海防林叶片形态、光合特征和枝条抗性对长期风胁迫的响应

为了解长期风胁迫对海防林的影响,对沿海6种林龄为8～10 a的防护林树种的叶片形态、光合特征及枝条抗性对长期风胁迫的响应特征进行了研究.结果表明,与未受风胁迫海防林相比,长期风胁迫显著降低了女贞(Ligustrum lucidum)、无患子(Sapindus mukorossi)、红楠(Machilus thunber...     

The temperature optima for tree seedling photosynthesis and growth depend on water inputs

Understanding how tree growth is affected by rising temperature is a key to predicting the fate of forests in future warmer climates. Increasing temperature has direct effects on plant physiology, but there are also indirect effects of increased water limitation because evaporative demand increases with temperature in many systems. In this study, we experimentally resolved the direct and indirect effects of temperature on the response of growth and photosynthesis of the widely distributed species Eucalyptus tereticornis. We grew E. tereticornis in an array of six growth temperatures from 18 to 35.5°C, spanning the climatic distribution of the species, with two watering treatments: (a) water inputs increasing with temperature to match plant demand at all temperatures (W_incr), isolating the direct effect of temperature; and (b) water inputs constant for all temperatures, matching demand for coolest grown plants (W_const), such that water limitation increased with growth temperature. We found that constant water inputs resulted in a reduction of temperature optima for both photosynthesis and growth by ~3°C compared to increasing water inputs. Water limitation particularly reduced the total amount of leaf area displayed at T_opt and intermediate growth temperatures. The reduction in photosynthesis could be attributed to lower leaf water potential and consequent stomatal closure. The reduction in growth was a result of decreased photosynthesis, reduced total leaf area display and a reduction in specific leaf area. Water availability had no effect on the response of stem and root respiration to warming, but we observed lower leaf respiration rates under constant water inputs compared to increasing water inputs at higher growth temperatures. Overall, this study demonstrates that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures.     

Light acclimation at the end of the growing season in two broadleaved oak species

The ability of plants to increase their net CO₂ assimilation rate in response to increased irradiance is due to morphological and physiological changes, which might be related to their shade tolerance and leaf ontogeny, but few studies have considered morphology and physiology. Two sympatric oak species (the shade-tolerant Q. petraea and the comparatively shade-intolerant Q. pyrenaica) were grown in hydroponic solution in low-light (LL) and high-light (HL) conditions. 5 months after leaf expansion under these conditions, half of the LL plants were transferred to high light (TLH). Transfer of Q. pyrenaica, from low- to high light led to photoinhibition and after 21 days in higher light there was little acclimation of the maximum rate of carboxylation (V_Cmax) or the maximum rate of electron transport (J_max). Q. pyrenaica TLH plants showed lower stomatal conductance at all times compared to plants growing in LL. Stomatal closure was the main limitation to photosynthesis after transfer in Q. pyrenaica. The increase in evaporative demand upon TLH did not affect hydraulic conductivity of Q. pyrenaica. In contrast, the more shade-tolerant Q. petraea showed a greater degree of acclimation of gas exchange in TLH than Q. pyrenaica and two weeks after transfer gas-exchange rates were as high as in LL plants. In Q. petraea, the most important changes occurred at the level of leaf biochemistry with significant increase in V_Cmax that decreased the J_max/V_Cmax ratio below values recorded in HL plants. However, this potential increase in photosynthesis was at least partially hamstrung by a decrease in internal conductance, which highlights the importance of internal conductance in acclimation to higher light in mature leaves. Neither oak species reached the photosynthetic rates of HL plants; however a trend towards leaf acclimation was observed in Q. petraea while the transfer was harmful to the leaves of Q. pyrenaica developed in the shade.     

CARBOHYDRATE MOBILIZATION AND MOVEMENT IN ALPINE PLANTS

Starch retention and disappearance from leaves and carbon movement under various temperatures were studied in two alpine species, Oxyria digyna (L.) Hill and Phleum alpinum L., and in two low-elevation species, Helianthus annuus L. and Elymus canadensis L. The alpine species exhibited starch disappearance from the leaf following cool night temperatures, whereas starch retention was noted under similar conditions for the low-elevation species. The alpine species, Oxyria, exhibited the highest rates of starch disappearance from the leaf under cool temperatures as well as the highest carbohydrate translocation under cool temperatures. The low-elevation species had low rates of starch disappearance and carbohydrate translocation under low temperatures, but exhibited relatively higher rates with an increase in temperature. Such a mechanism whereby alpine species can maintain relatively high rates of translocation under cold temperature represents a major form of physiological adaptation to the short, cool, growing season in the alpine tundra.     

Relationship between hydraulic resistance and leaf morphology in broadleaf Quercus species: a new interpretation of leaf lobation

We investigated the relationship between leaf shape and leaf hydraulic resistance in a set of broadleaf Quercus tree species (Q. cerris, Q. frainetto, Q. petraea, Q. pyrenaica, Q. robur, Q. rubra, Q. velutina). Seedlings of all the studied species were grown under uniform environmental conditions. A new high-pressure flowmeter was designed to measure leaf-blade hydraulic resistance. Leaf shape was characterised by the complexity of leaf outline which was regarded as an estimate of leaf lobation. This was done using the box-counting fractal dimension of the leaf silhouette. Leaf hydraulic resistance was negatively related to leaf lobation. It is suggested that the lower hydraulic resistance in deeply lobed leaves may constitute a mechanism for improving water balance under dry atmospheric conditions.     

Growth and leaf physiology of monkeyflowers with different altitude ranges

Every species is limited both geographically and ecologically to a subset of available habitats, yet for many species the causes of distribution limits are unknown. Temperature is thought to be one of the primary determinants of species distributions along latitudinal and altitudinal gradients. This study examined leaf physiology and plant performance under contrasting temperature regimes of sister species of monkeyflower, Mimulus cardinalis and Mimulus lewisii (Phrymaceae), that differ in altitude distribution to test the hypothesis that temperature is the primary determinant of differences in fitness versus altitude. Each species attained greatest aboveground biomass, net photosynthetic rate, and effective quantum yield of photosystem II when grown under temperatures characteristic of the altitudinal range center. Although both species exhibited greater stem length, stomatal conductance, and intercellular CO₂ concentration in hot than in cold temperatures, these traits showed much greater reductions under cold temperature for M. cardinalis than for M. lewisii. Survival of M. lewisii was also sensitive to temperature, showing a striking decrease in hot temperatures. Within each temperature regime, the species native to that temperature displayed greatest growth and leaf physiological capacity. Populations from the altitude range center and range margin of each species were used to examine population differentiation, but central and marginal populations did not differ in most growth or leaf physiological responses to temperature. This study provides evidence that M. cardinalis and M. lewisii differ in survival, growth, and leaf physiology under temperature regimes characterizing their contrasting low and high altitude range centers, and suggests that the species’ altitude range limits may arise, in part, due to metabolic limitations on growth that ultimately decrease survival and limit reproduction.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Thermotolerance of Photosystem 2 of Three Alpine Plant Species Under Field Conditions

The species specific response of photosystem 2 (PS2) efficiency and its thermotolerance to diurnal and seasonal alterations in leaf temperature, irradiance, and water relations were investigated under alpine field conditions (1 950 m) and in response to an in situ long-term heat treatment (+3 K). Three plant species were compared using the naturally occurring microstratification of alpine environments, i.e. under contrasting leaf temperatures but under similar macroclimatic conditions. Thermotolerance of PS2 showed a high variability in all three species of up to 9.6 K. Diumal changes (increases or even decreases) in PS2 thermotolerance occurred frequently with a maximum increase of +4.8 K in Loiseleuria procumbens. Increasing leaf temperatures and photosynthetic photon flux density influenced thermotolerance adjustments. Under long-term heating (+3 K) of L. procumbens canopies with infra-red lamps, the maxima of the critical (T_c) and the lethal (T_p) temperature of PS2 increased by at least 1 K. Thermotolerance of the leaf tissue (LT₅₀) increased significantly by +0.6 K. The effects of slight water stress on thermotolerance of PS2 were species specific. High temperature thresholds for photoinhibition were significantly different between species and increased by 9 K from the species in the coldest microhabitat to the species in the warmest. Experimental heating of L. procumbens canopies by +3 K caused a significant (p>0.01) upward shift of the high temperature threshold for photoinhibition by +3 K. Each species appeared to be very well adapted to the thermal conditions of its microhabitat as under the most frequently experienced daytime leaf temperatures no photoinhibition occurred. The observed fine scale thermal adjustment of PS2 in response to increased leaf temperatures shows the potential to optimise photosynthesis under varying environmental conditions as long as the upper thermal limits are not exceeded.     

Differences in leaf thermoregulation and water use strategies between three co‐occurring Atlantic forest tree species

Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.     

Changing fitness of a necrotrophic plant pathogen under increasing temperature

Warmer temperatures associated with climate change are expected to have a direct impact on plant pathogens, challenging crops and altering plant disease profiles in the future. In this study, we have investigated the effect of increasing temperature on the pathogenic fitness of Fusarium pseudograminearum, an important necrotrophic plant pathogen associated with crown rot disease of wheat in Australia. Eleven wheat lines with different levels of crown rot resistance were artificially inoculated with F. pseudograminearum and maintained at four diurnal temperatures 15/15°C, 20/15°C, 25/15°C and 28/15°C in a controlled glasshouse. To quantify the success of F. pseudograminearum three fitness measures, these being disease severity, pathogen biomass in stem base and flag leaf node, and deoxynivalenol (DON) in stem base and flag leaf node of mature plants were used. F. pseudograminearum showed superior overall fitness at 15/15°C, and this was reduced with increasing temperature. Pathogen fitness was significantly influenced by the level of crown rot resistance of wheat lines, but the influence of line declined with increasing temperature. Lines that exhibited superior crown rot resistance in the field were generally associated with reduced overall pathogen fitness. However, the relative performance of the wheat lines was dependent on the measure of pathogen fitness, and lines that were associated with one reduced measure of pathogen fitness did not always reduce another. There was a strong correlation between DON in stem base tissue and disease severity, but length of browning was not a good predictor of Fusarium biomass in the stem base. We report that a combination of host resistance and rising temperature will reduce pathogen fitness under increasing temperature, but further studies combining the effect of rising CO₂ are essential for more realistic assessments.     

Diurnal variation in the temperature response of leaf extension of two bunchgrass species in the field

The effect of temperature on short-term leaf extension rates was studied for two cool-season tussok grasses, Agropyron desertorum and Pseudoroegneria spicata, growing in the field under a variety of water stress and defoliation conditions. Leaf extension rates and air temperatures were monitored every half hour during numerous 12- to 65-h periods in three growing seasons using auxanometers constructed of precision resistors. For both species, a three-phase relationship between leaf extension rate and temperature was observed during diurnal cycles. Leaf extension rate increased linearly with temperature from dawn until midday (phase 1). Leaf extension then increased rapidly, reaching maximum rates in the early evening (approximately 1900h), despite decreasing temperatures during this period (phase 2). Finally, leaf extension rate declined with temperature from evening until dawn (phase 3). This diurnal cycle was described by linear (phase 1) and quadratic (phases 2 and 3 combined) regression models. Although the rate of leaf extension and daily integrals were affected by the water stress and defoliation treatments, the diurnal pattern was consistently observed. Temperature was probably a major factor governing leaf extension rates at night (phase 3), but it appeared unimportant in controlling leaf extension between dawn and midday. The relative importance of physiological and environmental factors controlling leaf extension rate appears to shift during the day in these species under field conditions.     

Stem xylem features in three Quercus (Fagaceae) species along a climatic gradient in NE Spain

 Stem xylem features in two evergreen Quercus species (Q. coccifera and Q. ilex) and a deciduous one (Q. faginea) were analysed along an Atlantic-Mediterranean climatic gradient in which rainfall and winter cold experience strong variation. Mean maximum vessel diameter, vessel density, vessel element length, xylem transverse sectional area, Huber value (xylem transverse sectional area per leaf area unit), theoretical leaf specific conductivity (estimated hydraulic conductance per leaf area unit) and total leaf area were determined in 3-year-old branches. Q. faginea presented the widest vessels and the highest theoretical leaf specific conductivity while Q. coccifera showed the lowest total leaf area and the highest Huber value. Studied features did not exhibit significant correlations with mean minimum January temperature in any species but did show significant relationships with rainfall. In Q. coccifera, mean maximum vessel diameter, vessel element length and theoretical leaf specific conductivity increased with higher rainfall while vessel density decreased. Mean maximum vessel diameter and total leaf area in Q. ilex increased with precipitation whereas variables of Q. faginea did not show any significant trend. Results suggest that aridity, rather than minimum winter temperature, controls stem xylem responses in the studied evergreen species. Q. faginea traits did not show any response to precipitation, probably because this species develops deep roots, which in turn makes edaphic and topographic factors more important in the control of soil water availability. In response to aridity Q. coccifera only exhibits adjustment at a xylem level by reducing its water transport capacity through a reduction of vessel diameter without changing the amount of xylem tissue or foliage, whereas Q. ilex adjusts its water transport capacity in parallel to the foliage area. Received: 13 January 1997 / Accepted: 8 April 1997     

7种木莲属植物抗寒性评价及其生理机制

为建立木莲属(Manglietia)植物的抗寒性综合评价体系,对7种木莲属植物扦插苗进行低温胁迫处理,测定叶片的抗寒性关键生理指标,拟合Logistic方程计算低温半致死温度,隶属函数法筛选抗寒性关键指标并综合评价.结果表明,7种木莲属植物的低温半致死温度依次为木莲<红花木莲<荷花木莲<巴东木莲<球果木莲<锥花木莲<粗...     

Stomatal encryption by epicuticular waxes as a plastic trait modifying gas exchange in a Mediterranean evergreen species (Quercus coccifera L.)

The adaptive benefit of stomatal crypts remains a matter of controversy. This work studies the effect on gas exchange of cuticular rims that overarch the stomatal pore in the Mediterranean species Quercus coccifera L. growing under Mediterranean (lower relative humidities and high summer temperatures) or oceanic conditions (higher daily relative humidities and mild temperatures). After microscopic assessment of the leaf surfaces and stomatal architecture, the impact of the cuticular ‘cup’ on gas exchange was evaluated by employing three‐dimensional finite element models. Here, we provide evidence for a high plasticity of the Q. coccifera cuticular cup, with much larger vents under oceanic conditions compared to small vents under Mediterranean conditions. This structure adds a substantial fixed resistance thereby strongly decreasing gas exchange under Mediterranean conditions. The cuticular cup, which also increases leaf internal humidity, might buffer the rapid changes in vapour pressure deficit (VPD) often observed under Mediterranean conditions. Since water loss of guard and adjacent epidermal cells regulates stomatal aperture, we suggest that this structure allows an efficient regulation of stomatal conductance and optimum use of resources under high VPD. This study provides evidence that plasticity of stomatal architecture can be an important structural component of hydraulic adaptation to different climate conditions.     

Effects of day and night temperature and temperature variation on photosynthetic characteristics

Net photosynthetic rates and mesophyll conductances were measured under standardized conditions for leaves of two C₃ and one C₄ annual species grown at temperatures of 20 to 32°C. Plants were grown with varying day and night temperatures, and also at constant temperatures equal to all the day and night temperatures used. Plants were grown with 8, 12, and 16 hours of light per day. This design allowed determination of whether photosynthetic characteristics were best correlated with day, night, mean, or time-weighted mean temperatures, The results showed that for Glycine max (L.) Merr. (C₃) night temperature was most important in determining photosynthetic characteristics, while in Helianthus annuus L. (C₃) and Amaranthus hypochondriacus L. (C₄) the time-weighted mean temperature was most important. The results for all species were consistent with the hypothesis that development of photosynthetic characteristics is related to a balance between the rate of leaf expansion and the rate of photosynthesis under the growth conditions.     

Characteristics of gas exchange and morphology of a spring ephemeral, Erythronium japonicum, in comparison with a sun plant, Glycine max

Morphological characteristics and responses of gas exchanges to light intensity were examined in a typical vernal species, Erythronium japonicum Decne (E. japonicum), grown (i) on the floor of a deciduous broad-leaved Quercus mongolica forest (one of its native habitats, the Q. mongolica stand); (ii) bare land left undisturbed for 9 years after forest clearing (the bare stand); and (iii) in a sun crop, soybean, grown for 110 days in an experimental field and for 17 days in pots, in order to evaluate the adaptability of the photosynthetic process of this vernal species to its shady native habitats. The daytime solar radiation, ai and leaf temperatures and leaf–air vapor pressure difference (VPD) were significantly higher at the bare stand than at the Q. mongolica stand. When environmental factors observed at the Q. mongolica and bare stands were reproduced in an assimilation chamber, leaf temperatures of E. japonicum plants increased markedly with increased radiation, whereas those of soybean plants differed little from the respective air temperatures. The photosynthetic and transpiration rates and stomatal conductance in the former plants placed under conditions at the Q. mongolica stand increased with radiation and reached respective steady state values at maximum radiation at the site; whereas, under the conditions at the bare stand, they also increased and reached respective steady state values, but then continuously decreased to be lower than the respective value at the Q. mongolica stand. However, both rates and the conductance in the soybean plants under both conditions increased significantly with radiation and reached much higher respective values at the respective maximum radiations. Water use efficiency for E. japonicum plants was much higher under conditions at the Q. mongolica stand than at the bare stand and was practically equal to those for soybean plants under both conditions. Water potential in the leaves of E. japonicum at maximum radiation at the bare stand was one-third that of those at the Q. mongolica stand. The potential in soybean leaves differed little between both conditions and was roughly equal to the low value in E. japonicum leaves at the bare stand conditions. The stomatal densities on upper and lower leaf surfaces and the ratio of root weight to leaf area (R : L) differed little between E. japonicum plants grown at both stands as well as between young and adult soybean plants. However, the densities on the upper and lower surfaces of E. japonicum were 25% and 60% of the respective values of both soybean plants. The ratios of R : L of the E. japonicum plants were only one-quarter that of the young and adult soybean plants. The cooperation between these morphological and gas exchange characteristics in E. japonicum plants is discussed in relation to adaptation to the environment in native habitats.     

Estimation of Growth and Photosynthetic Performance of Two C₄ Species (<i>Pennisetum spicatum</i> (L.) Körn. and <i>Zea mays</i> L.) under a Low Temperature Treatment

Pearl millet (Pennisetum spicatum (L.) Körn.) and maize (Zea mays L.) are C₄ grass species grown for feeding humans and animals in Almadinah Almunawwarah, which is in the western part of Saudi Arabia. During the winter, the mean temperature, which drops to 14°C, represents a major problem for the growth of these species in this region. Therefore, the objectives of this research were to investigate the growth response and the photosynthetic performance of P. spicatum and Z. mays under a low temperature stress. The treatments involved daytime and nighttime temperatures of 14/12°C (low temperature) and 24/22°C (optimum temperature). The results indicated that low temperature significantly reduced all growth and physiological parameters, including seed germination, leaf expansion, leaf area, shoot length and root length of the two species compared to those of the control. Additionally, the low temperature significantly decreased the light-saturated assimilation rate (A_sat), quantum yield (ϕ), saturated rate of carbon dioxide uptake (A_max) and efficiency of carboxylation on both species compared to those of the control. Moreover, the values of Fv/Fm and the chlorophyll contents of both species were significantly reduced by low temperature compared to those of the control. It can be concluded that both species had little tolerance to low temperatures.     

Photosynthesis and micro-environmental factors in a spring ephemeral,Erythronium japonicum, from native and open habitats

Environmental factors affecting photosynthetic activity of the typical vernal speciesErythronium japonicum Decne were examined on the floor of a deciduous broad-leavedQuercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 4 years after forest clearing (bare stand). Daytime solar irradiation and air and leaf temperatures at the bare stand were significantly higher than those at theQ.m. stand. The relative air humidity was very low and did not differ much between the stands, although the leaf-air vapor pressure differences (VPD) at the bare stand were about twice as high as those at theQ.m. stand. The plants at both stands were supplied with sufficient soil water throughout their growing season by a large snowmelt. However, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress, caused by the strong radiations and high leaf temperatures, and water stress, caused by the highly transpiring conditions, than those at theQ.m. stand. When the radiation on leaves, leaf temperatures and VPD in the assimilation chamber were changed from those observed at theQ.m. stand to those at the bare stand, the photosynthetic rate and stomatal conductance fell significantly. However, the rate and conductance were immediately restored to the respective values near those measured under the conditions at theQ.m. stand when only VPD was dropped to the value similar to that observed at theQ.m. stand. These results indicate that the photosynthetic rate of the plants at the bare stand was lowered largely by a decrease in stomatal conductance. The internal CO₂ partial pressure was considerably greater in leaves placed under environmental conditions similar to those at the bare stand.     

Soil Temperature Influences on Root Resistance of Pinus contorta Seedlings

The influence of low temperature in the root zone on water uptake in lodgepole pine (Pinus contorta Dougl. ex Loud.) was studied under laboratory conditions. To remove soil hydraulic influences, two-year-old seedlings were transferred to solution cultures and maintained in temperature controlled water baths. Short term measurements of leaf conductance, leaf water potential and tritiated water movement were taken at root temperatures from 22 C down to 0 C. Root resistance was calculated to be 67% of total plant resistance at 7 C and 93% at 0 C. In addition an Arrhenius break was found in a plant resistance versus temperature plot, suggesting a significant change with temperature in the membrane pathway in the root water uptake system.     

油葵苗期抗旱相关性状的QTL定位及候选基因筛选

干旱是限制向日葵生长发育的重要因素之一。为探究向日葵苗期抗旱性分子机制,该研究以向日葵K55与K58杂交构建的150个F₇重组自交系群体为材料,对其在正常浇水和干旱胁迫两种水分处理条件下的叶片相对电导率、叶绿素含量、叶面积、叶片相对含水量、根长进行表型测定,利用前期建立的SNP、SSR分子标记遗传连锁图谱,通过复合区间作图法对5个抗旱相关的性状进行QTL定位。结果表明：(1)共定位到向日葵QTL位点11个,其中正常浇水条件下5个,干旱胁迫条件下6个,表型贡献率为0.768%~7.547%,且5号连锁群上定位到的QTL位点最多(3个)。(2)QTL置信区间内共筛选到62个与干旱相关的候选基因,包括位于qLA 8 1上的rna23019、rna23004、rna22661、rna22193、rna23294、rna22783和位于qCC 13 1上的rna40140,这些基因可作为后续基因克隆及功能研究的重点候选基因。该研究结果为向日葵抗旱性研究及其遗传改良奠定了基础。     

STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub-steppe communities of the American West, big sagebrush (Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO₂ exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO₂ exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub-optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO₂ exchange characteristics is discussed.