Field water relations of a compass plant, Lactuca serriola L.

Abstract Field water relations of Lactuca serriola serriola and L. serriola integrifolia were examined. Leaf conductance to water vapour was high early in the morning and declined rapidly during the midday hours. Leaf water potentials decreased to their minima early in the morning and remained low all day. Afternoon recovery of leaf conductance occurred occasionally. Leaf conductance was shown to have a linear response to vapour concentration difference. No differences were seen between L. serriola serriola and L. serriola integrifolia. The pattern of diurnal gas-exchange activity appeared to be complemented by the pattern of intercepted solar irradiance which results from the compass plant leaf orientation observed in L. serriola.     

Leaf temperature effects on gas exchange in Quercus ilex L. growing under field conditions

ABSTRACT

Gas exchange temperature dependence in Quercus ilex shrubs growing in the Mediterranean maquis was analysed. The gas exchange trend was monitored during the year: photosynthetic activity (A _net) reached the highest average rates in early spring and autumn (12.5 µmol m^-2s^-1 was the absolute maximum A _net measured) and the lowest rates were monitored in the middle of June. There was a good correlation (r = 0.72) between A _net and g _s (A _net = 4.1246 ln g _s + 4316; P < 0.01), indicating that stomatal control of CO₂ diffusion plays an important role in controlling photosynthetic activity. Leaf temperature allowing the highest photosynthetic and stomatal conductance rates of Quercus ilex were in the range 17.5 – 29°C. A _net and g_s dropped below half its maximum value when leaf temperatures were below 11.5°C and above 35.7°C. Transpiration rates (E) were strongly related to leaf temperature; E increased as leaf temperature increased and the highest E rates were monitored in June, despite a 46% decrease in g _s. Leaf water loss from transpiration, during the drought period, could result in leaf water stress which would exacerbate heat effects on photosynthesis. During summer, the increase in leaf temperatures decreased g _s which in turn decreased A _net. Consequently, stomatal control in Quercus ilex may be considered as an adaptive strategy during drought.     

Diurnal cycles of leaf extension in unsalinized and salinized Beta vulgaris

Abstract. Continuous high resolution measurement of sugar beet leaf extension over 5 d in growth chambers showed average leaf extension rates (LER) in darkness to be from three to six times those in light for plants growing in non-salinized media. The changes in LER in light-dark transitions occurred within seconds, a response which was more rapid than stomatal opening or closing. When the growth medium was salinized to 100 mol m⁻³ NaCl, LER's were reduced by about 50% in darkness and 90% in light, markedly increasing the ratio of dark to light LER.
A 2-d episode of root-zone salinity imposed midway through a 5-d period of measurement decreased LER and produced higher leaf temperatures. LER and diurnal leaf temperature patterns reverted to their pre-salinized levels when root-zone salinity was removed. Thus, the effects of short episodes of high sodium chloride in the growth medium appear to be reversible, suggesting a water stress mechanism of growth reduction rather than toxicity effects of salt.     

Leaf demography of <Emphasis Type="Italic">Festuca pallens</Emphasis> in dry grassland communities

Leaf blade parameters and leaf demography of Festuca pallens Host were studied in two types of dry grasslands. The field work was carried out in the Považsky Inovec Mts (Western Carpathians) during 1993–1995. The permanent plot in the Poo badensis-Festucetum pallentis was located on a steep, strongly eroded S-facing slope covered with dolomite outcrops, scree and sparse vegetation (20%) dominated by Festuca pallens. The permanent plot in the Festuco pallentis-Caricetum humilis was located on the even ridge plateau with shallow stony soil and vegetation covering about 70% dominated by Carex humilis and Festuca pallens. In comparison to other grasses Festuca pallens had a very low rate of leaf turnover. The highest leaf birth rates and the lowest leaf death rates were observed in June. Leaf mortality was uniformly distributed in time without a distinct minimum or maximum. For the surviving tillers the leaf production exceeded the leaf mortality during the whole growing season. The steady net gain of leaves in tillers was not interrupted by the parallel process of tillering. Among the leaf cohorts the leaves produced in May had the longest leaf blades. Leaves grew during the whole year. The winter cold and summer drought might slow down the growth rate or interrupt the growth. The growth of a leaf blade took five to eight weeks. Leaf life span was estimated to 150–200 days (time from leaf appearance at the apex to the complete loss of its green assimilating parts). In comparison to other grasses Festuca pallens belongs to the species with the longest leaf life span. The effect of environmental factors on leaf demography was followed by the comparison of two populations belonging to two phytosociological associations differing mostly in habitat xericity. Differences were revealed in the following characteristics: length of leaf blade in cohorts born during May and June, maximum length of a leaf blade in a tiller and daily increments in May and June. The course of leaf natality and mortality was similar in the studied populations.     

东灵山地区辽东栎叶的生长及其光合作用

测定辽东栎叶在不同发育时期的长度,面积和干重,应用红外ＣＯ２技术测定叶的净光合速率和暗呼吸速率的季节变化和日变化,并根据叶的平均生长速率和净光合速率推算叶生长过程中碳的输入和输出的变化趋势。结果表明：⑴辽东栎叶的长度、面积和干重的增加有共同趋势,即在叶生长早期增加很快,其后渐渐降低。叶长度、面积约在６月初达极大值;叶干重稍后达极大值。⑵净光合速率在整个生长季里随叶的生长发育是先上升,至７月中旬达极     

Environmental control of canopy dynamics and photosynthetic rate in the evergreen tussock grass Stipa tenacissima

Seasonal changes in leaf demography and gas exchange physiology in the tall evergreen tussock grass Stipa tenacissima, one of the few dominant plant species in the driest vegetation of Europe, were monitored over a period of two years at a field site in semi-arid south-eastern Spain. Three age-classes of leaves – young, mature and senescent – were distinguished in the green canopy. Production of new leaves and extension growth of older leaves occurred exclusively from October–November to May–June. The rate of extension was significantly correlated with gravimetric soil water content. Leaf growth ceased after gravimetric soil water content fell below 0.015 g g^–1 at the beginning of the dry season which corresponded to pre-dawn leaf water potentials of -3.0 MPa. Leaf senescence and desiccation reduced green leaf area by 43–49% during the dry season. Diurnal changes in the net photosynthetic rate of all three cohorts of leaves were bimodal with an early morning maximum, a pronounced midday depression and a small recovery late in the afternoon. Maximum photosynthetic rates of 10–16 mol CO₂ m^–2 s^–1 were attained from November 1993 to early May 1994 in young and mature leaves. Photosynthetic rate declined strongly during the dry season and was at or below compensation in September 1994. Gas exchange variables of young and mature leaves were not significantly different, but photosynthetic rate and diffusive conductance to water vapour of senescing leaves were significantly lower than in the two younger cohorts. Leaf nitrogen content of mature leaves varied seasonally between 2.9 and 5.2 g m^–2 (based on projected area of folded leaves), but was poorly correlated with maxima of the photosynthetic rate. There was a stronger linear relationship between the daily maxima of leaf conductance and pre-dawn leaf water potential than with atmospheric water vapour saturation deficit. Seasonal and between-year variation in daily carbon assimilation were caused mainly by differences in climatic conditions and canopy size whereas the effect of age structure of canopies was negligible. Since water is the most important limiting factor for growth and reproduction of S. tenacissima, any future rise in mean temperature, which might increase evapotranspiration, or decrease in rainfall, may considerably reduce the productivity of the grasslands, particularly at the drier end of their geographical distribution.     

Non-random leaf orientation in Lactuca serriola L.

Abstract Leaf shapes and leaf orientation of Lactuca serriola serriola and serriola integrifolia were studied. Leaf shapes in L. serriola serriola differed greatly from those of L.serriola integrifolia, but leaf surface areas were similar. In exposed habitats, leaf orientation of cauline leaves of both forms was non-random, with leaves almost vertical and tending to orient with their lamina normal to the east and west. In the shade, cauline leaves oriented randomly. An experiment demonstrated that the orientation of leaves did not change significantly once they were fully expanded. The leaf orientation in L. serriola affected the diurnal distribution of solar irradiance intercepted by a leaf. Peak solar radiation fluxes are incident on the rosette leaves at midday, but on the cauline leaves the peak solar radiation flux occurs early in the morning and again late in the afternoon. The significance of this unusual leaf orientation is discussed in relation to water loss and carbon gain.     

VARIATIONS IN THE LEAF OILS OF THREE SPECIES OF JUNIPERUS

Leaf oils from Juniperus occidentalis, J. osteosperma, and J. califomica were analyzed qualitatively and quantitatively for characteristic chemical composition. Seasonal, diurnal, and local microcline variation was quantified. Neither microcline nor seasonal variation appeared to have a significant effect on the leaf oil composition, although some variation was observed. A definite trend toward diurnal cycling was detected and tentatively linked to air temperature fluctuation from dawn to dark.     

Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LA_max) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small. Specific leaf nitrogen (SLN, g N m^-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA_max) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels. Leaf nitrogen accumulation after 35% LA_max continued up to achievement of LA_max; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LA_max) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m^-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m^-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m^-2. Thus, once the area of a leaf exceeds 35% of LA_max, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.     

Effects of an Experimental Increase of Temperature and Drought on the Photosynthetic Performance of Two Ericaceous Shrub Species Along a North–South European Gradient

Plant ecophysiological changes in response to climatic change may be different in northern and southern European countries because different abiotic factors constrain plant physiological activity. We studied the effects of experimental warming and drought on the photosynthetic performance of two ericaceous shrubs (Erica multiflora and Calluna vulgaris) along a European gradient of temperature and precipitation (UK, Denmark, The Netherlands, and Spain). At each site, a passive warming treatment was applied during the night throughout the whole year, whereas the drought treatment excluded rain events over 6–10 weeks during the growing season. We measured leaf gas exchange, chlorophyll a fluorescence, and leaf carbon isotope ratio (¹³C) during the growing seasons of 1999 and 2000. Leaf net photosynthetic rates clearly followed a gradient from northern to southern countries in agreement with the geographical gradient in water availability. Accordingly, there was a strong correlation between net photosynthetic rates and the accumulated rainfall over the growing season. Droughted plants showed lower leaf gas exchange rates than control plants in the four sites. Interestingly, although leaf photosynthetic rates decreased along the precipitation gradient and in response to drought treatment, droughted plants were able to maintain higher leaf photosynthetic rates than control plants in relation to the accumulated rainfall over the months previous to the measurements. Droughted plants also showed higher values of potential photochemical efficiency (F _v/F _m) in relation to controls, mainly at midday. The warming treatment did not affect significantly any of the studied instantaneous ecophysiological variables..     

Influence of Drought,Rain and Artificial Irrigation on Photosynthesis,Gas Exchange and Water Relations of the Fynbos Plant Protea acaulos (L.) Reich at the End of the Dry Season

Protea acaulos, a prostrate fynbos shrub, often experiences very low air humidity at leaf temperatures over 10°C higher than mean air temperature. We determined to what degree this particular microclimate influenced photosynthetic performance, leaf conductance and water relations of non-irrigated and trickle-irrigated plants. Measurements were made at the end of the dry summer season in the sand plain lowland fynbos on the west coast of South Africa. Independent of water supply, plants showed a pronounced midday depression of gas exchange. While in non-irrigated plants leaf water potential dropped to ? 2.0 MPa around noon, it never fell below ?1.0 MPa in irrigated plants. On the other hand minimum pressure potential was similar in irrigated and non-irrigated plants. The latter showed higher turgor after rain, due to osmotic acclimation, which resulted from a reduction in maximum water volume. The main osmoticum was 1,5-anhydro-D-glucitol. Leaf temperature, directly or via the vapour pressure deficit between leaf and air (Δw), rather than plant water status, was the determinant of the midday depression of gas exchange. High Δw caused stomatal closure during times of saturating light, thus limiting photosynthetic CO₂ uptake and availability and enhancing the susceptibility for photoinhibition. This, as well as high leaf temperature per se, decreased the efficiency of photochemistry of photosystem II. Initial fluorescence remained constant until temperatures exceeded 35 °C, above which changes in fluorescence indicated both photoinhibition and heat stress. Unlike other fynbos plants, Protea acaulos could not use the improved soil water supply to increase carbon gain under hot summer condition.     

迁地保护的4种龙脑香冠层叶光合速率和叶绿素荧光参数的日变化

龙脑香科植物是东南亚热带雨林冠层的优势树种,在生态和经济上具有重要地位,而我国西南地区被认为是龙脑香科植物分布的最北端。该文于2004年雨季测定了西双版纳热带植物园迁地保护区内引种的4种国产龙脑香科植物:望天树(Parashorea chinensis)、云南龙脑香(Dipterocarpus retusus)、版纳青梅(Vatica xishuangbannaensis)和海南坡垒(Hopea hainanensis)冠层叶片的光响应曲线、光合速率的日进程、叶绿素荧光参数、叶绿素含量及叶片平均面积、气孔保卫细胞长度和分布状况等。结果表明,4种植物的最大光合速率(P_max)(7.5~18.1 μmol·m ^-2·s^-1,用单位叶面积表示;89.1~150.8 nmol·g^-1 DW·s^-1,用单位干重表示)、暗呼吸速率(R_d)、光补偿点(LSP)、光饱和点(LCP)、表观量子效率和叶片特征差异显著。在正午时4个树种均出现光合作用显著降低的现象,其中海南坡垒光合速率下降最少。虽然高的叶片温度对光合作用有一定的影响,但正午光合作用的下降主要是由于正午高的叶片-空气的水蒸气压力差(LAVPD)引起的气孔关闭造成的。4个树种正午的光系统Ⅱ线性电子传递的量子效率Φ_PSⅡ显著降低,表明遭受了强烈的光抑制。云南龙脑香、版纳青梅和海南坡垒的非光化学猝灭系数(NPQ)在正午时升高的幅度最大,而望天树的NPQ上升较少,表明热耗散是前3个树种的主要光保护机制。中午前后望天树的表观电子传递速率(ETR)一直保持在很高的水平,但其光合速率显著降低,表明大量的电子被分配到光呼吸上,也说明望天树主要通过光呼吸来保护光合机构。     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

The dynamics of leaf extension in plants with diverse altitudinal ranges

Summary The dynamics of leaf extension in five species of Poa were studied with electronic auxanometers (LVDTs) along an elevational gradient from 600 to 3200 m in the Austrian Alps. Extension rates peak at midday at all elevations and rates at 20°C are almost twice as high at low elevation as compared with those from the highest sites. The low temperature threshold for leaf extension drops by 7 K over this range of elevation, with plants from the highest sites showing some extension around freezing point. Thus, there is a substantial adaptive adjustment in response of leaf extension to declining mean temperatures with increasing altitude, which is not paralleled by known altitudinal trends of photosynthetic responses in herbaceous plants.     

Comparison of physiological responses of guard cell protoplasts of Nicotiana glauca isolated from leaves collected before dawn or at midday

We observed that guard cell protoplasts isolated from leaves collected at midday from Nicotiana glauca Graham (tree tobacco) did not give the same physiological responses to light as those isolated from leaves collected in early morning. Based on that observation, we attempted to determine whether there were significant differences between the physiological responses of guard cell protoplasts isolated from leaves collected before dawn (with closed stomata) and those isolated from leaves collected at midday (with open stomata). We isolated guard cell protoplasts from leaves collected before dawn and at midday and compared (1) rates of red and blue light-induced pH changes in weakly buffered media caused by changes in their metabolism, (2) their rates of oxygen consumption in darkness and oxygen evolution in light and (3) relative rates of decay of variable chlorophyll a fluorescence in their chloroplasts. Studies with the vital stain fluorescein diacetate failed to reveal any significant differences in the viabilities of protoplast preparations from leaves collected before dawn and at midday. Furthermore, protoplasts from leaves collected at these times swelled to similar extents in an osmotic medium containing 10 µM fusicoccin and 5 mM KCI. Nevertheless, rates of light-induced pH changes, rates of oxygen consumption and evolution and rates of decay of variable chlorophyll a fluorescence were all lower in preparations of guard cell protoplasts from leaves collected at midday than in preparations from leaves collected before dawn. Initial volumes of guard cell protoplasts isolated from leaves collected at midday were 150% of those of guard cell protoplasts isolated from leaves collected before dawn. We suggest that the differences in responses of guard cell protoplasts isolated from leaves collected before dawn and at midday may be caused by (1) nonoptimal isolation conditions for guard cell protoplasts prepared from leaves collected at midday, (2) the lower surface-to-volume ratio of guard cell protoplasts isolated from leaves collected at midday or (3) diurnal and/or circadian regulation of guard cell metabolism over the course of a day.     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

A comparative analysis of the temperature response of leaf elongation in Bromus stamineus and Lolium perenne plants in the field: intrinsic and size-mediated effects

BACKGROUND AND AIMS: Growth of grass species in temperate-humid regions is restricted by low temperatures. This study analyses the origin (intrinsic or size-mediated) and mechanisms (activity of individual meristems vs. number of active meristems) of differences between Bromus stamineus and Lolium perenne in the response of leaf elongation to moderately low temperatures. METHODS: Field experiments were conducted at Balcarce, Argentina over 2 years (2003 and 2004) using four cultivars, two of B. stamineus and two of L. perenne. Leaf elongation rate (LER) per tiller and of each growing leaf, number of growing leaves and total leaf length per tiller were measured on 15-20 tillers per cultivar, for 12 (2003) or 10 weeks (2004) during autumn and winter. KEY RESULTS: LER was faster in B. stamineus than in L. perenne. In part, this was related to size-mediated effects, as total leaf length per tiller correlated with LER and B. stamineus tillers were 71% larger than L. perenne tillers. However, accounting for size effects revealed intrinsic differences between species in their temperature response. These were based on the number of leaf meristems simultaneously active and not on the (maximum) rate at which individual leaves elongated. Species differences were greater at higher temperatures, being barely notable below 5 degrees C (air temperature). CONCLUSIONS: Bromus stamineus can sustain a higher LER per tiller than L. perenne at air temperatures > 6 degrees C. In the field, this effect would be compounded with time as higher elongation rates lead to greater tiller sizes.     

Seasonal variations on water relations ofAmygdalus communis L. under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L.) of the Garrigues variety were grown in the field drip irrigated and rainfed. Leaf water potential (Ψ) and leaf conductance (g₁) were determined throughout one growing season. Pre-dawn measurement for Ψ in the irrigated treatment was consistent through the growing season, whereas in the rainfed treatment it decreased gradually. Ψ values at midday (Ψ minimum) was closely dependent on atmospheric evaporative demand, and their recovery was quicker in the wet treatment than in the dry. The g₁ values were higher in the wet than dry treatments, decreasing in both cases by leaf ageing. Maximum values for g₁ were reached when evaporative demand was highest in the day. The relationship between Ψ and g₁ revealed a decrease in the hysteresis throughout the growing season, being most marked in the dry treatment. The results highlight the close dependence of Ψ and g₁ on evaporative demand, leaf ageing and irrigtion treatment during the growing season.     

The use of a temperature-profiled position transducer for the study of low-temperature growth in Gramineae

A device is described for measuring linear extension of grass leaves during controlled cooling and heating of the growing region. The instrument was employed to investigate the sensitivity to temperature of the expanding third and fourth leaves of Lolium temulentum L. seedlings. Using a stepped temperature profile it was established that there was no lag in the response of growth rate to rapid changes in temperature below 16°C. If cooling was continued to the point where growth ceased (1°C) but no further, then rates of growth on rewarming were enhanced over the chilling range and reverted to the original rate at 20°C. Cooling to successively lower subzero temperatures before rewarming abolished the hysteretic enhancement, progressively raised the temperature at which growth resumed and decreased the rate of extension until, at-5.3°C, no recovery occurred. The temperature sensitivity of growth, measured as Q₁₀, was essentially constant when cooling from 20°C to 5°C, with 5°C-grown leaf tissue exhibiting a higher mean Q₁₀ than tissue developed at 20°C. The possible physiological significance of these data is discussed.Abbreviations LVDT linear variable displacement transformer - Pe, Fx temperatures at which growth ceases during cooling and resumes during rewarming     

Gas exchange of Agropyron desertorum: diurnal patterns and responses to water vapor gradient and temperature

Summary The response of leaf gas exchange to environmental variables were measured at different levels of drought stress for Agropyron desertorum, a naturalized perennial bunchgrass of the semiarid shrub steppes of western North America. Leaf conductance (stomatal plus boundary layer) was more sensitive to changes in water vapor gradient than to changes in leaf temperature. Assimilation was sensitive to both temperature and vapor gradient, and also appeared to be affected by conductance and high transpiration rates. The magnitudes of both assimilation and conductance decreased with increased drought conditions. Diurnal patterns of gas exchange were measured during 3 growing seasons. For a typical spring day with moderate leaf temperature and vapor gradient, diurnal patterns were similar for plants at different levels of soil water availability. Assimilation was relatively constant during most of the day, but conductance decreased during the afternoon. Total daily carbon gain was decreased to a lesser extent than daily water loss as soil water was depleted. Consequently, the ratio of daily carbon gain to daily water loss, i.e. daily water use efficiency, increased with decreased soil water content for diurnals under spring conditions. Diurnal patterns of assimilation for a typical summer day with high leaf temperature and vapor gradient differend from those for a spring day. An afternoon decrease in assimilation was typical during a summer day. Daily carbon gain, water use, and water use efficiency for summer diurnals decreased only under severe drought conditions. Almost complete recovery of assimilation and conductance occurred if leaf microclimate was ameliorated during the afternoon of either spring or summer diurnals. Thus, conditions responsible for a midday depression in assimilation during a single day did not have persistent effects on leaf gas exchange. Daily carbon gain of a typical summer day was restricted by leaf microclimate during the afternoon, but daily water use efficiency was not relatively increased by the amelioration of leaf microclimate.