Yield and persistency of contrasting white clover populations grown in pure swards and in mixed swards with S.23 perennial ryegrass

Ten contrasting white clover populations were grown as spaced plants, in pure clover swards and in mixed swards with S.23 perennial ryegrass. Four of the populations were also tested for tolerance of low temperatures. In the establishment year, the autumn yields of populations were correlated with leaf size. However, during the severe winter which followed, populations with large leaves, particularly those of Mediterranean origin, suffered extensive stolon kill. This winter damage reduced the spring yields and total annual yields of populations with large leaves, so that leaf size and total annual yield were not correlated in the year after sowing. Stolon kill was positively correlated with autumn growth activity as measured by leaflet size and stolon internode length in October. Stolon kill during winter was correlated with cold tolerance of naturally-hardened stolon apices, assessed in artificial cold tests.     

Effect of temperature, irradiance and photoperiod on morphological characters of seedlings of contrasting white clover populations

Seedlings of 10 contrasting white clover populations differing in leaf size and origin were grown in a glasshouse in spring and autumn, and in controlled environments in which temperature (10°, 15°, 20°C), photoperiod ( 8 , 16-h) and irradiance ( 13, 47 , 57 Wm^-2) were varied. There were large effects of these environmental variables on a range of morphological characters. Population differences were closely related to classification of the populations by leaf size. Within the large leaf types there was also an effect of origin. In particular, Mediterranean types differed from those of northern origin. Rate of leaf appearance increased with temperature but was less affected by photoperiod. Population differences were greater at lower temperatures. Petiole length was more sensitive to temperature than was leaflet length, particularly in the range 10° to 15°C. Similarly, petiole length increased appreciably with photoperiod extension; smaller leaf types responded more than large leaf types with the result that there were no population differences in the 16-h photoperiod. Population differences in the other environments were closely related to leaf size classification. The onset of stolon production was delayed by lower temperatures and by short photoperiods but stolon number at the sixth leaf stage of development was not closely related to this character in the temperature environments since stolon numbers increased at lower temperatures. In low temperature and short day conditions the large leaf types including those of Mediterranean origin showed a desirable combination of long petioles and large leaves compared with the smaller leaf types. However, these differences between the two groups were eliminated for petiole length, and much reduced for leaflet length, by the greater response to temperature and photoperiod of the smaller leaf types. The implications of these responses for seasonal production are discussed. There were large effects of irradiance on the lengths of leaflets and petioles with longer ones at higher irradiance levels. There were also faster rates of leaf appearance at higher light levels. Stolon production was faster at 57 Wm^-2 than at 47 Wm^-2 while at 13 Wm^-2 no stolons were produced. After an extended period in the lowest light environment there was some plant mortality.     

Variation in growth attributes of contrasting populations of Trifolium repens

A range of growth attributes was measured in seedlings of 10 Trifolium repens populations, differing in leaf size and origin, grown in three temperature and two glasshouse environments. Growth rates of large leaf types of Mediterranean origin were higher than those of smaller leaf types at 10°C. However, the greater temperature response of the smaller leaf types resulted in higher growth rates for S.100 and S.184 than for a large leaf type from Israel at 20°C. The increase of growth rate with temperature was associated with changes in leaf area ratio and net assimilation rate between 10° and 15°C but only with changes in net assimilation rate between 15° and 20°C. Within each temperature environment, population differences in growth rate were related to differences in net assimilation rate rather than leaf expansion. At low temperature a greater proportion of dry matter was distributed to leaf tissue in large leaf types particularly those of Mediterranean origin but they showed a proportionately smaller increase in allocation to leaves with increasing temperature compared with small leaf types. In the glasshouse environments growth rates in spring were more than double those in the autumn. This difference was associated with net assimilation rates which were about five times greater in the spring environment. However, leaf area ratios in the spring were only half those in the autumn. These differences in leaf area ratio between the glasshouse environments were closely related to differences in specific leaf area but not to differences in distribution of dry matter to leaf tissue which was greater in the spring environment.     

Morphological and phenological differences in Nothofagus pumilio from contrasting elevations: Evidence from a common garden

Abstract We tested the hypothesis that contrasting elevations select distinct growth patterns and vegetative phenology in Nothofagus pumilio, a winter deciduous tree that dominates mountain forests of Patagonia. Analysis of saplings maintained under common‐garden conditions for 4 years showed a significant decrease in shoot annual growth, leaf size, and a delay in bud‐break, and leaf expansion with increased elevation of their site of origin. Rapid gain in height seems to be advantageous at low elevation in such light‐demanding species. Lower stature high‐elevation plants have wider branching angles and greater branching ratios (number of branches/number of internodes) than low‐elevation plants. Compact growth at high elevation may be related to strong winds and irradiance. Plants from different elevations had distinct growth patterns during the common‐garden experiment. This could be of importance in Mediterranean‐climate areas characterized by highly unpredictable precipitation regimes. Also, liberation of growth‐suppressed seedlings may follow different environmental signals in low‐ and high‐habitats, which might explain such time‐dependent responses to optimal conditions under cultivation. While these greenhouse‐grown N. pumilio saplings showed heritable differences in plant architectural traits and leafing phenology, it was not clear how the genotypes characteristic of particular elevations would respond to longer growing seasons such as those predicted under global warming.     

Grasstree (Xanthorrhoea preissii ) leaf growth in relation to season and water availability

Abstract Water stress usually arrests growth of even the most deep‐rooted species during summer drought in Mediterranean‐type climates. However, scant evidence suggests that grasstrees may represent an unusual exception. We used weather data and plant water potential to investigate the relationship between leaf growth and season in the grasstree, Xanthorrhoea preissii Endl. (Xanthorrhoeaceae). Leaf production in two contrasting habitats revealed continuous annual growth, oscillating between maximum rates (2.5–3.2 leaves/d) in late‐spring to autumn, to a minimum rate of 0.5 leaf/d during winter but never stopping. While the rate of leaf production during the fast‐growth season was positively correlated with temperature above 17–18°C, leaf elongation commenced substantially earlier in the year (from 12°C). Leaf water potentials cycled annually, with predawn readings commonly measured as zero during winter–spring and as low as ?1.26 MPa during summer, but never indicating stress by exceeding the turgor loss point. Leaf death was synchronized with summer drought. The fast (summer) growth period was characterized by rapidly fluctuating leaf production, particularly in banksia woodland, where plant growth reliably responded quickly to >18 mm of rainfall. Within 24 h of 59 mm of simulated rainfall, grasstrees in banksia woodland showed a marked increase in water potential, and leaf production reached 7.5 times the controls, confirming their capacity to respond to temporary spasmodic summer rains. Rainfall was the best climatic variable for predicting woodland grasstree leaf production during summer, whereas leaf production of forest grasstrees was most closely correlated with daylength. This plastic response of grasstrees between seasonal weather extremes is relatively rare among other mediterranean floras, and has implications for a recently proposed technique for ageing grasstrees.     

Leaf area and crop growth in relation to phenology of barley in Mediterranean environments

In a barley/livestock farming system of northern Syria, high biomass production in addition to high grain yield is desirable. The aim of this study was to assess the effect of environment and phenology on growth and yield of barley in northern Syria. Leaf area duration (LAD), specific leaf area (SLA), crop growth rate (CGR) on a thermal time basis, and biological and grain yields were compared for entries representing three contrasting development patterns: early heading spring types (pattern A), medium early heading winter types (pattern B), and late heading spring types (pattern C). The experiment was conducted in 1988/89 at two sites: Tel Hadya (TH, 250 mm precipitation) and Breda (BR, 180 mm). Cold damage occurred in winter and, especially at BR, drought stress developed in spring.At the two sites, development was similar, but yields at TH were twice those at BR. This was related to a longer LAD and a faster CGR in spring. Development pattern affected growth. A long vegetative phase (pattern B) resulted in small leaves with a low SLA in winter, probably due to a slow leaf extension rate. Since cold tolerance and profuse tillering compensated for the small leaf size, pattern B had on average a longer LAD than pattern A. Pattern C had a longer LAD than pattern A because of a longer crop duration. This long duration had a negative effect on yield, so LAD was poorly related to yield. Development in spring was associated with CGR. Pattern C had a slow CGR and low yields; pattern B had the fastest CGR, but the yield advantage over pattern A was not significant. These results suggest that early heading winter barley, which combines long LAD with fast spring CGR, may give the best performance in a barley/livestock farming system in northern Syria.     

Spring water deficit and soil conditions matter more than seed origin and summer drought for the establishment of temperate conifers

In anticipation of more severe summer droughts, forestry in temperate Europe is searching for drought-resistant ecotypes of native tree species that might maintain ecosystem services in the future. We investigated how spring precipitation and soil conditions interact with summer drought and affect the establishment of conifer seedlings from different climatic origin. Emergence, establishment and subsequent performance of seedlings originating from autochthonous, Central Alpine, continental Eastern European, and Mediterranean Pinus sylvestris and Picea abies populations were studied in the dry Alpine Rhine valley, Switzerland, at three sites with differing soil water holding capacities and in 3 years with contrasting weather conditions. In addition to this natural inter-annual variation, precipitation was manipulated within sites with throughfall reduction roofs. Seedling establishment and growth were principally affected by the spring weather in the year of emergence. In years with average to positive spring water balance, seedlings grown at the site with the highest water holding capacity had 2–5 times more aboveground biomass than seedlings grown at sites with less favourable soils. Effects of seed origin were marginal and only detectable at the drier sites: contrary to our expectations, seedlings from the Central Alpine Rhone valley, where the climatic spring water deficit is large, outperformed those from the Mediterranean. Consequently, plantation of non-native populations from dryer origin will mitigate the effects of increased summer drought at driest sites only, while the inter-annual variability of spring precipitation will continue to enable temperate conifers to regenerate on a wide range of forest soils independent of seed origin.     

Non-structural carbohydrates and nitrogen dynamics in mediterranean sub-shrubs: an analysis of the functional role of overwintering leaves

Previous studies have led to contrasting results about the role of overwintering leaves as storage sites, which is related to leaf longevity and life-form. The aim of this study was to evaluate the functional role of the leaves of four species of Mediterranean sub-shrubs, with different leaf phenology, as sources of nitrogen (N) and non-structural carbohydrates (NSC) for shoot growth. The seasonal dynamics of the concentrations and pools of N and NSC were assessed monthly in the leaves and woody organs of each species. Overwintering and spring leaves served as N and NSC sources for shoot growth in the evergreen species analyzed, providing up to 73 % and 324 % of the N demand for spring and autumn growth, respectively. Excess autumn N was stored in woody structures which contributed to the N and NSC requirements of spring growth. In the winter deciduous species, woody organs were the main N source for spring growth, while current photosynthesis from immature brachyblasts seemed to be the main carbon (C) source. Due to their short lifespan, overwintering and spring leaves did not show several translocation processes throughout their life time, their contribution to new growth being made during senescence. The successive exchange of leaf cohorts displayed by Mediterranean sub-shrubs might serve as a mechanism to recycle N and C between consecutive cohorts as plants perform the pheno-morphological changes needed to adapt their morphology to the seasonality of their environment.     

Comparison of the feeding apparatus and diet of European sardines Sardina pilchardus of Atlantic and Mediterranean waters: ecological implications

In this study, the feeding apparatus (gill rakers, GR) and the diet composition of European sardine Sardina pilchardus populations living in two contrasting environments were compared: the upwelling area off western Iberia and the comparatively less productive region of the north‐western Mediterranean Sea. The importance of local adaptations in the trophic ecology of this species was estimated. Sardina pilchardus from the Atlantic Iberian coast and from the north‐western Mediterranean Sea have clear differences in the feeding apparatus and diet compositions. Those from the Atlantic Iberian coast have significantly more GRs than S. pilchardus of the same size range in the Mediterranean Sea. While S. pilchardus from the Mediterranean Sea mostly depend on prey ranging between 750–1500 and 3000–4000 µm, corresponding mostly to cladocerans, decapods and copepods, those from the Atlantic depend on smaller prey (50–500 and 1000–1500 µm) that include phytoplankton and copepods, particularly during summer months, and S. pilchardus eggs during the winter. The marked difference between the trophic ecology of S. pilchardus in the two areas studied appears to have originated from different dietary strategies that the two populations have adopted in contrasting feeding environments. These differences are shown to profoundly affect the size and quality of prey consumed, and the effect of cannibalism on the populations.     

Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during drought

A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (g_max) and maximum leaf photosynthesis (A_max) values than Grenache at lower predawn leaf water potentials, Ψ_leaf, throughout the season. The Ψ_leaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which Ψ_leaf did not drop significantly below the minimum Ψ_leaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, K_l, incorporating leaf transpiration rate and whole‐plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure‐flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two‐ to three‐fold larger hydraulic conductance per unit path length (K_h) and a four‐ to six‐fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air‐dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry‐down. It is hypothesized that the differences in water‐conducting capacity of stems and especially petioles may be at the origin of the near‐isohydric and anisohydric behaviour of g.     

Influences of microclimatic conditions and water relations on seasonal leaf dimorphism of Prosopis glandulosa var. torreyana in the Sonoran Desert,California

Summary Seasonal measurements of microclimatic conditions were compared to seasonal indices of leaf structural components and plant water relations in Prosopis glandulosa var. torryana. P. glandulosa had two short periods of leaf production which resulted in two distinct even aged cohorts of leaves. The two leaf cohorts (summer, winter) were concurrent in the summer and fall, contrasting to previous studies on other species in which one leaf form replaces a previous leaf type. The structural characteristics of these two cohorts differed significantly in two replicate year cycles. The leaves of the spring cohort were larger in weight and area but similar to the summer cohort in specific leaf weight and leaflet number. The second growth period leaves constituted only a small proportion of the total plant leaf area. The dimorphism between the two cohorts was best associated with plant water relations and not energy load. Second growth period leaves maintained turgor to greater water deficits but lost turgor at higher leaf water potentials. Seasonal osmotic adjustment occurred for first growth period leaves but not second growth period leaves. The small leaves produced during the hot climate were most likely the result of low turgor potential during development rather than an adaptation to tolerate stressful environments.     

Relationship between acorn size and seedling morphological and physiological traits of <Emphasis Type="Italic">Quercus ilex</Emphasis> L. from different climates

Effects of Quercus ilex acorn size on seedling morphological and physiological traits were analysed. The study was carried out with five populations from different geographical areas and covering an aridity gradient. Seedling morphological traits (height and total leaf area) were analyzed during the first growing year. Physiological traits (net photosynthesis, stomatal conductance, leaf transpiration, and intrinsic water-use efficiency) were analysed at different air temperatures during spring, summer, and winter months, and in response to water stress. There were significant correlations among the considered acorn (volume, fresh mass) and seedling traits. Differences in growth and physiological traits among the considered populations were found. The larger differences in the growth parameters were observed during the first growing season, and they could be justified by the significant differences among acorn size. On the contrary, the physiological response to air temperature and aridity was more related to the geographical origin of the considered populations than to acorn size. Q. ilex acorn and seedling traits more tolerant to drought might have a high potential for vegetation recovery in afforestation projects and restoration programmes particularly under water-limited environments or in degraded areas.     

Changes in Stolon Carbohydrates During the Winter in Four Varieties of White Clover (Trifolium repensL.) with Contrasting Hardiness

Miniswards of white clover (Trifolium repensL.) varieties Menna,Huia, AberCrest and AberHerald, with contrasting winter hardiness,were established from cloned material in flat boxes. The miniswardswere placed in an unheated glasshouse with natural lightingfor the winter of 1995/96. During this time there were severalperiods of cold weather. The plants experienced temperaturesat least as severe as those experienced by unprotected plantsduring preceding mild winters. Growth measurements carried outmonthly suggested the cloned genotypes were representative ofthe varieties from which they were derived. Growth continuedthroughout the experiment, particularly when expressed as appearanceof new internodes. During the coldest weather these internodeswere short, and the amount of tissue produced was small. Starchconcentration showed a complex pattern of changes during thewinter. Older internodes were the major site of starch storagein the autumn; starch was lost from these internodes duringthe winter and early spring. Starch synthesis also occurred,even during the coldest periods. Starch accumulated in all newstolon tissue. There were significant differences between varieties.AberCrest and AberHerald retained higher concentrations of starchin older stolon tissue, synthesized higher concentrations ofstarch in young tissue and retained these for longer. Therewere few differences in soluble sugar concentrations betweenvarieties, but there were significant changes during the winter.In mid-winter, sucrose and pinitol were at their highest concentrations,whereas reducing sugars were at their lowest concentrations.This study further supports the important role of starch reservesin supporting winter survival.Copyright 1998 Annals of BotanyCompany Trifolium repens; white clover; winter hardiness; starch; sugar; pinitol.     

The effect of temperature on the response of canes of red raspberry cv. Mailing Jewel to infection by Didymella applanata

A single population of container-grown red raspberry plants cv. Mailing Jewel was divided and grown in two environments prior to wound inoculating young canes with Didymella applanata in late July. The plants were then sub-divided and held in one of three contrasting post-inoculation environments. Various assessments were made to examine the effect on spur blight of the different environmental treatments. Inoculation of internodes showed that D. applanta produced longer lesions and more pseudothecia on canes grown in spring in an unheated glasshouse than on canes grown outside, but post-inoculation conditions had little effect on these aspects of disease. Petiole inoculations showed that dwarfing of axillary buds in the first season and lateral shoot failure in the year after infection were most serious on canes grown outside in spring but held at relatively high temperatures in August and September after inoculation. The results indicate that spur blight may have the potential to cause higher yield losses in areas with warmer spring and autumn conditions than those usually experienced in north-east Scotland.     

Flowering phenology of <Emphasis Type="Italic">Ulex europaeus</Emphasis>: ecological consequences of variation within and among populations

Reproductive phenology of gorse (Ulex europaeus L., Genisteae, Fabaceae) is unusual in that the onset and duration of flowering vary greatly among individuals within populations: some plants initiate flowering in autumn or winter and continue flowering through spring, others initiate flowering in early spring. To understand the origin of this diversity and its ecological consequences, we investigated flowering phenology of randomly sampled individuals from five different natural populations in Brittany (France). Reproductive success was evaluated for individuals with contrasting flowering patterns, from 16 natural populations. Flower production, pod production, seed production and seed predation were estimated. Plants initiating flowering in spring produced larger numbers of flowers and pods over a shorter period than plants flowering from winter to spring, which produced few flowers and pods at a time but over a longer period. Pod production of long-flowering plants did not differ significantly between winter and spring, but their pods were more intensively attacked by seed predators in spring than in winter. We discuss our results in relation to biotic and abiotic parameters. We postulate that long-flowering can be interpreted as a bet-hedging strategy, spreading the risk of pod failure (rotting or freezing) in winter and of seed predation in spring.     

Gender-Related developmental instability and herbivory ofPistacia atlantica across a steep environmental gradient

To examine gender-related developmental instability and herbivory of a plant across a steep climatic gradient, we studiedPistacia atlantica DESF., a dioecious tree of the Irano-Turanian zone in the Middle East. Leaves were sampled from five populations along a climatic gradient in Israel from mesic Mediterranean to xeric (and geographically peripheral) sites. Leaves of the xeric populations were ca. 25% smaller in size (length) than leaves of other populations. Leaf developmental instability (an indicator of stress) was examined using the measurement of leaf asymmetry (LA), estimated by the deviation from the origin of paired lateral leaflets and by the presence vs. absence of the terminal leaflet. LA was not correlated with the climatic (site) gradient and had the highest, though not significantly, levels in the intermediate ecotonal zone between the Mediterranean and the arid ecosystems. Male trees were 13% higher in LA than females. Density of two specialist gall-forming aphids varied with site but was not affected by plant gender. Gender-related developmental instability is discussed in relation to resource allocation in early spring. Because the study was limited to five populations further research is clearly needed.     

Environmental and genetic control of phenotypic adaptation to drought in Potentilla glandulosa Lindl.

Summary Two populations of Potentilla glandulosa (Rosaceae) have strongly contrasting abilities to seasonally modify their phenotypic sensitivity to drought by low temperature-induced changes in leaf morphology. One population is native to an inland continental climate with unpredictable droughts occurring at any time during the year. The second population is native to a coastal mediterranean climate with a highly predictable annual cycle of winter rain and summer drought. In response to low temperatures in the autumn, the inland plants produce a compact rosette of small leaves and shed their large summer leaves, thereby reducing the total plant leaf area. The inland plants begin growth in the spring in the compact rosette phenotype and are much less sensitive to drought, maintaining higher values of stomatal conductance and leaf water potential, than when in the largeleaved summer phenotype. The coastal plants do not exhibit the low temperature-induced change in leaf morphology and are relatively sensitive to drought when grown at high or low temperatures. The F₁ hybrids of a coastal x inland cross exhibit an intermediate response to low temperature and drought. In the F₂ generation the inland parental class of individuals fully responding to low temperature segregates in a 1:63 ratio.     

The nature of migration in the red admiral butterfly Vanessa atalanta: evidence from the population ecology in its southern range

1. The migrant Vanessa atalanta (L.) occurs throughout Europe and North Africa. In autumn, populations emigrate from northern and central Europe to the Mediterranean region to overwinter. In the spring, the northern range is recolonised by migrants from the south. The dynamics of the species in the winter range is poorly known. 2. From 1994 to 1999, adults and immatures of V. atalanta were monitored all year round in Mediterranean habitats in north‐east Spain. 3. Data showed that the Catalonia lowlands is an area to which V. atalanta migrates to breed during the winter. Migrants arrive in October and early November and initiate a period of intensive breeding. Larval development occurs throughout the winter until a first annual generation of adults appears in early spring. 4. Most of the butterflies emerging in the spring emigrate and leave the area without breeding. The data suggest strongly that recolonisation of the northern range is by these butterflies not by wintering adults. Altitudinal migration also seems to be a common phenomenon, allowing a further summer generation of adults to occur at high elevations within the Mediterranean region. 5. The complex phenology of V. atalanta in its southern range has evolved as a strategy to track larval resources through space and time. Autumn migration coincides with the greatest availability of the main food plant, Urtica dioica L. Late spring migration occurs by the time food quality is decreasing.     

Influence of weather factors on population dynamics of two lagomorph species based on hunting bag records

Weather conditions can have a significant influence on short-term fluctuations of animal populations. In our study, which is based on time series of hunting bag records of up to 28?years from 26 counties of The Netherlands and Germany, we investigated the impact of different weather variables on annual counts of European rabbits (Oryctolagus cuniculus) and European hares (Lepus europaeus). Overall, the long-term dynamics of both species could be described by higher-order polynomials. On a smaller time scale, the number of European hares shot was lower in years with higher amounts of precipitation during late summer/autumn, and the number of European rabbits shot was lower in years with high precipitation in spring of the respective year. We suggest that rainy weather conditions might have lowered the survival of young rabbits in spring and might have generally facilitated the outbreak or spread of diseases in rabbits as well as in hares, specifically in autumn. In addition, the results showed a time-delayed, interactive effect between precipitation in spring and winter weather on European rabbit dynamics: rabbit numbers were limited by low temperatures of the prior winter season, but only when precipitation was high during spring of the previous year. The latter result might be explained by the lowering effects of rainy spring weather on the body condition of the animals, leading to higher sensitivity to harsh winter conditions. In conclusion, our study provides evidence for the impact of weather conditions on the population dynamics of both study species and particularly highlights complex interactions between the prevailing weather conditions during different seasons in the European rabbit.     

Ecophysiology of two solar tracking desert winter annuals

Summary The seasonal course of water relations in field populations of two leaf solar tracking desert winter annuals was examined. Measurements were made of leaf movements in relation to leaf conductance and water potential. Malvastrum rotundifolium maintained solar tracking movements up to the wilting point of the plant (-4 MPa). Lupinus arizonicus altered its morphology through paraheliotropic leaf movements as leaf water potentials declined to-1.8 MPa. Diurnal patterns of leaf conductance showed marked seasonal trends, with gas exchange activity being restricted to early morning hours as water availability declined. Studies of potted plants showed that L. arizonicus was not able to alter its osmotic potential in response to drought, while M. rotundifolium underwent a 1.86 MPa reduction in osmotic potential. The significance of the two contrasting patterns is discussed in terms of observed plant distribution and origin.