Shoot Temperature Research Articles

AIM1-dependent high basal salicylic acid accumulation modulates stomatal aperture in rice.

The basal levels of salicylic acid (SA) vary dramatically among plant species. In the shoot, for example, rice contains almost 100 times higher SA levels than Arabidopsis. Despite its high basal levels, neither the biosynthetic pathway nor the biological functions of SA are well understood in rice. Combining with metabolite analysis, physiological, and genetic approaches, we found that the synthesis of basal SA in rice shoot is dependent on OsAIM1, which encodes a beta-oxidation enzyme in the phenylalanine ammonia-lyase (PAL) pathway. Compromised SA accumulation in the Osaim1 mutant led to a lower shoot temperature than wild-type plants. However, this shoot temperature defect resulted from increased transpiration due to elevated steady-state stomatal aperture in the mutant. Furthermore, the high basal SA level is required for sustained expression of OsWRKY45 to modulate the steady-state stomatal aperture and shoot temperature in rice. Taken together, these results provide the direct genetic evidence for the critical role of the PAL pathway in the biosynthesis of high basal level SA in rice, which plays an important role in the regulation of steady-state stomatal aperture to promote fitness under stress conditions.

Temperature changes in the root ecosystem affect plant functionality

Climate change is increasing the frequency of extreme heat events that aggravate its negative impact on plant development and agricultural yield. Most experiments designed to study plant adaption to heat stress apply homogeneous high temperatures to both shoot and root. However, this treatment does not mimic the conditions in natural fields, where roots grow in a dark environment with a descending temperature gradient. Excessively high temperatures severely decrease cell division in the root meristem, compromising root growth, while increasing the division of quiescent center cells, likely in an attempt to maintain the stem cell niche under such harsh conditions. Here, we engineered the TGRooZ, a device that generates a temperature gradient for in vitro or greenhouse growth assays. The root systems of plants exposed to high shoot temperatures but cultivated in the TGRooZ grow efficiently and maintain their functionality to sustain proper shoot growth and development. Furthermore, gene expression and rhizosphere or root microbiome composition are significantly less affected in TGRooZ-grown roots than in high-temperature-grown roots, correlating with higher root functionality. Our data indicate that use of the TGRooZ in heat-stress studies can improve our knowledge of plant response to high temperatures, demonstrating its applicability from laboratory studies to the field.

Shoot and root zone temperatures are critical in bidirectional regulation of tuberization in potato

Multiple signals regulate tuberization in potato and among them, photoperiod and temperature are most crucial. Optimum temperature for tuberization is around 20 °C and elevated temperatures above optimum drastically reduces tuberization. Our study therefore aims at understanding and establishing a system to study the effect of shoot and root zone temperatures on tuberization. Towards this, a less photosensitive cultivar, Kufri Jyoti was identified initially by analysing the allelic variations in CDF1 gene. Using the aeroponic systems housed inside the poly house, the effect of temperature on tuberization was studied. Following staggered sowing on aeroponic chambers during rabi season (Oct-Dec), the shoot zone (Sz) and root zone (Rz) were exposed to elevated mean cumulative day temperatures with one set of chambers regulated to maintain optimum temperature of 19 ± 2 °C in Rz (Rz-R) irrespective of date of sowing/ planting. With delayed planting, plants were exposed to high temperatures and our study demonstrated that, elevated temperatures in Sz and/or Rz had significant effect on growth attributes, tuber number and tuber weight per plant. While the tuber number was significantly higher under optimum root and shoot zone temperature, it was rather decreased by 3.5 and 2.3 for every degree rise in Rz (irrespective of Sz) and Sz (irrespective of Rz) temperature, respectively. However, regulating the Rz temperature inside the aeroponic chambers, reasonably a good number of mini-tubers per plant was produced regardless of the shoot zone temperatures suggesting that, root temperature is more crucial than shoot temperature. Furthermore, reduction in tuberization at elevated temperature was correlated well with reduction in expression of key positive regulators (StSP6A, StBEL5) and enhanced expression of key negative regulator, StSP5G in both leaves and solons. Taken together, our data suggest that, regulation of both root and shoot zone temperatures are important for effective tuberization and tuber yield in potato. • Elevated shoot zone (Sz) and root zone (Rz) temperature above optimum significantly affected plants growth and tuber yield. • Every degree rise in Sz and Rz zone temperature above optimum resulted in significant reduction in tuber number and weight. • Root zone temperature is more critical in regulating tuber number and tuber weight than shoot zone temperature. • At elevated Sz and Rz temperatures, expression of positive and negative regulators decreased and increased respectively. • Regulating Rz temperature would likely to help maintain mini-tuber production even under non-optimal shoot zone conditions.

Responses of Solanum tuberosum L. to Water Deficit by Matric or Osmotic Induction

To select potato genotypes tolerant to water deficit, systems to simulate this stress have been used. Polyethylene glycol (PEG) is the main osmotic agent used for this purpose, but it causes an excessively severe stress. However, it is difficult to carry out an experiment that aims to compare plant responses under water deficit by osmotic or matric induction, and, thus, few studies compare these stress-inducing mechanisms. Therefore, the objective of this study was to compare the responses of Agata, BRS Clara, C2406-03 and Cota genotypes to water deficit in both induction methods (matric or osmotic). The tests were carried out in a greenhouse, one using hydroponics (osmotic induction) and the other in pots with soil (matric induction). In both tests, the application of stressful conditions occurred at the beginning of tuberization. Assessments of gas exchange and shoot temperature were made throughout the exposure to stress. Also, samples were collected from leaves for analysis of osmotic potential and leaves and tubers for analysis of metabolite content. At the end of the potato plant cycle, the number and weight of tubers were evaluated. In both stress conditions, there were significant reductions in photosynthesis and transpiration rate compared to the respective normal hydration conditions. In addition, indicators such as metabolite levels (proline and soluble sugars) were significantly altered in plants exposed to different stress inductions. These data, together with the significant limitations in the growth of stressed plants, indicate that the experimental models induce similar responses. However, the water deficit by osmotic induction was more severe for the potato plants when compared to stress due to matric induction, mainly affecting tuber production. Therefore, the water deficit osmotic induction model can be recommended for phenotyping tolerance to this stress, due to the hydroponic system inducing greater tuber production per plant under optimal cultivation conditions.

Bottom-cold stress was less harmful than cold-air stress on tomato seedling production treated with boric acid

Boron is beneficial element can help plants face chilling stress. This research was arranged using factorial experiment based on randomized complete block design on Lycopersicon esculentum var. Infinity in Isfahan University of Technology due to having a new idea about bottom-cold and cold-air stress tomato seedling with boron application. There were two treatments having different boron (B1) concentrations (B1) 50.5 mM, and (B2) 75.82 mM. Three temperature treatment used as following: (1) the cold treatment used with vegetation chambers with low temperature (10 °C) (cold air), (2) the low nutrient solution (10 °C) temperature but the aerial part exposed to optimum temperature (bottom-cold) and (3) the last part, was the control plant (opt) keep in optimum root and shoot temperature (22 °C). Some physiological and biochemical characteristics were measured. The results were shown that boron uptake decreased in cold or stress as well as the water status of the plant which is suffering from cold-air stress greater than bottom-cold. Boron application, especially in higher concentration, improved some deleterious effect of cold stress, especially in the bottom-cold. The reason may refer to keeping photosynthesis traits in better level with B application. Cold-air stress, increased stress indices such as antioxidant and proline as well as glucose level and saturated/unsaturated fatty acid greater than bottom-cold stress. It was concluded that tomato was more resistant to bottom-cold stress than cold-air stress. Boron application increased the boron of leaves more effectively in the bottom-cold consequently increase plant tolerance to a chilling condition at the bottom-cold too.

A Mobile Auxin Signal Connects Temperature Sensing in Cotyledons with Growth Responses in Hypocotyls.

Plants have a remarkable capacity to adjust their growth and development to elevated ambient temperatures. Increased elongation growth of roots, hypocotyls, and petioles in warm temperatures are hallmarks of seedling thermomorphogenesis. In the last decade, significant progress has been made to identify the molecular signaling components regulating these growth responses. Increased ambient temperature utilizes diverse components of the light sensing and signal transduction network to trigger growth adjustments. However, it remains unknown whether temperature sensing and responses are universal processes that occur uniformly in all plant organs. Alternatively, temperature sensing may be confined to specific tissues or organs, which would require a systemic signal that mediates responses in distal parts of the plant. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings show organ-specific transcriptome responses to elevated temperatures and that thermomorphogenesis involves both autonomous and organ-interdependent temperature sensing and signaling. Seedling roots can sense and respond to temperature in a shoot-independent manner, whereas shoot temperature responses require both local and systemic processes. The induction of cell elongation in hypocotyls requires temperature sensing in cotyledons, followed by the generation of a mobile auxin signal. Subsequently, auxin travels to the hypocotyl, where it triggers local brassinosteroid-induced cell elongation in seedling stems, which depends upon a distinct, permissive temperature sensor in the hypocotyl.

Canopy structure affects temperature distributions and free convection in moss shoot systems.

Nonvascular plants play important roles in exchange of water and heat at the soil-atmosphere interface. Differential evaporative cooling may cause temperature gradients within bryophyte canopies, influencing convective heat and mass transport. Understanding mechanisms that affect fluxes through moss layers should improve models of forest floor function. A three-dimensional thermal imaging system measured temperature distributions within moss shoot systems that were used to explore relationships among canopy structure, temperature gradients, evaporation, and conductance to water vapor (gs ). We studied five moss species under dark and light conditions in the lab. Also, these properties were measured in two species that differed in canopy structure during drying. Differential evaporative cooling led to a 1.4 to 5.0°C range in shoot temperatures within canopies. Samples displayed -0.5 to -0.9°C/cm temperature gradients with cooler apical temperatures. Gradient magnitudes did not differ among species, but taller canopies expressed greater temperature differences. Light enhanced both the gradient and the temperature difference. Rates of evaporation were significantly related to canopy height in the light, but not in the dark, although gs was positively associated with canopy height in both. Rayleigh (Ra) numbers characterize whether temperature gradients likely generate free convection. In tall canopies, Ra numbers exceeded the value indicative of free convection and turbulent flow. As plants dried, temperature gradients decreased. When moss canopies are wet, cooler apical temperatures create thermal instabilities within the canopies that appear sufficient to enhance convective transport of water vapor and heat in tall canopies with low bulk density.

Adaptation of Arabidopsis Plants to Tropical Aeroponics Using Cool Root Zone Temperatures

Arabidopsis thaliana (L.) Heyhn. is a well known model plant in plant research. However, its growth conditions and diminutive stature associated with low biomass at maturity make it a challenging species for physiological studies. While in the tropical countries, it can only be grown either by tissue cultures or in growth chambers under controlled conditions. An aeroponic technique with 20°C ± 2°C and 30°C ± 2°C root-zone temperatures (RZT) was used to grow Arabidopsis (Columbia ecotype) in a tropical greenhouse with natural irradiance and high ambient temperature (38°C/28°C day/night). Seedlings germinated in growth chambers at 20°C or 30°C. At 6 to 8 leaf stage, they were transferred to the aeroponic troughs with their roots exposed to constant temperature of 20°C ± 2°C and 30°C ± 2°C while their aerial parts were subjected to fluctuating ambient temperature from 28°C to 38°C. After a week, plants have acclimatised to both RZTs and started developing normal rosettes, bolted and yielded viable seeds. However, 20°C ± 2°C RZT allowed them to recover from turgor pressure despite of wilting, and significantly increased biomass. Mature plants grown in each RZTs were compared morphologically and physiologically to the plants grown in growth chamber (GC) at 20°C (root and shoot) temperature with 60% relative humidity. Aeroponically grown plants did not experience photoinhibition, and also exhibited higher photosynthetic light usage efficiency and higher capacities of heat dissipation, compared to GC plants. This aeroponics with cool RZTs can allow the use of Arabidopsis as a model plant even under tropical climate.

Photosynthetic and Antioxidant Responses of Jatropha curcas Plants to Heat Stress: On the Relative Sensitivity of Shoots and Roots

This study aimed to evaluate the photosynthetic and antioxidant responses of Jatropha curcas plants exposed to heat stress applied simultaneously in both shoots and roots and solely in each tissue. Thirty-day-old plants were transferred to two growth chambers, where they were subjected to 27 or 42 °C. In each growth chamber, the root system was isolated and then subjected to 27 or 42 °C. Then, four groups of plants were formed according to shoot/root temperature: 27/27; 27/42; 42/27; and 42/42 °C. Plants under 27/27 °C were considered as control and plants were exposed to each thermal treatment for 12 h. The leaf CO2 assimilation rate, stomatal conductance, and mesophyll conductance were decreased by high temperature, regardless of plant tissue. However, primary photochemistry given by effective quantum yield of PSII, electron transport rate, photochemical and non-photochemical quenchings, and maximum electron transport rate driving RuBP regeneration revealed that high shoot temperature was more deleterious for photosynthesis than high root temperature. J. curcas presented efficient mechanisms to avoid photo-damage and photo-inhibition through increases in non-photochemical quenching, maintenance of the maximum quantum yield of PSII, and increases in total superoxide dismutase, ascorbate peroxidase, and catalase activities when shoots were exposed to high temperature, regardless of root temperature. In general, our data suggest that J. curcas present an efficient antioxidant protection system in photosynthetic tissues and acclimation of PSII avoiding photo-damage and photo-inhibition under heat stress. On the other hand, high root temperature inhibits antioxidant metabolism, leading to higher oxidative damage in root tissues and affecting primary photochemistry in leaf tissues.

Clouds homogenize shoot temperatures, transpiration, and photosynthesis within crowns of Abies fraseri (Pursh.) Poiret.

Multiple studies have examined the effects of clouds on shoot and canopy-level microclimate and physiological processes; none have yet done so on the scale of individual plant crowns. We compared incident photosynthetically active radiation (PAR), leaf temperatures, chlorophyll fluorescence, and photosynthetic gas exchange of shoots in three different spatial locations of Abies fraseri crowns on sunny (clear to partly cloudy) versus overcast days. The field site was a Fraser fir farm (1038m elevation) in the Appalachian mountains, USA. Ten saplings of the same age class were marked and revisited for all measurements. Sunny conditions corresponded with 5-10× greater sunlight incidence on south-facing outer shoots compared to south-facing inner and north-facing outer shoots, which were shaded and received only indirect (diffuse) sunlight. Differences in spatial distribution of irradiance were mirrored in differences in shoot temperatures, photosynthesis, and transpiration, which were all greater in south-facing outer shoots compared to more shaded crown locations. In contrast, overcast conditions corresponded with more homogeneous sunlight distribution between north and south-facing outer shoots, and similar shoot temperatures, chlorophyll fluorescence (ΦPSII), photosynthesis, and transpiration; these effects were observed in south-facing inner shoots as well, but to a lesser extent. There was no significant difference in conductance between different crown locations on sunny or overcast days, indicating spatial differences in transpiration under sunny conditions were likely driven by leaf temperature differences. We conclude that clouds can affect spatial distribution of sunlight and associated physiological parameters not only within forest communities, but within individual crowns as well.

Root contraction helps protect the “living rock” cactus Ariocarpus fissuratus from lethal high temperatures when growing in rocky soil

We investigated how the "living rock" cactus Ariocarpus fissuratus, like other low-growing desert plants, can endure potentially lethal high temperatures at the soil surface. Specifically, we examined how shoot descent by root contraction in the presence or absence of soil rocks influences shoot temperatures and transpiration. • Root contraction was identified by measuring shoot descent and anatomical analysis. Temperatures and transpiration were measured for plants at two heights in sandy and rocky soil, and temperature tolerances were determined by vital staining. • Plants embedded in rocky soil survived an extreme heat episode, unlike plants in sandy soil, though rocks did not moderate low temperatures. Root contraction occurred regardless of season and soil moisture. Xylem conduits (wide-band tracheids) formed a compressible lattice that decreased root length as rays enlarged the root base radially. Plant position in the soil did not affect transpiration. • Contractile roots pulled plants of A. fissuratus into the soil at rates of 6-30 mm yr(-1). Maintaining shoots level with the soil surface kept plant temperatures below the high lethal temperature and improved survivorship in soil shaded by surface rocks.

EFFECTS OF EMERGING SHOOT SIZE, TEMPERATURE, AND BENZYLADENINE ON GROWTH AND FLOWERING OF ZYGOPETALUM REDVALE 'FIRE KISS'

Zygopetalum is an epiphytic and terrestrial orchid genus that originates from mountainous regions of South America and has exotic, fragrant flowers. We performed experiments to determine how emerging shoot length, temperature, and application of benzyladenine (BA) influence growth and flowering of Zygopetalum Redvale 'Fire Kiss'. In the first experiment, the length of one immature vegetative shoot was measured on each 4-year-old plant at week 0 and plants were assigned to groups based on emerging shoot length: 1.0 to 2.0 cm, 3.0 to 4.0 cm, 6.0 to 7.0 cm, or 9.0 to 10.0 cm. Plants were transferred to environmental chambers with constant temperatures of 12.5, 15.0, 20.0, or 25.0°C and a 9-h photoperiod and grown for 8 weeks. Plants were then grown in a greenhouse at a constant temperature of 23°C with a 16-h photoperiod. After 73 days, 63 or 75% of plants had developed a visible inflorescence (VI) if emerging shoots between 3.0 and 7.0 cm in length were exposed to 12.5 or 15.0°C. No plants developed a VI when grown at 20 or 25°C, regardless of shoot length. In the second experiment, vegetative 5.5-year-old plants were grown in a greenhouse with a constant temperature of 14 or 20°C and a 16-h photoperiod. Plants received a foliar spray application (volume of 0.2 L·m -2 ) of BA at 200 or 400 mg/L at weeks 0 and 4 or weeks 4 and 8. Control plants did not receive a BA application. At week 8, plants were transferred to a greenhouse at a constant temperature of 20°C with a 16-h photoperiod. After 4 weeks, plants that were treated with BA had developed a mean of 7.2 to 10.7 new immature shoots per plant, while untreated plants had only 0.4 to 0.8 new shoots. Thus, application of BA could be used to stimulate shoot development to increase vegetative growth of Zygopetalum.

Experimental branch warming alters tall tree leaf phenology and acorn production

An increasing number of field experiments have been initiated worldwide to study simulated effects of global warming. To experimentally determine how tall, mature trees respond to warming, we developed a new technique that heated canopy-level branches with electric heating cables. Using a canopy crane, we attached electric cables to top canopy branches of tall, mature Quercus crispula Blume (18–20 m in height) trees; shoot temperature was elevated 5 °C above ambient. Branch warming extended the length of the growing season of canopy leaves by later leaf fall. Moreover, branch warming increased acorn production. Application of this technique should lead to a better understanding of how tall, mature trees respond to global warming.

Horsetails get the wind up

Horsetails get the wind up

A model of vegetative flush development and its potential use managing macadamia (Macadamia integrifolia) tree canopies

We examined the relationship between shoot growth and temperature and solar radiation in macadamia (Macadamia integrifolia Maiden and Betche, M. integrifolia × tetraphylla Johnson) as an aid to developing pruning strategies for this crop. Trees growing at Alstonville (28.9°S) in northern NSW, Australia, were pruned at various times to promote vegetative flushing under a range of environmental conditions. Flush development in macadamia is cyclic: bud release and stem elongation followed by a period of dormancy, before bud release of the subsequent flush. The rate of bud release after pruning was best correlated with the product of the mean temperature and solar radiation (r2 = 0.75, P < 0.0001), whereas the rate of flush development was best correlated with the mean temperature (r2 = 0.76, P < 0.0001). The number of buds released per pruned stem was greater under higher temperatures and solar radiation (r2 = 0.37, P < 0.001), but the length of the flush after pruning decreased with increasing temperatures (r2 = 0.32, P < 0.01). The descriptive models were combined with long-term weather data to predict the duration and characteristics of flushes following pruning at various times of the year along Australia’s eastern seaboard, from Mareeba (17.0°S) to Coffs Harbour (30.3°S). Flush duration and stem length following June pruning were predicted to be greater than following early autumn or September pruning and to vary from year to year, and with location (latitude). We discuss the implications of the model predictions for productivity and propose pruning times intended to optimise flowering and yield. Further research is required to test these proposed pruning strategies.

Influence of High Shoot and Root‐Zone Temperatures on Growth of Three Wheat Genotypes during Early Vegetative Stages

AbstractHigh temperatures, whether of shoot or root, are reported to affect shoot and root growth of various plant species. The scanty information available on the differential response of wheat genotypes to high shoot and root‐zone temperatures triggered this investigation to study the response and adaptation of shoot and root growth of three wheat genotypes to high shoot and root‐zone temperatures during early growth stages. Three wheat genotypes; Fang (heat tolerant), Siete Cerros (heat sensitive) and Imam (recent cultivar adapted to a hot irrigated environment) were grown in soil and hydroponically. Three shoot/root‐zone temperatures (23/23, 23/35 and 35/35 °C for the soil experiment and 22/22, 22/38 and 38/38 °C for the hydroponic experiment) were applied at three‐leaf growth stage. High root‐zone temperature alone or combined with high shoot temperature reduced xylem sap flow rate, root dry weight, root length and root/shoot ratio. Unexpectedly, shoot fresh and dry weights and relative growth rate (RGR) were not significantly affected by the high root‐zone temperature except for the susceptible genotype, Siete Cerros, after prolonged exposure in the hydroponic experiment. In contrast, high shoot/root‐zone temperature significantly reduced shoot fresh and dry weights from as early as the first week of the hydroponic experiment. The 38/38 °C treatment also caused significant reduction in RGR and net assimilation rate during the first 2 weeks, but no significant differences were found during the last 2 weeks compared with 22/22 °C. Interesting responses were observed among genotypes in terms of shoot and root dry weights and root/shoot ratio at 38/38 °C treatment. The heat‐sensitive Siete Cerros showed the least reduction in these traits during the first 2 weeks while the heat‐tolerant Fang and Imam responded by greatly reducing their shoot and root weights. The situation was almost reversed with the duration of treatments such that Siete Cerros became the most affected genotype while Fang and Imam were better adapted to high shoot/root‐zone temperature. Specific root weight was the exceptional trait that increased under high temperature treatments. Results indicate that despite the reduction in root length and weight observed under high root‐zone temperature, shoot growth was not much affected suggesting that the use of suitable cultivar coupled with proper management could alleviate most of high root‐zone temperature effects during early growth stages.

Effects of foliar and root applied nitrogen on nitrogen uptake and movement in white clover at low temperature

‘Grasslands Huia’ white clover plants were grown at a constant root and shoot temperature of 8°C in a controlled environment room. Nitrate or urea labelled with 15N was applied to the foliage, foliage plus the roots, or to the roots only. After 21 days, plants supplied with foliar nitrogen (N) or with N to the foliage plus roots, produced significantly (P < 0.05) more lamina dry weight than plants supplied with N to the roots only. Plants supplied with N to the foliage contained significantly (P < 0.05) more lamina 15N than plants supplied with N to the foliage plus the roots or to the roots only. From 14 days onwards, plants supplied with foliar N, or N to the foliage plus the roots, had significantly (P < 0.05) higher leaf areas than plants supplied with N to the roots only, irrespective of the form of N applied. By 21 days, plants supplied with foliar nitrate or urea had significantly (P < 0.05) higher photosynthesis, transpiration, stomatal conductance rates, and lamina sucrose concentrations, than plants supplied with N to the roots. The experiment demonstrated the importance of N availability as a rate‐limiting factor on starch and sucrose synthesis and, in turn, on foliage growth at low temperatures in white clover.

Rock associations, root depth, and temperature tolerances for the “rock live-forever,” Dudleya saxosa, at three elevations in the north-western Sonoran Desert

At three elevations in the north-western Sonoran Desert, Dudleya saxosa (Crassulaceae) occurred predominantly where the ground surface was covered >90% by rocks. Its roots were extremely shallow, with a mean depth of only 46 mm and essentially none below 120 mm, for a north-facing dolomitic rock outcropping at an elevation of 300 m, exposed quartz veins at 825 m (Agave Hill), and a north-facing granitic outcropping at 1395 m. For a mild spring day at Agave Hill, the maximum ground surface temperature of a quartz vein was 11 °C cooler than that of the adjacent bare soil, which was a consequence of the high thermal conductivity coefficient (8.2 W m −1 °C −1) and low shortwave absorptance (0.23) of the quartz. The leaves and the roots of D. saxosa had only modest low-temperature tolerances and acclimation, as is typical of succulent plants. Although its root cortical cells were relatively tolerant of high temperatures, D. saxosa from Agave Hill could not tolerate shoot temperatures above 60 °C, whereas eight other succulent species endemic to the north-western Sonoran Desert can. The average increase in the temperature killing 50% of their shoot cells as the ambient temperature is increased by 10 °C was 2.4 °C for D. saxosa but averaged 5.2 °C for the eight other species. Thus, the relatively poor high-temperature tolerance and acclimation ability of its mesophyll cells may cause D. saxosa to favor cooler north-facing microhabitats, such as when it occurs on granitic outcroppings with their high shortwave absorptance (0.73).

An experimental method for varying root temperature independently of shoot temperature

In the field, roots and shoots of plants are seldom exposed to the same temperature. Methods for varying root temperature independently of shoot temperature have relied upon hydroponic systems or field‐based studies, and both approaches have limitations for investigating the physiological affects of temperature on nutrient uptake. A low‐cost method for varying root and shoot temperature independently using a solid root medium was developed, and its utility demonstrated in an experiment with white clover. The “temperature gradient trough” enabled the effects of a range of root temperatures from 6 to 24°C on nitrogen (N) uptake and distribution to be evaluated. Results showed that at an 8°C shoot temperature, an increase in root temperature resulted in an increase in 15N distribution to the laminae. At a 24°C shoot temperature, and root temperatures greater than 10°C, plants had significantly more 15N in laminae and petioles than at shoot temperatures of 8°C. The temperature gradient trough enabled independent varying of root and shoot temperatures in a controlled environment, allowing rapid exploration of the effects of temperature on N distribution in white clover.

A Test of Treeline Theory on a Montane Permafrost Island

Trees have a common high elevation distribution limit at similar soil temperatures across the globe. Here we tested whether low temperature in the root zone alone can induce the well known dwarfing at the low temperature growth limit of trees by using a “natural experiment” with trees growing on low elevation permafrost ground. At the natural high elevation treeline, both air (shoot) and soil (root) temperature are low, while at the montane permafrost site in the Swiss Jura mountains, roots are cold, but not shoots. Soil temperature records confirmed that the low elevation study site resembles thermal conditions typical for the high elevation treeline. The warm air conditions have no ameliorating effect on tree growth. Irrespective of shoot temperatures, the root zone temperature and the associated metabolism appear to determine tree growth at this site. The test revealed a critical role of soil temperature, which by itself is sufficient to explain a growth limit of trees associated with a seasonal mean soil temperature at 10 cm depth of around 6°C.