Breathe softly, beetle: continuous gas exchange, water loss and the role of the subelytral space in the tenebrionid beetle, Eleodes obscura

Flightless, diurnal tenebrionid beetles are commonly found in deserts. They possess a curious morphological adaptation, the subelytral cavity (an air space beneath the fused elytra) the function of which is not completely understood. In the tenebrionid beetle Eleodes obscura, we measured abdominal movements within the subelytral cavity, and the activity of the pygidial cleft (which seals or unseals the subelytral cavity), simultaneously with total CO2 release rate and water loss rate. First, we found that E. obscura has the lowest cuticular permeability measured in flow-through respirometry in an insect (0.90 microg H2O cm(-2) Torr(-1) h(-1)). Second, it does not exhibit a discontinuous gas exchange cycle. Third, we describe the temporal coupling between gas exchange, water loss, subelytral space volume, and the capacity of the subelytral space to exchange gases with its surroundings as indicated by pygidial cleft state. Fourth, we suggest possible mechanisms that may reduce respiratory water loss rates in E. obscura. Finally, we suggest that E. obscura cannot exchange respiratory gases discontinuously because of a morphological constraint (small tracheal or spiracular conductance). This "conductance constraint hypothesis" may help to explain the otherwise puzzling phylogenetic patterns of continuous vs. discontinuous gas exchange observed in tracheate arthropods.     

Discontinuous gas exchange and water loss in the keratin beetle Omorgus radula: further evidence against the water conservation hypothesis?

Discontinuous gas exchange cycles are demonstrated in Omorgus radula (Erichson) (Coleoptera, Trogidae) for the first time, thus extending evidence for such cycles to another family of beetles. The closed, flutter and open phases of the cycle were clearly distinguishable in this species, and the duration of these phases was 221 ± 28, 1403 ± 148 and 755 ± 43 s (mean ± SE), respectively. No evidence for significant intraspecific mass scaling of VCO₂ or any of the components of the cycle was found. Although the prolonged F‐phase recorded here is unusual for many insects, it has previously been found in other scarabaeoid beetles, especially those from xeric environments. It has been suggested that such modulation of the discontinuous gas exchange cycle may result in a reduced VCO₂ and, consequently, reduced water loss. In O. radula VCO₂ (15.25 ± 1.49 μl/h) was considerably lower than that predicted from its body mass (0.207 ± 0.006 g). However, the small relative contribution of respiratory transpiration (6.5%) to total water loss indicated that reduced VCO₂ has little to do with water economy. Rather, it may be a consequence of generally low activity levels of these beetles. The low respiratory water loss, but distinct subterranean component in the adult life of O. radula, lend some credence to the hypothesis suggesting that regular use of subterranean habitats might have been responsible for the evolution of discontinuous gas exchange cycles. However, non‐adaptive hypotheses can still not be discounted.     

Neural regulation of discontinuous gas exchange in Periplaneta americana

Patterns of gas exchange among terrestrial arthropods are highly variable from continuous to discontinuous with discretely partitioned phases. The underlying initiation and co-ordination of these patterns is relatively poorly understood. Here we present a novel method for the simultaneous measurement of central nervous system (CNS) activity of the metathoracic ganglion and VCO(2) in medium to large sized live terrestrial arthropods. Using Periplaneta americana at four oxygen levels (40%, 21%, 10% and 2% at 25 degrees C; n=6 per treatment), we present minimally invasive visualization of nervous output relative to typical resting discontinuous gas exchange (DGE) data for the first time. DGE was maintained when cockroaches were exposed to hyperoxia or moderate hypoxia, but was lost in severe hypoxia. CNS activity was manifested in three signal types: large CNS output coinciding with peak CO(2) production during a burst, moderate CNS output coinciding with CO(2) sawtoothing and fluttering, and minimal CNS activity during the closed phase of DGE in normoxia. Large and moderate CNS outputs were associated with observed abdominal pumping and congruent CO(2) peaks. At 10% oxygen, VCO(2) was significantly elevated during the inter-burst period in association with almost constant moderate CNS output between the periodic large CNS output. At 2% oxygen, DGE and large CNS output are lost to continuous CO(2) release and largely continuous moderate CNS output. As previously reported for this species, a central pattern generator for ventilation in the metathoracic ganglion is supported and we infer the presence of localized oxygen chemoreceptors based on clear CNS response to a change in oxygen tension.     

The role of the subelytral cavity in respiration in a tenebrionid beetle,Onymacris multistriata (Tenebrionidae: Adesmiini)

This study measured the respiratory patterns in the tenebrionid beetle, Onymacris multistriata, using flow-through respirometry to measure carbon dioxide emission from the mesothoracic spiracles separately and simultaneously with that from around the elytral case. 96% of the total CO(2) emitted was via the mesothoracic spiracles. These spiracles used a discontinuous gas exchange cycle similar to that measured from other tenebrionid beetles. Although the circadian rhythm of the beetles resulted in changes to the period durations and cycle frequencies in the discontinuous gas exchange cycles, the mesothoracic spiracle remained the major site for gas exchange. Thus the subelytral cavity plays a different role in respiration other than the elimination of CO(2) build-up. It is expected that other arid dwelling flightless beetles will also be shown to use the mesothoracic spiracle as the major route for CO(2) emission.     

Pump out the volume—The effect of tracheal and subelytral pressure pulses on convective gas exchange in a dung beetle, Circellium bacchus (Fabricus)

Many flightless beetles like the large apterous dung beetle Circellium bacchus, possess a subelytral cavity (SEC) providing an extra air space below the elytra which connects to the tracheal system (TS) via metathoracic and abdominal spiracles. By measuring subelytral and intratracheal pressure as well as body movements and gas exchange simultaneously in a flow-through setup, we investigated the contribution of convection on Circellium respiratory gas exchange.No constriction phase was observed. TS and SEC pressures were always around atmospheric values. During interburst phase open abdominal spiracles and a leaky SEC led to small CO₂-peaks on a continuous CO₂ baseline, driven by intermittent positive tracheal pressure peaks in anti-phase with small negative subelytral pressure peaks caused by dorso-ventral tergite action.Spiracle opening was accompanied by two types of body movements. Higher frequency telescoping body movements at the beginning of opening resulted in high amplitude SEC and TS pressure peaks. High frequency tergite movements caused subelytral pressure peaks and led to a saw tooth like CO₂ release pattern in a burst. We propose that during the burst open mesothoracic spiracles increase the compliance of the subelytral cavity allowing big volumes of tracheal air being pulled out by convection.     

Reactive oxygen species production and discontinuous gas exchange in insects

While biochemical mechanisms are typically used by animals to reduce oxidative damage, insects are suspected to employ a higher organizational level, discontinuous gas exchange mechanism to do so. Using a combination of real-time, flow-through respirometry and live-cell fluorescence microscopy, we show that spiracular control associated with the discontinuous gas exchange cycle (DGC) in Samia cynthia pupae is related to reactive oxygen species (ROS). Hyperoxia fails to increase mean ROS production, although minima are elevated above normoxic levels. Furthermore, a negative relationship between mean and mean ROS production indicates that higher ROS production is generally associated with lower . Our results, therefore, suggest a possible signalling role for ROS in DGC, rather than supporting the idea that DGC acts to reduce oxidative damage by regulating ROS production.     

A test of the oxidative damage hypothesis for discontinuous gas exchange in the locust Locusta migratoria

The discontinuous gas exchange cycle (DGC) is a breathing pattern displayed by many insects, characterized by periodic breath-holding and intermittently low tracheal O(2) levels. It has been hypothesized that the adaptive value of DGCs is to reduce oxidative damage, with low tracheal O(2) partial pressures (PO(2) ≈ 2-5 kPa) occurring to reduce the production of oxygen free radicals. If this is so, insects displaying DGCs should continue to actively defend a low tracheal PO(2) even when breathing higher than atmospheric levels of oxygen (hyperoxia). This behaviour has been observed in moth pupae exposed to ambient PO(2) up to 50 kPa. To test this observation in adult insects, we implanted fibre-optic oxygen optodes within the tracheal systems of adult migratory locusts Locusta migratoria exposed to normoxia, hypoxia and hyperoxia. In normoxic and hypoxic atmospheres, the minimum tracheal PO(2) that occurred during DGCs varied between 3.4 and 1.2 kPa. In hyperoxia up to 40.5 kPa, the minimum tracheal PO(2) achieved during a DGC exceeded 30 kPa, increasing with ambient levels. These results are consistent with a respiratory control mechanism that functions to satisfy O(2) requirements by maintaining PO(2) above a critical level, not defend against high levels of O(2).     

Discontinuous gas exchange in dung beetles: patterns and ecological implications

This study correlates a distinctive pattern of external gas exchange, referred to as the discontinuous gas exchange cycle (DGC), observed in the laboratory, with habitat associations of five species of telecoprid dung beetles. The beetles were chosen from a variety of habitats that would be expected to present different amounts of water stress. All five species exhibited DGC. Sisyphus fasciculatus has been recorded only in woodland areas, and does not have strict spiracular control during its DGC. Anachalcos convexus and Scarabaeus rusticus are associated with open mesic habitats. Both species exhibit a distinct DGC, previously found in some other insect species, but intermediate within this study group. Sc. flavicornis and Circellium bacchus are typically found in arid regions, and have the most unusual form of DGC, with spiracular fluttering during the burst phase. These results support the hypothesis that spiracular fluttering reduces respiratory water loss. From this study we conclude that the DGC is an ancestral adaptation, most probably as a result of anoxic environments in underground burrows, but that spiracular control is enhanced to reduce respiratory water loss in beetle species that live in arid habitats. Received 4 August 1999 / Accepted: 7 October 1999     

多枝柽柳气体交换特性研究

在美国柽柳（Tamarix L.）被列为臭名昭著的十大我来杂草之一,而在中国,它则作为一种濒危物种生存状况堪忧。对多枝柽柳（Tamarix ramosissima L bd.）气体交换进行研究,探讨其蒸汽压差（VPD）、叶水势、气孔导度（g）和水分利用效率（WUE）之间的联系,结果表明,柽柳之所以能在沙漠生存又能在美国河岸地带疯狂扩展,主要原因可能是：①柽柳属地下水湿生植物,发达的根系能直达地下水。②柽柳适应性生境很广,作为一种阳性树种,柽柳非常耐旱,能够在很低的叶水势（φ）条件下（-4.59Mpa）进行光合作用,而较低的光补偿点和暗呼吸速率又反映出其耐阴湿的特点。③灵敏的气孔适应性调整：其g对VPD的灵敏性随着干湿季其体内的水分状况变化而变化,调整蒸腾速率,从而影响其WUE;gs与φ成显著正相关（P≤0.05）;gs与WUE显著相关（P≤0.05）,但只有当gs处于正在下降的时候,光合作用和蒸腾作用两个交换过程受轻微限制时,可得到水消耗和CO2吸收的最优协调,WUE达到最大值。这些适应性调整可能是柽柳能在美国河岸地带疯狂扩展的主要原因。实验结果表明,柽柳完全能够适应荒漠生长环境,因而导致柽柳在我国处于濒危状况的根本原因不是传统认为的干旱的自然环境,而主要是人为因素造成地下水位下降以及过度砍伐所致。     

意大利蝗不连续气体交换循环(DGC)呼吸周期历时对高温胁迫的响应

【目的】在前期研究高温胁迫下意大利蝗Calliptamus italicus呼吸代谢的变化特征基础上,为探明相关变化机制,本研究进一步从不连续气体交换循环(DGC)呼吸周期历时阐明其对高温胁迫的响应机制。【方法】以源自新疆伊犁察布查尔蝗区野生种群的意大利蝗成虫为材料,应用多通道昆虫呼吸仪测定意大利蝗DGC暴发间期、暴发期和呼吸周期三者历时随温度变化(21~42℃区间,3℃温度梯度)的响应特征。【结果】雌雄意大利蝗DGC呼吸周期中暴发间期历时随温度升高而缩短,21℃时暴发间期历时最长,雌雄分别为7.29±0.31和7.05±0.37 min,与24,27,30和33℃差异不显著(P0.05),超过33℃后迅速缩短;至39℃时最短,雌雄分别为0.42±0.01和0.70±0.03 min,与其他温度下的历时差异显著(P0.05)。暴发期历时亦随温度升高而缩短,21和39℃时,雌虫暴发期历时分别达到最长和最短,依次为1.55±0.14和0.45±0.01 min,与其他温度下的历时差异显著(P0.05);24和39℃时,雄虫暴发期历时分别达到最长和最短,依次为1.61±0.10和0.68±0.02 min,与其他温度下的历时差异显著(P0.05)。随温度升高,意大利蝗DGC呼吸周期历时呈减少趋势,且主要由暴发间期历时减少所致。21~33℃范围内,每升高3℃雌雄虫暴发间期历时分别平均减少0.174和0.121 min,DGC呼吸频率增幅一致,均为0.0022 Hz,不同温度间差异不显著(P0.05);超过33℃后,每升高3℃雌雄虫暴发间期历时分别平均减少2.197和2.189 min,DGC呼吸频率分别增加0.0120和0.0085 Hz,不同温度间差异显著(P0.05);39℃时暴发间期历时接近零,42℃时DGC呼吸模式消失。【结论】随温度升高,意大利蝗呈现出O2吸收量和CO2释放量增加的变化特征,这是通过减少暴发间期历时和增加DGC呼吸频率等机制而实现的。     

Gas exchange pattern transitions in the workers of the harvester termite

The evolutionary genesis and the current adaptive significance of the use of the discontinuous gas exchange cycle (DGC) for respiration by insects is the subject of intense debate. Years of research have resulted in several leading hypotheses, one of which is the emergent-property hypothesis. This hypothesis states that DGC is an emergent property or consequence of interactions between the O₂ and CO₂ set points that regulate spiracular function, i.e. opening and closing. Workers of the harvester termite, Hodotermes mossambicus were selected as a model to test this hypothesis. The respiratory patterns of major workers, investigated using flow-through respirometry, were obtained at 100% relative humidity (RH) under varying temperature to evaluate the assumptions of the emergent-property hypothesis. Metabolic rate, measured as VCO₂ increased significantly after 15 °C. As VCO₂ increased in response to increasing temperature and activity, the gas exchange pattern displayed by workers transitioned to a continuous gas exchange. A true DGC, defined as showing all three phases and a coefficient of variation value close to 2, was not expressed under the experimental conditions. The conclusion drawn from this study of termite workers is that changes in respiratory patterns are most likely an emergent property of the insects’ nervous and respiratory system.     

热带柚木(Tectona grandis L. f.)人工林中不同邻体干扰强度下林木CO2气体交换特征

研究以海南岛尖峰岭21a生的珍贵树种柚木人工林为对象,以邻体干扰指数为干扰强度评价指标,用Li-6400(Li-cor, Inc., USA)便携式光合作用测定系统研究了邻体干扰强度对柚木个体CO2气体交换的日动态和季节变化特性,光合-光响应特性及光合-CO2响应特性的影响.结果表明:邻体干扰并未改变个体叶片光合作用日进程,但对日净光合速率(Pn)的影响差异较大,表现为弱、中、强和极强度干扰下日Pn时段均值比为2.5 : 2.3 : 1.7 : 1.0,弱干扰下日最大净光合速率为极强干扰下的2.8倍;随着干扰强度的增加,柚木叶片的蒸腾速率、气孔导度等特征参数也出现不同程度的下降.邻体干扰对光合-光响应和光合-CO2响应特征参数的影响明显,弱干扰下柚木叶Asat、Qsat 、αA、CE、Vcmax和Jmax分别是极强干扰下的2.7、1.3、1.4、2.7、1.9和2.8倍,Qcomp和Rd则受邻体干扰的影响不明显.干扰强度对光合光响应和光合CO2响应特征的影响随光环境和CO2浓度的改变而有所差异,光环境的改变对弱干扰下柚木个体的影响明显要强于强度以上干扰的个体,而CO2浓度的改变对弱干扰个体的影响则明显要弱于强度干扰以上个体的影响.因此,在林分结构相对单一、经营管理措施基本一致的人工林内,邻体干扰是个体竞争的主要影响因素之一,干扰强度的不同将影响种内个体光合碳同化能力出现差别,导致个体生长差异.可以基于邻体干扰指数评价柚木人工林内种内竞争程度的大小,并以此为依据合理调整柚木人工林的林分结构,达到对环境资源的最合理利用.     

The length of discontinuous gas exchange cycles in lepidopteran pupae may serve as a mechanism for natural selection

Gas exchange is studied in diapausing pupae of Mamestra brassicae L., whose larvae are reared under identical conditions. The release of CO₂ gas is recorded with infrared gaseous analyzers. Oxygen convective uptake into the tracheae and oxygen consumption rates are recorded by means of a constant‐volume coulometric respirometer. Outputs from both of these respirometry systems are combined with infrared actographs. All 3‐month‐old pupae of M. brassicae display a pattern of discontinuous gas exchange (DGE) cycles of CO₂ gas release by bursts, although the lengths of these cycles varies between individuals. Some pupae exhibit long DGE cycles of at least 20 h in duration, with negligible CO₂ gas release during interburst periods, and there is presumed to be a convective gas exchange at this time. As a result of a partial vacuum inside the tracheae, a large oxygen convective uptake always occurs at the start of the spiracular opening phase. Other pupae have short DGE cycles of less than 3 h in duration, with elevated CO₂ gas release during the interburst period, when gas exchange is predominantly diffusive. The spiracular open phase in these pupae consists of frequent separate convective bursts of CO₂ gas release, with the opening–closing rhythms of the spiracles, which are considered as O phase fluttering. The pupae with long DGE cycles exhibit extremely low metabolic rates and very low total water loss rates, whereas those with short DGE cycles have higher metabolic and total water loss rates. The pupae with long DGE cycles live approximately twice as long as those with short cycles; thus, the present study demonstrates that long DGE cycles confer a fitness benefit on pupae as a result of a lower metabolic rate associated with water economy, conferring on them a longer life.     

Response of gas exchange to neighborhood interference in leaves of teak (Tectona grandis L. f.) in a tropical plantation forest

The growth performance of individual plants in a population can be affected by the plant-plant interaction, which has been well recognized and is called neighborhood interference. Though mechanisms are still unclear, the variations in gas exchange parameters in relation to the neighborhood interference between individual plants are crucial for evaluating the effects of plant-plant interaction. CO₂ assimilation in leaves of teak under natural conditions of dry seasons and wet seasons as well as its response to variations in light flux density and CO₂ concentration in different neighborhood interferences were simultaneously measured with Li-6400 portable photosynthesis system in a 21-year-old tropical plantation forest in Jianfengling, Hainan Island, China. This article dealt with the change rule of neighborhood interference on tree characteristics of gas exchange and its dynamic response to light en-vironments of individual plants. Diurnal courses of photosynthesis of individual leaves were not affected by neighborhood interfer-ence, but net photosynthetic rates showed a negative relationship with the intensity of neighborhood interference. The ratio of daily average P_n in weak, moderate, strong and heavy neighborhood interferences was 2.5:2.3:1.7:1.0, and the daily maximum P_n in weak interference was 2.8 times that in heavy interference. Leaf transpiration and stomatal conductance decreased with the increasing of interference intensity. Characteristics of photosynthetic light response and CO₂ response changed with the neighborhood interference, and values of leaf gas exchange parameters including A_sat, Q_sat, α_A, CE, V_cmax and J_max in weak interference were enhanced by 2.7, 1.3, 1.4, 2.7, 1.9 and 2.8 times, respectively, than those in heavy interference. But changes in those parameters partly depended on light environment and CO₂ concentration, and the influence of changes in light environment on weak interference individuals was sig-nificantly stronger than that on heavy interference. Beyond the growth CO₂ concentration, the influence resulting from changes in CO₂ concentration on heavy interference individuals was obviously stronger than that on weak interference. Neighborhood interference can be described as a major means of intraspecific competition of population in a plantation forest with uniform forest structure and consistent management. Carbon assimilation can be affected by the neighborhood interference, and result in divergence in growth performance. Indices of neighborhood interference can be used to evaluate the intraspecific competition, and based on this point, we can make maximum use of resources after stand structure has been well adjusted.     

Effects of flow rate and temperature on cyclic gas exchange in tsetse flies (Diptera, Glossinidae)

Air flow rates may confound the investigation and classification of insect gas exchange patterns. Here we report the effects of flow rates (50, 100, 200, 400 ml min⁻¹) on gas exchange patterns in wild-caught Glossina morsitans morsitans from Zambia. At rest, G. m. morsitans generally showed continuous or cyclic gas exchange (CGE) but no evidence of discontinuous gas exchange (DGE). Flow rates had little influence on the ability to detect CGE in tsetse, at least in the present experimental setup and under these laboratory conditions. Importantly, faster flow rates resulted in similar gas exchange patterns to those identified at lower flower rates suggesting that G. m. morsitans did not show DGE which had been incorrectly identified as CGE at lower flow rates. While CGE cycle frequency was significantly different among the four flow rates (p < 0.05), the direction of effects was inconsistent. Indeed, inter-individual variation in CGE cycle frequency exceeded flow rate treatment variation. Using a laboratory colony of closely related, similar-sized G. morsitans centralis we subsequently investigated the effects of temperature, gender and feeding status on CGE pattern variation since these factors can influence insect metabolic rates. At 100 ml min⁻¹ CGE was typical of G. m. centralis at rest, although it was significantly more common in females than in males (57% vs. 43% of 14 individuals tested per gender). In either sex, temperature (20, 24, 28 and 32 °C) had little influence on the number of individuals showing CGE. However, increases in metabolic rate with temperature were modulated largely by increases in burst volume and cycle frequency. This is unusual among insects showing CGE or DGE patterns because increases in metabolic rate are usually modulated by increases in frequency, but either no change or a decline in burst volume.     

Abdominal movements, heartbeats and gas exchange in pupae of the Colorado potato beetle, Leptinotarsa decemlineata

The rhythms of abdominal movements, heartbeats and gas exchange in the pupae of Leptiontarsa decemlineata (Say) were recorded simultaneously using an electrolytic respirometer and infrared gas analyser, both combined with contact thermography. Abdominal pulsations and heartbeat occurred periodically with little variance among individuals. The abdominal pulsations and heartbeat pauses varied individually within large limits, with the frequency of abdominal pulsations being six to seven times lower than that of the heart pulses. A proportion of the pupae (20%) showed discontinuous gas exchange with large, actively ventilated CO₂ bursts, whereas others (≈ 25%) exhibited continuous regular microcycles (flutter) with abrupt intake of air into the tracheae before discrete microbursts of carbon dioxide. The abdominal pulsations exerted only a minor influence on ventilation during the microcycles. More than 90% of the bursts of abdominal movement coincided with a series of forward directed heartbeats, but interspersed between the bouts of abdominal movement commonly two to three heartbeat pulses were observed that were not associated with abdominal movements. A period of abdominal movement associated with a heartbeat pulse was commonly initiated by one or two vigorous strokes of abdominal rotation.     

Lack of discontinuous gas exchange in a tracheate arthropod, Leiobunum townsendi (Arachnida, Opiliones)

Abstract The discontinuous gas exchange cycle, characterized by stringent spiracular control and periods of near-zero external CO₂ emission separated by 'bursts' of CO₂ emission, has evolved independently in several taxa of tracheate arthropods. These include the hexapoda, diplopoda, and several arachnid taxa; ticks, pseudoscorpions and solphugids. This paper presents the first data on gas exchange kinetics in a harvestman (Arachnida; Opiliones). The experimental animal, Leiobunum townsendi Weed, from an arid area of the south-western United States, displayed a metabolic rate similar to those of other arthropods at 25 °C (129 ± 22 µW). Their CO₂ emission kinetics showed, when the animals were motionless, only minor variations about a mean value of 0.0217 ± 0.0037 mL/h ( n  = 6, mean body mass 86 mg). Expressed on an intra-recording basis, the coefficient of variation of CO₂ emission (= SD/MEAN), which is an index of short-term gas emission fluctuations and thus of spiracular control, had a mean value of only 0.082. In contrast, the coefficient of variation of animals employing a discontinuous gas exchange cycle is > 1.5. Gas exchange in opilionids, unlike the case with most other tracheate arthropods, may therefore be dominated by simple diffusion without a prominent role for wide modulations of spiracular conductance. Contributory to this conservative spiracular control strategy may be the weak degree of tracheation in opilionids, combined with circulating haemocyanin, which acts as both a transport medium and a buffering reservoir for respiratory gas exchange.     

Bias, precision and accuracy in the estimation of cuticular and respiratory water loss: a case study from a highly variable cockroach, Perisphaeria sp

We compared the precision, bias and accuracy of two techniques that were recently proposed to estimate the contributions of cuticular and respiratory water loss to total water loss in insects. We performed measurements of VCO2 and VH2O in normoxia, hyperoxia and anoxia using flow through respirometry on single individuals of the highly variable cockroach Perisphaeria sp. to compare estimates of cuticular and respiratory water loss (CWL and RWL) obtained by the VH2O-VCO2 y-intercept method with those obtained by the hyperoxic switch method. Precision was determined by assessing the repeatability of values obtained whereas bias was assessed by comparing the methods' results to each other and to values for other species found in the literature. We found that CWL was highly repeatable by both methods (R0.88) and resulted in similar values to measures of CWL determined during the closed-phase of discontinuous gas exchange (DGE). Repeatability of RWL was much lower (R=0.40) and significant only in the case of the hyperoxic method. RWL derived from the hyperoxic method is higher (by 0.044 micromol min(-1)) than that obtained from the method traditionally used for measuring water loss during the closed-phase of DGE, suggesting that in the past RWL may have been underestimated. The very low cuticular permeability of this species (3.88 microg cm(-2) h(-1) Torr(-1)) is reasonable given the seasonally hot and dry habitat where it lives. We also tested the hygric hypothesis proposed to account for the evolution of discontinuous gas exchange cycles and found no effect of respiratory pattern on RWL, although the ratio of mean VH2O to VCO2 was higher for continuous patterns compared with discontinuous ones.     

Respiratory physiology and water relations of three species of Pogonomyrmex harvester ants (Hymenoptera: Formicidae)

The respiratory physiology and water relations of three harvester ant species (Pogonomyrmex rugosus Emery, P. occidentalis[Cresson] and P. californicus[Buckley]) were examined at three temperatures (15, 25 and 35°C) using a flow-through respirometry system. As intact ants tended to be active during testing, we performed a parallel set of experiments on individuals rendered motionless by decapitation. Both intact and decapitated ants exhibited discontinuous ventilation. Decapitation caused metabolic rate (V˙CO₂) and burst frequency to decrease in all three species. Burst volume either remained constant or increased after removal of the head, though mass-specific V˙CO₂ was unaffected except in P. rugosus. Mass-specific V˙CO₂s of headless harvesters were comparable with published values derived from motionless specimens of other ant species. The mean Q₁₀ for intact ants of all three species was 2.37; for decapitated insects the mean was 2.32. Respiratory water constituted a small (< 5%) fraction of total loss, and we believe that discontinuous ventilation does not act to conserve water in these organisms, although it may serve other functions.     

Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment

This study was conducted to determine the response in leaf growth and gas exchange of soybean (Glycine max Merr.) to the combined effects of water deficits and carbon dioxide (CO₂) enrichment. Plants grown in pots were allowed to develop initially in a glasshouse under ambient CO₂ and well-watered conditions. Four-week old plants were transferred into two different glasshouses with either ambient (360 μmol mol^-1) or elevated (700 μmol mol^-1) CO₂. Following a 2-day acclimation period, the soil of the drought-stressed pots was allowed to dry slowly over a 2-week period. The stressed pots were watered daily so that the soil dried at an equivalent rate under the two CO₂ levels. Elevated [CO₂] decreased water loss rate and increased leaf area development and photosynthetic rate under both well-watered and drought-stressed conditions. There was, however, no significant effect of [CO₂] in the response relative to soil water content of normalized leaf gas exchange and leaf area. The drought response based on soil water content for transpiration, leaf area, and photosynthesis provide an effective method for describing the responses of soybean physiological processes to the available soil water, independent of [CO₂].