The relationship between isoprene emission rate and dark respiration rate in white poplar (Populus alba L.) leaves

In past studies, it was hypothesized that reductions in chloroplast isoprene emissions at high atmospheric CO(2) concentrations were caused by competition between cytosolic and mitochondrial processes for the same substrate, possibly phosphoenolpyruvate (PEP). We conducted field and laboratory experiments using leaves of white poplar (Populus alba L.) to identify whether an inverse relationship occurs between the dark respiration rate (a mitochondrial process) and the isoprene emission rate. Field experiments that were carried out in a free-air CO(2)-enriched (FACE) facility showed no clear effect of elevated CO(2) on either isoprene emission rate or respiration rate by leaves. In young, not yet fully expanded leaves, low isoprene emission and high dark respiration rates were measured in both ambient and elevated CO(2). In these leaves, isoprene emission was inversely correlated with dark respiration. It is possible to interpret from these results that, in young leaves, high rates of growth respiration compete with isoprene biosynthesis for the same substrate. However, it is also possible that the negative correlation reflects the contrasting reductions in growth respiration and increases in expression of the enzyme isoprene synthase at this final stage of leaf maturation. In contrast to our observations on young leaves, respiration rate and isoprene emission rate were positively correlated in older, fully expanded leaves (8 and 11 from apex). A positive correlation was also found between respiration rate and isoprene emission rate when these parameters were modulated using different ozone exposure, growth light intensity, growth temperature and exposure to different leaf temperatures in laboratory experiments. These data show that competition for substrate between isoprene biosynthesis and leaf respiration does not determine the rate of isoprene emission in most circumstances that affect both processes. A negative correlation was observed across all experiments between isoprene emission rate and the activity of phosphoenolpyruvate carboxylase (PEPc), a cytosolic enzyme that competes with isoprene biosynthesis for substrate. The cytosolic metabolite, PEP, occurs at a metabolic branch point from which substrate flows into three processes: (1) the production of pyruvate for mitochondrial respiration, (2) the production of oxaloacetate (OAA) by PEPc for anabolic support of mitochondrial respiration and (3) transport into the chloroplast to support chloroplastic demands for pyruvate, including isoprenoid biosynthesis. The results of our observations suggest that only the second process competes for substrate with isoprenoid synthesis, while the partitioning of PEP between mitochondrial respiration and chloroplast isoprenoid biosynthesis is controlled in a way that retains balance in substrate demand.     

The relationship between CO2 assimilation and electron transport in leaves

The inter-relationships between the quantum efficiencies of photosystems I (_I) and II (_II) and the quantum yield of CO₂ fixation % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] were investigated in pea (Pisum sativum (L)) leaves with differing rates of photosynthesis using both photorespiratory and non-photorespiratory conditions, and in a leaf of Hedera helix (L) under photorespiratory conditions. The results indicate that under photorespiratory conditions the relationship between % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] and both _I and _II is non-linear and variable. The relationship between _I and _II under these circumstances remains predominantly linear. Under non-photorespiratory conditions, leaves with a low rate of photosynthesis due to sink limitation exhibit a non-linear relationship between _I and _II, though the relationship between _I and _II remains linear suggesting a close relationship between linear electron flow and CO₂ fixation. Leaves irradiated at the CO₂ compensation point also exhibit a non-linear relationship between _I and _II. These results suggest that for leaves in air linear electron flow is the predominant source of energy for metabolism. The role of cyclic electron transport is considered when the requirement for the products of linear electron transport is depressed.Abbreviations q_p the coefficient for photochemical quenching of chlorophyll fluorescence - _exe the quantum efficiency of excitation energy capture by open PS II traps - _II the quantum efficiency for electron transport by PS II - _I the quantum efficiency (for electron transport) by PS I - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] the quantum yield for CO₂ fixation (obtained as the gross rate of CO₂ fixation divided by the irradiance) - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabgs5aenaaBa% aaleaacqaH8oqBdaWgaaadbaGaamisamaaCaaabeqaaiabgUcaRaaa% aeqaaaWcbeaaaaa!3CB0!\[\Delta _{\mu _{H^ + } } \] trans-thylakoid proton potential difference - PAQF photosynthetically active quantum flux     

The relationship between photosystem II efficiency and quantum yield for CO(2) assimilation is not affected by nitrogen content in apple leaves

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO(2) assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O(2) (2%) and saturated CO(2) (1300 micromol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO(2) assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO(2) mol(-1) quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO(2) assimilation and photorespiration in different N leaves in response to intercellular CO(2) concentration (C(i)) under normal O(2) conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO(2) or O(2) increased with increasing leaf N at any given C(i). The percentage of non-cyclic electron flow to CO(2) assimilation, however, remained the same regardless of leaf N content. As C(i) increased, the percentage of non-cyclic electron flow to CO(2) assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO(2) assimilation and the partitioning of electron flow between CO(2) assimilation and photorespiration are not affected by N content in apple leaves.     

意杨苗木耗水特征与水分利用效率

研究杨树耗水量的变化特征、水分利用效率及其影响因子对杨树生理生态研究、造林树种的选择和林业生态工程建设具有重要的指导价值。以意杨(Populus euramevicana cv.‘I-214’)为研究对象进行盆栽试验,设定了4个处理组,分别为T1处理组(种植意杨,密封处理),T2处理组(种植意杨,非密封处理),T3处理组(不种植意杨,非密封处理),C处理组(不种植意杨,密封处理),定量分析了意杨耗水规律、水分利用效率及土壤蒸发量与植株生理特性、气象环境因子之间的关系。结果表明:(1)4个处理组耗水量变化曲线均呈"单峰型",且在7月份达到最大值,2月份降到最低值。(2)栽植意杨的土壤水分蒸发量占总耗水量的15.9%,全年波动状态稳定,本底流失量占30.4%。(3)在地表覆盖物下的意杨蒸腾耗水量占总耗水量的53.7%,年变化曲线为单峰型;栽培意杨的土壤水分总流失量是不栽培意杨土壤总流失量2.77倍;在裸地上种植意杨的土壤水分总蒸发量仅比没有意杨的裸地土壤多流失7.9%水分。(4)在有地表覆盖物下和裸地上的意杨叶面平均蒸腾强度分别为30.8 g cm~(-2)a~(-1),9.5 g cm~(-2)a~(-1);平均每克生物量耗水量为39.61 g。综上所述,意杨具有很强的蒸腾耗水能力,种植意杨可能会造成造林地区土壤水分大量流失,使该地区深层土壤干燥化,不利于土壤储水调节作用的发挥。     

Interactions between temperature and intercellular CO2 concentration in controlling leaf isoprene emission rates

Plant isoprene emissions have been linked to several reaction pathways involved in atmospheric photochemistry. Evidence exists from a limited set of past observations that isoprene emission rate (I_s) decreases as a function of increasing atmospheric CO₂ concentration, and that increased temperature suppresses the CO₂ effect. We studied interactions between intercellular CO₂ concentration (C_i) and temperature as they affect I_s in field‐grown hybrid poplar trees in one of the warmest climates on earth – the Sonoran Desert of the southwestern United States. We observed an unexpected midsummer downregulation of I_s despite the persistence of relatively high temperatures. High temperature suppression of the I_s:C_i relation occurred at all times during the growing season, but sensitivity of I_s to increased C_i was greatest during the midsummer period when I_s was lowest. We interpret the seasonal downregulation of I_s and increased sensitivity of I_s to C_i as being caused by weather changes associated with the onset of a regional monsoon system. Our observations on the temperature suppression of the I_s:C_i relation are best explained by the existence of a small pool of chloroplastic inorganic phosphate, balanced by several large, connected metabolic fluxes, which together, determine the C_i and temperature dependencies of phosphoenolpyruvate import into the chloroplast.     

Studies of the relationship between isoprene emission rate and CO2 or photon-flux density using a real-time isoprene analyser

Abstract. Studies of the isoprene emission rate in response to changes in photon-flux density and CO₂ partial pressure were conducted using a recently developed on-line isoprene analyser combined with a gas exchange system and chlorophyll fluorometer. Upon darkening, the isoprene emission rate from leaves of aspen ( Populus tremuloides Michaux.) began to decline immediately, demonstrating that the internal pool of isoprene, or its precursors, is small and that the instantaneous emission rate is tightly coupled to the rate of synthesis. A post-illumination burst of isoprene was observed within 5 min after darkening and lasted for 15–20 min in four isoprene-emitting species that were examined. In leaves of eucalyptus ( Eucalyptus globulus Labill.), the magnitude of the post-illumination burst was dependent on the photon-flux density that existed before darkening, but not on ambient CO₂ partial pressure. The dependence of the post-illumination burst on photon-flux density paralleled that for the steady-state rate of isoprene emission. A step-wise increase in intercellular CO₂ partial pressure from 24.5 to 60 Pa resulted in an immediate decrease in isoprene emission rate and non-photochemical fluorescence quenching, but an increase in CO₂ assimilation rate. Given the several recent studies that link isoprene emission to chloroplastic processes, the results of this study indicate that the linkage is not dependent on the rate of CO₂ flux through the reductive pentose phosphate pathway, but rather on more complex relationships involving metabolites not appreciably influenced by CO₂ partial pressure.     

Enhanced assimilation rate and water use efficiency with latitude through increased photosynthetic capacity and internal conductance in balsam poplar (Populus balsamifera L.)

In outdoor common gardens, high latitude populations of deciduous tree species often display higher assimilation rates ( A ) than low latitude populations, but they accomplish less height. To test whether trends in A reflect adaptation to growing season length or, alternatively, are garden growth artefacts, we examined variation in height increment and ecophysiological traits in a range-wide collection of Populus balsamifera L. populations from 21 provenances, during unconstrained growth in a greenhouse. Rooted cuttings, maintained without resource limitation under 21 h photoperiod for 90 d, displayed increasing height growth, A , leaf mass per area and leaf N per area with latitude whereas stomatal conductance ( g _s) showed no pattern. Water-use efficiency as indicated by both gas exchange and δ ¹³C increased with latitude, whereas photosynthetic nitrogen-use efficiency decreased. Differences in δ ¹³C were less than expected based on A/g _s, suggesting coextensive variation in internal conductance ( g _m). Analysis of A – C _i curves on a subset of populations showed that high latitude genotypes had greater g _m than low-latitude genotypes. We conclude that higher peak rates of height growth in high latitude genotypes of balsam poplar are supported by higher A , achieved partly through higher g _m, to help compensate for a shorter growing season.     

不同土壤水分条件下杨树人工林水分利用效率对环境因子的响应

运用涡度相关(Eddy covariance,EC)开路系统和微气象观测系统,于2007年对位于北京市大兴区永定河沙地杨树(Populus euramertcana)人工林与大气间碳、水和能量交换进行了连续测定.通过分析总生态系统生产力(GEP)、蒸发散(ET)以及水分利用效率(WUE=GEP/ET)随相对土壤含水量(REW)的变化趋势,探讨杨树人工林不同土壤水分条件下水分利用效率对气象因子以及下垫面因素的响应,为杨树人工林经营管理提供理论依据.研究结果表明:当REW＜0.1时,GEP和ET受到严重水分胁迫的影响维持在较低水平,环境因子对GEP、ET和WUE的影响较小;当0.1＜REW＜0.4时,GEP和ET随着土壤体积含水量(VWC)的增加而增大,WUE随VWC的增大而减小;REW＞0.4时,气象因子是影响碳固定和水分损耗的主要原因,由于ET对气象因子变化的响应较GEP更为敏感,因此,WUE随空气饱和水汽压差(VPD)的增大而减小.沙地土壤保水能力较差,不能保证土壤水分被植物有效利用,因此当VWC处于5.2%-8.8％(0.1＜REW＜0.4)范围时,碳固定与水分消耗达到最高效率.研究表明杨树人工林WUE随降水变化而变化,未来气候变化和变异有可能影响杨树林耗水和生产力之间的关系.     

Photosynthetic efficiency and rate of CO2 assimilation by Arthrospira (Spirulina) platensis continuously cultivated in a tubular photobioreactor

Similar to other photosynthetic microorganisms, the cyanobacterium Arthrospira platensis can be used to produce pigments, single cell proteins, fatty acids (which can be used for bioenergy), food and feed supplements, and biofixation of CO₂. Cultivation in a specifically designed tubular photobioreactor is suitable for photosynthetic biomass production, because the cultivation area can be reduced by distributing the microbial cells vertically, thus avoiding loss of ammonia and CO₂. The aim of this study was to investigate the influence of light intensity and dilution rate on the photosynthetic efficiency and CO₂ assimilation efficiency of A. platensis cultured in a tubular photobioreactor in a continuous process. Urea was used as a nitrogen source and CO₂ as carbon source and for pH control. Steady‐state conditions were achieved in most of the runs, indicating that continuous cultivation of this cyanobacterium in a tubular photobioreactor could be an interesting alternative for the large‐scale fixation of CO₂ to mitigate the greenhouse effect while producing high protein content biomass.     

CO2浓度、氮和水分对春小麦光合、蒸散及水分利用效率的影响

研究了不同土壤氮和土壤水分条件下,大气CO2浓度升高对春小麦光合作用、气孔导度、蒸散和水分利用效率的影响。结果表明,CO2浓度升高,干旱处理的春小麦（Triticum aestivum L.）叶片光合作用速率幅度增加大于湿润处理,随着氮肥用量增加光合速率相应增加,而不施氮脂增加有限;干旱处理气孔导度幅度减少大于湿润处理,不施氮肥的大于氮肥充足的CO2浓度升高,干旱处理的蒸散量减少比湿润处理多,不施氮肥的蒸散量减少较为明显;但干旱处理单叶WUE增加大于湿润处理;随着氮肥用量增加,冠层WUE提高,而不施氮肥的冠层WUE最低。因而CO2浓度升高、光合速率增加和蒸散量减少会减缓干旱的不利影响,增强作物对干旱胁迫的抵御能力。     

Long-term relationships among atmospheric CO2, stomata, and intrinsic water use efficiency in individual trees

Leaf-level responses to increases in atmospheric carbon dioxide (CO(2)) concentrations could have large implications for water and carbon cycles. We investigated whether stomatal density, guard cell length, and intrinsic water use efficiency (iWUE) of 27 individual trees growing at the Arnold Arboretum in Boston, Massachusetts have responded to changing environmental conditions over the last 100 years. We examined leaves from 74 herbarium specimens collected from three genera-Acer (maples), Quercus (oaks), and Carpinus (hornbeams)-from 1893 to 2006. During this period, global average atmospheric CO(2) concentrations increased by approximately 29% (86 ppm), and temperatures in Boston increased by 1.8°C. Stomatal density and guard cell length were negatively correlated in oaks and hornbeams. Although stomatal density declined and guard cell length increased over time, the changes were not dependent on the magnitude of changes in CO(2) concentrations. Intrinsic WUE did not change significantly over time. Our findings suggest that iWUE may not respond to changes in CO(2) concentrations over the lifetimes of individual trees, possibly because of compensating changes in stomatal density and guard cell size. We provide an example of a method that can enable researchers to differentiate between genetic and plastic responses to global change in long-lived trees.     

The relationship between the activation state of sucrose-phosphate synthase and the rate of CO2 assimilation in spinach leaves

The relationship between the gas-exchange characteristics of spinach (Spinacia oleracea L.) leaves and the activation state of sucrose-phosphate synthase was examined at different intercellular partial pressures of CO₂ at two different photon flux densities. There was a strong positive correlation between the activation state of sucrose-phosphate synthase and the assimilation rate. The relationship was the same at both photon flux densities, indicating that the activation state of the enzyme is determined by a product of carbon assimilation, rather than directly by light.Abbreviations A assimilation rate for CO₂ - p _i intercellular CO₂pressure - PFD photon flux density - SPS sucrose-phosphate-synthase - Glc6P glucose-6-phosphate - Fru6P fructose-6-phosphate A.B. was the recipient of a visiting fellowship from the National Research Council of the Italy. This work was also supported by the Science and Engineering Research Council and the Agricultural and Food Research Council, UK.     

作物群体CO2通量和水分利用效率的快速测定

In this paper, Eddy Correlation (EC) method was employed to measure the latent heat and CO2 flux density and to calculate Water Use Efficiency (WISE) of winter wheat community in Yucheng district, Shandong Province in 1997. The results showed that the CO2 flux density had an obvious diurnal change, with a maximum aboutl. 5 mg·s^-1·m^-2, which appeared at about 9 : 00-10 : 00 am in general. The WUE of wheat community presented a fall trend from morning to afternoon, and the CO2 flux density and WUE also had an obvious seasonal change, being lower in the early and late growth stages, and higher in the middle growth stage. The ranges of daily mean CO2 flux density and WUE were 0.2 - 0.9 mg·s^-1·m^-2 and 5 - 20 gCO2·kg^-1 1H2O, respectively.     

北京永定河沿河沙地杨树人工林光能利用效率

光能利用效率(LUE)是影响生态系统生产力大小和质量的主要因素。以位于北京市大兴区永定河沿河沙地的杨树(欧美107/108,Populus euramericana cv.)人工林生态系统作为研究对象,依托涡度相关观测系统,对该生态系统的LUE进行研究,从而确定LUE在不同时间尺度上的影响因子,并确定最大光能利用利用效率(LUEmax)。结果表明:LUE存在明显的季节变化趋势,4月份生长季开始后LUE迅速升高,到7—8月达到最大,而后逐渐降低;在生长季不同阶段,LUE日动态的影响因子不同:4月份气温(Ta)、蒸散比(EF)和饱和水汽压差(VPD)是影响LUE日动态的主要因子,7、8月份光合有效辐射(PAR)和冠层导度(gc)是主要影响因子,5—6月与9—10月LUE日动态则与土壤水分(VWC)有较大关系;而LUE月动态则与月蒸散比(EFm)和月平均土壤温度(Tsm)有关。由于该人工林各月光能利用最适宜环境条件不同,各月LUEmax也各有差异,该生态系统年LUEmax为0.44 g C/MJ PAR,7、8月LUEmax最大,分别为0.66和0.69 g C/MJ PAR。研究结果表明,在利用光能利用模型进行区域乃至全球初级生产力估算时需要根据研究的不同时间尺度确定LUEmax。     

Comparisons of biomass,water use efficiency and water use strategies across five genomic groups of Populus and its hybrids

Genetic improvement and hybridization in the Populus genus have led to the development of genotypes exhibiting fast growth, high rooting ability and disease resistance. However, while large biomass production is important for bioenergy crops, efficient use of resources including water is also important in sites lacking irrigation and for maintaining ecosystem water availability. In addition, comparison of water use strategies across a range of growth rates and genetic variability can elucidate whether certain strategies are shared among the fastest growing and/or most water use efficient genotypes. We estimated tree water use throughout the second growing season via sapflow sensors of 48 genotypes from five Populus taxa; P. deltoides W. Bartram ex Marshall × P. deltoides (D × D), P. deltoides × P. maximowiczii A. Henry (D × M), P. deltoides × P. nigra L. (D × N), P. deltoides × P. trichocarpa Torr. & Gray (D × T) and P. trichocarpa × P. deltoides (T × D) and calculated average canopy stomatal conductance (G_S). We regressed G_S and atmospheric vapor pressure deficit (VPD) wherein the slope of the relationship represents stomatal sensitivity to VPD. At the end of the second growing season, trees were harvested, and their dry woody biomass was used to calculate whole tree water use efficiency (WUE_T). We found that D × D and D × M genotypes exhibited differing water use strategies with D × D genotypes exhibiting high stomatal sensitivity while retaining leaves while D × M genotypes lost leaf area throughout the growing season but exhibited low stomatal sensitivity. Across measured taxa, biomass growth was positively correlated with WUE_T, and genotypes representing each measured taxa except D × N and T × D had high 2-year dry biomass of above 6 kg/tree. Overall, these data can be used to select Populus genotypes that combine high biomass growth with stomatal sensitivity and WUE_T to limit the negative impacts of bioenergy plantations on ecosystem water resources.     

The assimilation efficiency of Asellus aquaticus L. (Crustacea, Isopoda)

(1) Comparative studies of the ash-ratio and gravimetric methods of determining assimilation efficiency in Asellus aquaticus showed that the differential use of minerals by this species rendered the ash-ratio method unreliable. Results obtained by the gravimetric method were therefore employed in the analysis of further experiments. (2) The assimilation efficiencies estimated for A. aquaticus ranged between 26 and 44%, and varied according to population density and reproductive condition. (3) Individual winter males had a significantly lower assimilation efficiency (26%) than grouped animals (35%), but the assimilation efficiency of summer males (33%) did not differ significantly from that of winter males at the same density. It is concluded that density affects assimilation efficiency in A. aquaticus. (4) The assimilation efficiency of summer males (33 %) is significantly different from that of summer females (non-ovigerous, 41 %; ovigerous, 44%). A mean assimilation efficiency of 40% is proposed for the summer period whereas an overall annual mean of 30% is suggested. (5) Despite the various assimilation efficiencies reported within any one season consumption rate per unit weight is fairly constant (winter, 0·04–0·05 cal/24 h; summer, 0·36–0·38 cal/24 h) and it is suggested that the different assimilation rates are a mechanism whereby the additional energy and material requirements of females for breeding can be met without increasing food intake.     

Differentiation among populations of Sedum wrightii (Crassulaceae) in response to limited water availability: water relations,CO2 assimilation,growth and survivorship

Summary Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO₂ assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO₂ exchange patterns, in biomass ¹³C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO₂ uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.