a temperature of 22°C. Quantitative measurements of GUS activity in DWF4pro:GUS revealed that growth at 29°C is similar to the level of growth after addition of 100 nM IAA to the medium. Our results suggest that temperature-dependent synthesis of free auxin stimulates BR biosynthesis, particularly via the key biosynthetic gene DWF4, and that the BRs thus synthesized are involved in hypocotyl growth at high temperature.