Extensive summer water pulses do not necessarily lead to canopy growth of Great Basin and northern Mojave Desert shrubs

Plant species and functionally related species groups from arid and semi-arid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation, and subsequently respond with ecophysiological changes (e.g. water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and thus alter competitive interactions that structure plant communities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia, and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters (leaf N, ¹⁵N, ¹³C, gas exchange, sap flow) were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons, despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, co-limitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth, for different habitats and species, is critical for determining the significance of summer precipitation for desert community dynamics.     

Changes in winter snow depth affects photosynthesis and physiological characteristics of biological soil crusts in the Tengger Desert

Water availability is a major limiting factor in desert ecosystems. However, a winter snowfall role in the growth of biological soil crusts is still less investigated. Here, four snow treatments were designed to evaluate the effects of snow depth on photosynthesis and physiological characteristics of biological soil crusts. Results showed that snow strongly affected the chlorophyll fluorescence properties. The increased snow depth led to increased contents of photosynthetic pigments and soluble proteins. However, all biological soil crusts also exhibited a decline in malondialdehyde and soluble sugar contents as snow increased. Results demonstrated that different biological soil crusts exhibited different responses to snow depth treatment due to differences in their morphological characteristics and microhabitat. In addition, interspecies differentiation in response to snow depth treatment might affect the survival of some biological soil crusts. Further, this influence might lead to changes in the structural composition and functional communities of biological soil crusts.

     

Photosynthetic and phenological responses of dwarf shrubs to the depth and properties of snow

Snow is known to have a major impact on vegetation in arctic ecosystems, but little is known about how snow affects plants in boreal forests, where the snowpack is uneven due to canopy impact. The responses of two dwarf shrubs, the evergreen Vaccinium vitis‐idaea and the deciduous V. myrtillus, to snow conditions were studied in a snow manipulation experiment in southern Finland. The thermal insulation of the snowpack was expected to decrease with partial removal or compression of the snow, while addition of snow was expected to have the opposite effect. The penetration of light was manipulated by partial removal of snow or by formation of an artificial ice layer in the snowpack. CO₂ exchange measurements that were carried out at the time of maximum snow depth in late March indicated significant photosynthetic activity in the leaves of V. vitis‐idaea under snow. Net gain of CO₂ was observed in the daytime on all the manipulation plots, excluding the snow addition plots, where light intensity was very low. The subnivean photosynthesis compensated for a substantial proportion (up to 80%) of the respiratory CO₂ losses. Chlorophyll fluorescence measurements indicated reduced potential capacity of photosystem II in the leaves of V. vitis‐idaea on those plots where snow cover was thin. Neither V. vitis‐idaea nor V. myrtillus suffered from frost damage (assessed as electrolyte leakage) when thermal insulation was reduced by means of snow manipulations. No phenological responses were observed in V. vitis‐idaea, but in V. myrtillus bud burst, leaf unfolding and flowering were advanced by 1–3 days on the addition plots. The results of the present study show that dwarf shrubs respond to not only the thickness of snow but also the physical properties of snow, both of which are expected to change due to climatic warming.     

Intraspecific versus interspecific variation in Miocene Great Basin mylagaulids: implications for systematics and evolutionary history

The Mylagaulidae are a family of burrowing rodents abundant in Miocene faunas from western North America. Recent taxonomic revisions of mylagaulids from the Great Plains suggest that their systematics may be best understood on a regional basis. Previous studies addressed the taxonomy and evolutionary history of mylagaulids from the Great Basin, but recent discoveries of specimens, new phylogenetic data, and more detailed stratigraphical information necessitate a thorough reanalysis of their relationships and occurrences. We present a revision of the systematics of the mylagaulids from the Great Basin. In addition to rare large mylagaulids of uncertain taxonomic affinity, we recognize four species of mylagaulids distributed throughout Oregon and Nevada from the late Hemingfordian through to the early late Hemphillian: Alphagaulus vetus, Hesperogaulus gazini, Hesperogaulus wilsoni, and a new species from the genus Hesperogaulus. All species are known from large sample sizes of isolated premolars, allowing consideration of ontogenetic variation in determining the key morphological differences that allow recognition of different species. Although the number of enamel lakes varies within a given taxon, the presence of some of these lakes is taxonomically significant. This result emphasizes the importance of understanding ontogeny in describing species of fossil hypsodont mammals. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164 , 427–450.     

毛乌素沙地生物土壤结皮对油蒿水分吸收的影响模拟

植物可利用水分是决定沙生灌木生长的主要因子,生物土壤结皮(简称生物结皮)在降雨期影响降水入渗,而在干旱期改变土壤蒸发,从而影响土壤水分分布,最终可能影响灌木水分吸收。然而,关于不同降水条件下生物结皮对灌木水分吸收和水分胁迫的影响机制认识不清。以油蒿为研究对象,基于试验数据和1990—2019年气象数据,采用数学模拟,定量研究了毛乌素沙地不同降水条件下生物结皮对土壤水分分布和油蒿水分吸收的影响,评价干旱期生物结皮对油蒿水分胁迫的影响。结果表明：与无结皮处理相比,生物结皮处理的土壤蒸发降低了5.1%;生物结皮改善了干旱期的土壤水分条件;生物结皮降低了植物水分胁迫的比例,平均降低比例为8.1%;生物结皮提高了植物水分吸收,平均增加比例为12.8%;生物结皮和对照植物水分吸收的比值随季节降水量的增加而降低,均值为1.13。综上,生物结皮的出现并未消极地影响沙生灌木的水分吸收。研究结果有助于理解生物结皮与灌木的共生或竞争关系。     

High temperature effects on photosynthesis and water relations of grain legumes

Effects of high temperature on photosynthesis, and its interaction with water relations in common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), faba bean (Vicia faba), and five cultivars of field pea (Pisum sativum) were investigated. Responses of all species were compared at 20/15, 30/15, or 30/25 °C day/night, and cowpea and pea were compared at 20/15 and 30/25 °C under well-watered and limited-water conditions. Response of pea to 20/15 and 30/25 °C during flowering was ascertained, and sensitivity of the photosystem of pea and faba bean to 40 °C was determined.High temperature decreased chlorophyll variable fluorescence (F_v), a measure of injury to photosynthesis, in all species except cowpea, which was highly tolerant. Leaf chlorophyll and most measures of growth were favored by high day temperature but not by high night temperature, and photosynthetic rates were enhanced by high temperatures that increased leaf chlorophyll and nitrogen (N) contents. High temperature diminished growth less than water deficiency and increased water use of all three species but only lowered the water potential in faba bean. Water deficiency generally decreased growth, water use, and water potential more at 30/25 °C than at 20/15 °C. Stress from high temperature during flowering of pea decreased all components of yield at maturity, particularly at nodes that flowered latest. Whole-chain photosynthetic activity in thylakoids of pea, faba bean, and wheat (Triticum aestivum) were equally sensitive to high temperature, suggesting that Photosystem Il was the most labile component. The results show that high temperature affects photosynthesis, growth, and water relations of grain legumes, and sensitivity to the stress differs among species and genotypes.     

Impact of flower and seed predators on seed-set in two Banksia shrubs

A non-systemic Insecticide was used to exclude flower and seed predators from Banksia ericifolia and B. oblongifolia Inflorescenses. In B. ericifolia this treatment doubled the number of inflorescences that set seeds, and increased by 40% the number of seeds produced per Inflorescence. Insecticide treatment did not alter either of these components of seed-set in B. oblongifolia partly because the experiment began too late to exclude flower predators In this species. The results obtained for B. ericifolia support the hypothesis that flower and seed predators explain, in part, the low incidence of seed-bearing cones and the low seed:flower ratios reported for many Banksia species. Each Banksia species supports two guilds of Inflorescence predators. Two Lepidopteran species feed on young flowers; one of these causes tunnel damage to the rachis. Each Banksia species also supports five Lepidopteran and two Curcullonid seed-predators. About 40% of these herbivorous Insects are common to both shrubs.     

风蚀和沙埋对塔克拉玛干沙漠南缘骆驼刺水分和光合作用的影响

塔克拉玛干沙漠南缘风沙活动十分频繁, 风蚀和沙埋是该地区自然植被生长发育的重要影响因子。该文以塔克拉玛干沙漠南缘策勒绿洲-沙漠过渡带为研究区, 以该区域主要建群种植物骆驼刺(Alhagi sparsifolia)为研究对象, 对一次强沙尘天气过后沙丘表面5种不同风蚀沙埋状况的骆驼刺植物进行标定(包括10 cm风蚀、5 cm风蚀、不蚀不积、10 cm沙埋、30 cm沙埋), 天晴后测定其叶水势、叶片含水量、光合参数和叶绿素荧光等参数, 分析研究自然环境条件下风蚀和沙埋对骆驼刺水分和光合作用的影响。结果表明: (1)风蚀显著降低了骆驼刺叶水势和叶片含水量, 进而导致植物气孔导度降低, 并引起植物光合速率和蒸腾速率的下降。风蚀的植物水分利用效率低于沙埋, 特别是在10 cm风蚀深度明显降低。 (2)沙埋增加了骆驼刺的叶水势、叶片含水量和气孔导度, 并引起植物光合速率和蒸腾速率的上升, 水分利用效率也得到提升。(3)风蚀条件下骆驼刺所受胁迫增加, 但可以通过增加活性反应中心的数量和光化学效率来抵消胁迫造成的不利影响。沙埋条件下骆驼刺受胁迫减轻, 反应中心吸收的光能和用于光化学反应的能量随着沙埋程度增加而减小, 这是骆驼刺适应风沙环境的一种生存策略。(4)与5 cm风蚀以及10 cm沙埋相比, 10 cm风蚀显著抑制骆驼刺的生长, 30 cm沙埋则会显著促进骆驼刺的生长。     

Effects of wind erosion and sand burial on water relations and photosynthesis in Alhagi sparsifolia in the southern edge of the Taklimakan Desert

塔克拉玛干沙漠南缘风沙活动十分频繁, 风蚀和沙埋是该地区自然植被生长发育的重要影响因子。该文以塔克拉玛干沙漠南缘策勒绿洲-沙漠过渡带为研究区, 以该区域主要建群种植物骆驼刺(Alhagi sparsifolia)为研究对象, 对一次强沙尘天气过后沙丘表面5种不同风蚀沙埋状况的骆驼刺植物进行标定(包括10 cm风蚀、5 cm风蚀、不蚀不积、10 cm沙埋、30 cm沙埋), 天晴后测定其叶水势、叶片含水量、光合参数和叶绿素荧光等参数, 分析研究自然环境条件下风蚀和沙埋对骆驼刺水分和光合作用的影响。结果表明: (1)风蚀显著降低了骆驼刺叶水势和叶片含水量, 进而导致植物气孔导度降低, 并引起植物光合速率和蒸腾速率的下降。风蚀的植物水分利用效率低于沙埋, 特别是在10 cm风蚀深度明显降低。 (2)沙埋增加了骆驼刺的叶水势、叶片含水量和气孔导度, 并引起植物光合速率和蒸腾速率的上升, 水分利用效率也得到提升。(3)风蚀条件下骆驼刺所受胁迫增加, 但可以通过增加活性反应中心的数量和光化学效率来抵消胁迫造成的不利影响。沙埋条件下骆驼刺受胁迫减轻, 反应中心吸收的光能和用于光化学反应的能量随着沙埋程度增加而减小, 这是骆驼刺适应风沙环境的一种生存策略。(4)与5 cm风蚀以及10 cm沙埋相比, 10 cm风蚀显著抑制骆驼刺的生长, 30 cm沙埋则会显著促进骆驼刺的生长。     

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

雪被斑块对高山森林两种灌木凋落叶质量损失的影响

高山/亚高山森林灌木层植物凋落物的分解对于系统物质循环等过程具有重要意义, 并可能受到冬季不同厚度雪被斑块下冻融格局的影响。该文采用凋落物分解袋法, 研究了高山森林典型灌层植物华西箭竹(Fargesia nitida)和康定柳(Salix paraplesia)凋落物在沿林窗-林下形成的冬季雪被厚度梯度(厚型雪被斑块、较厚型雪被斑块、中型雪被斑块、薄型雪被斑块、无雪被斑块)上在第一年不同关键时期(冻结初期、冻结期、融化期、生长季节初期和生长季节后期)的质量损失特征。在整个冻融季节, 华西箭竹和康定柳凋落叶的平均质量损失分别占全年的(48.78 ± 2.35)%和(46.60 ± 5.02)%。冻融季节雪被覆盖斑块下凋落叶的失重率表现出厚型雪被斑块大于薄型雪被斑块的趋势,而生长季节无雪被斑块的失重率明显较高。尽管如此, 华西箭竹凋落物第一年分解表现出随冬季雪被厚度增加而增加的趋势, 但康定柳凋落物第一年失重率以薄型雪被斑块最高, 而无雪被斑块最低。同时, 相关分析表明冻融季节凋落叶的失重率与平均温度和负积温呈极显著正相关, 生长季节凋落叶的失重率与所调查的温度因子并无显著相关关系, 但全年凋落物失重率与平均温度和正/负积温均显著相关。这些结果清晰地表明, 未来冬季变暖情境下高山森林冬季雪被格局的改变将显著影响灌层植物凋落物分解, 影响趋势随着物种的差异具有明显差异。     

Daily and seasonal variation in water relations of macchia shrubs and trees in France (Montpellier) and Turkey (Antalya)

Based upon two different research studies in the mediterranean regions of France and Turkey, drought resistance strategies were investigated in a broad group of species. The diurnal and seasonal patterns of the water relations of different lifeforms from the thermo-mediterranean to submediterranean lifezones were compared. Three sites near Montpellier, in Southern France, and five sites near Antalya, Turkey were used for this comparison. Xylem pressure potential and relative stomatal aperture were the key water relations parameters collected in France while these parameters as well as osmotic potential and leaf conductance were studied in Turkey.From the 26 different study species investigated in France, 7 distinct types of stomatal control were observed, with the deciduous lifeforms showing the least control, the sclerophyllous and coniferous evergreens the greatest control and the malacophyllous shrublets intermediate levels of control. Predawn water potential values provided a means of classifying species according to their temporal and spatial utilization of site water reserves. The comparison of turgor potentials (difference between water and osmotic potentials) gave an insight into leaf adaptations to site moisture. Species with high predawn water potentials generally maintain positive turgor even at midday during the summer, whereas species with low predawn values were frequently at zero turgor even at predawn. Phlomis grandiflora was the most extreme species with mid-summer predawns and midday water potentials of –6 MPa and osmotic potentials never more negative than –2.4 MPa.     

水氮添加对内蒙古温带典型草原植被氮含量季节和年际动态的影响

作为对全球变化响应最敏感的生态系统类型之一,草原生态系统植被氮含量的季节、年际变化及其对气候变化(氮沉降、降水格局改变)的响应研究相对匮乏。基于内蒙古温带典型草原5年的氮添加(10 g N m^-2 a^-1)和水添加(添加量80 mm,分2 mm×40次、5 mm×16次、10 mm×8次、20 mm×4次、40 mm×2次5种处理)控制试验分析了水氮添加后植被氮含量在生态系统和物种两个水平的季节和年际变化。结果表明,高强度水添加处理有降低(10 mm/次和40 mm/次)生态系统氮含量的趋势,但不显著,小强度水添加处理(2 mm/次、5 mm/次)在不同年份之间无一致的升高或降低趋势,但所有水添加处理有降低两种优势物种整个生长季氮含量的趋势。氮添加促进生态系统和两种优势物种整个生长季的氮含量,但该促进作用可被水添加抵消,且这种抵消作用随水氮添加年限的延长而加剧。水氮添加均增加了生态系统氮含量的年际变异,但对特定物种季节内变异的影响在湿润和干旱年份存在一定差异。本研究将为预测草原生态系统对未来氮沉降增加和降水格局改变的响应及模型改进提供科学依据...     

Effects of potassium deficiency on water relations and photosynthesis of the tomato plant

Potassium deficient (−K) and potassium sufficient (+K) plants were exposed to four days of water stress. Well watered −K and +K plants had comparable rates of transpiration. But +K plants had a larger leaf area and depleted the soil moisture to a greater extent on day 1 of stress. For days 2 and 3 their transpiration rate, leaf water potential and relative water content fell below those of −K plants. Well watered −K plants had a significantly lower rate of photosynthesis than +K plants. Photosynthesis of −K plants was more sensitive to reduction in plant water potential than that of +K plants. Reduction of photosythesis in −K leaves was due to impairment of photosynthetic capacity and not to stomatal closure. Growth was significantly reduced in −K plants.     

Changes in water relations for Acacia ancistrocarpa on natural and mine-rehabilitation sites in response to an experimental wetting pulse in the Great Sandy Desert

Vegetation plays an essential role in mine-site rehabilitation as it serves to reduce erosion and extract moisture from the soil cover, thus minimising deep drainage to potentially hazardous materials below. Plant water-use patterns and root development are likely to be impacted where subsoils are physically and/or chemically inhospitable and close to the surface, particularly in arid and semi-arid environments where pulses of rainfall are infrequent and interspersed with long dry periods. The aim of this investigation was to determine how plant water relations of the woody shrub species Acacia ancistrocarpa (Maiden and Blakely) were affected by a summer wetting pulse on a degraded mine-rehabilitation site and a pristine natural site in the Great Sandy Desert of north-western Australia. At both the natural and rehabilitation sites, 20 m³ of water was applied to emulate an 80-mm rainfall event comparable with cyclones that occur during the summer wet season. We found that plants responded with significant increases in stomatal conductance, leaf water potential and sap-flow in lateral roots within three days of irrigation at the natural site and two days at the rehabilitation site. At the rehabilitation site this response occurred despite the fact that sinker roots were stunted by impeding physical properties of the mine-waste material. Sap-flow velocity, using the heat ratio method, in lateral and primary sinker roots was close to zero at night and positive during the day for trees at both sites during the dry period before the irrigation. These data indicate hydraulic redistribution was not occurring between the primary sinker and lateral roots at night, and water was transported along the laterals towards the crown, despite the fact they were in dry soil. Excavations revealed that lateral roots extended up to 6 m and displayed secondary sinker roots accessing water from deeper soil layers, even from mine-waste material. These morphological traits likely improved the water relations and survival of A. ancistrocarpa at the rehabilitation site. Considering these traits and the rapid pulse responsiveness of this species, we recommend A. ancistrocarpa for future rehabilitation projects at this and other mine-sites in the region.     

Field studies of water relations and photosynthesis in Scots pine.

Needle water potential at noon and diurnal variation in needle water potential was measured with a pressure chamber during the growth seasons (1974, 1975 and 1976) in a 20-year-old stand of Scots pine (Pinus sylvestris L.), subjected to daily irrigation and nutrient treatments at Jädraås, in central Sweden (60°50'N).
In unstressed conditions there was little difference in water potential amongst the treatments. However, during a dry spell in 1976 the diurnal curves of water potential in the control and combined irrigated and fertilized treatment diverged by up to 6 bar in the middle of the day. Irrigation alone resulted in higher midday water potentials (Ψ_n) than in the controls only during this period. At other times in 1974, 1975 and early 1976, the values of Ψ_n in the irrigated treatment were similar to the controls. Fertilization alone resulted in higher Ψ_n than in the controls in both 1975 and 1976 during both wet and dry periods. However, the largest difference between treatment and control occurred in the combined, irrigated and fertilized treatment during the dry spell in 1976.
Several possible explanations for the effect of fertilization on leaf water potential are discussed, including changes in hydraulic conductivity and water storage. The most likely explanation is a reduced transpiration rate as a result of more effective stomatal control of water loss.     

Diurnal and seasonal variations in the photosynthetic performance and water relations of two co-occurring Mediterranean shrubs,Erica multiflora and Globularia alypum

Diurnal and seasonal fluctuations in the photosynthetic performance and water relations of two co-occurring Mediterranean shrubs, Erica multiflora and Globularia alypum were monitored throughout two consecutive years at Garraf Natural Park in north-east Spain. Leaf gas exchange rates, chlorophyll fluorescence and shoot water potentials were measured once each season. Leaf nitrogen and carbon concentrations, leaf delta13C and delta15N and specific leaf area (SLA) were also measured once a year (August) on well developed mature leaves. Globularia alypum experienced seasonal fluctuations in their water potential, with the lowest values recorded in summer, whereas E. multiflora did not show significant differences in water potential among seasons. Moreover, lower water potentials were found in G. alypum than in E. multiflora throughout the entire study, suggesting that the latter behaved as a drought-avoiding species, whereas the former tolerated lower water potentials. In both species, maximum leaf gas exchange rates were observed in autumn and secondarily in spring; in contrast, photosynthetic and transpiration rates reached absolute minima in summer. The stronger fluctuations in water potential and leaf gas exchange rates found in G. alypum compared to E. multiflora, suggest that G. alypum is, sensu Levitt (1980), a water spender, whereas E. multiflora is a water conservative. This hypothesis is further supported by a higher integrated water-use efficiency (higher delta13C values) and a higher degree of sclerophylly (lower SLA) in E. multiflora in comparison with G. alypum. Globularia alypum showed higher leaf gas exchange rates and higher predawn potential photochemical efficiency (Fv/Fm) than E. multiflora during most of the study. In spring and autumn, predawn Fv/Fm values were within the optimal range, whereas chronic photoinhibition in summer and winter was detected in both species. However, whereas both species could maintain positive photosynthetic rates in winter, frequent negative values were found in summer, suggesting higher levels of stress during the drought period. These results together with the high correlations that were found between the net photosynthetic rates and several parameters of water availability (accumulated rainfall, soil moisture or midday water potential) provided further evidence of the key role of water availability in the regulation of the photosynthetic rates in these Mediterranean species. Warmer and drier conditions in future decades, as a consequence of climate change, may alter the present, slight competitive advantage of G. alypum and the fitness of both shrub species within semi-arid Mediterranean environments.     

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.     

Drought stress effects on three cultivars of Eragrostis curvula: photosynthesis and water relations

Water stress effects were studied on three cultivars ofEragrostis curvula. Leaf water potential, RWC, total plantleaf area, green dry weight mass percentage and CO₂ gas-exchangeweremeasured during the onset of stress and after recovery. After 3 days of waterstress, RWC of cv Tanganyika plants was around 30–40% of controls,while RWC of cvs Ermelo and Consol was around 50–60% of controls.However midday and predawn water potentials were lower in cvs Tanganyka andErmelo than in cv Consol. After re-watering, RWC and water potentials recoveredonly in Consol plants. A strong decrease of leaf area was recorded in cvsErmeloand Consol during water stress (about 91–94% less than the leafarea of controls). Photosynthesis decreased as a function of the degree ofwaterstress severity in all cultivars. Also, light saturated photosynthesis,CO₂ quantum yield and light at which saturated photosynthesisoccurred, were strongly reduced by water stress. Recovery of photosynthesis wasfound in cv Consol after five days re-watering. Cv Consol showed a betterconservation of water and higher resistance to water stress than the other twocvs.     

Effects of soil water deficit and nitrogen nutrition on water relations and photosynthesis of pot-grown Coffea canephora Pierre

Coffea canephora plants (clone INCAPER-99) were submitted to low N (LN) or high N (HN) applications and two watering regimes (daily irrigation and irrigation every 5 days for a month). Although water potential was not altered significantly by N, HN plants showed higher relative water content than did LN plants under water deficit. Only HN plants exhibited some ability for osmotic adjustment. Plants from both N treatments increased their cell wall rigidity under drought, with a more pronounced augmentation in HN plants. In well-watered plants, carbon assimilation rate increased with increasing N while stomatal conductance did not respond to N supply. Under drought conditions, carbon assimilation decreased by 68-80% compared to well-watered plants, whereas stomatal conductance and transpiration rate declined by 35% irrespective of the N applications. Stable carbon isotope analysis, combined with leaf gas exchange measurements, indicated that regardless of the watering treatments, N increased the long-term water use efficiency through changes in carbon assimilation with little or no effect on stomatal behaviour.