Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4–6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.     

Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry.     

An undiscovered facet of hydraulic redistribution driven by evaporation—a study from a Populus tomentosa plantation

Maintaining the activity and function of the shallow root system of plants is essential for withstanding drought stress, but the associated mechanism is poorly understood. By investigating sap flow in 14 lateral roots (LRs) randomly selected from trees of a Chinese white poplar (Populus tomentosa) plantation receiving three levels of irrigation, an unknown root water transport mode of simultaneous daytime bi-directional water flow was discovered. This mode existed in five LRs confined to the surface soil without attached sinker roots. In the longer term, the bi-directional water flow was correlated with the soil water content. However, within the day, it was associated with transpiration. Our data demonstrated that bi-directional root sap flow occurred during the day, and was driven by evaporative demand, further suggesting the existence of circumferential water movement in the LR xylem. We named this phenomenon evaporation-driven hydraulic redistribution (EDHR). A soil-root water transport model was proposed to encapsulate this water movement mode. EDHR may be a crucial drought-tolerance mechanism that allows plants to maintain shallow root survival and activity by promoting root water recharge under extremely dry conditions.     

Redistribution of soil water by lateral roots mediated by stem tissues

Evidence is increasing to suggest that a major activity of roots is to redistribute soil water. Roots in hydraulic contact with soil generally either absorb or lose water, depending on the direction of the gradient in water potential between root and soil. This leads to phenomena such as "hydraulic lift" where dry upper soil layers drive water transfer from deep moist layers to the shallow rhizosphere and, after rain or surface irrigation, an opposite, downward water transfer. These transport processes appear important in environments where rainfall is strongly seasonal (e.g. Mediterranean-type climates). Irrigation can also induce horizontal transfers of water between lateral roots. Compared with transpiration, the magnitudes, pathways, and resistances of these redistribution processes are poorly understood. Field evidence from semi-arid eucalyptus woodlands is presented to show: (i) water is rapidly exchanged among lateral roots following rain events, at rates much faster than previously described for other types of hydraulic redistribution using sap flow methods; (ii) large axial flows moving vertically up or down the stem are associated with the horizontal transfer of water between roots on opposite sides of the stem. It appears that considerable portions of the stem axis become involved in the redistribution of water between lateral roots because of partial sectoring of the xylem around the circumference of these trees.     

Ecophysiology of a wild platyopuntia exposed to prolonged drought

The effects of reduced annual rainfall over a 7-year period on the daily patterns of gas exchange were analyzed for a wild platyopuntia (Opuntia robusta) growing in rocky soil. In spite of the prolonged water stress, stomata opened at night and net carbon gain was observed for eight 1-day sampling periods during 2000. Daily net CO₂ uptake increased more than five-fold from the end of the dry spring season to the middle of the summer wet season. Curtailment in the formation of both new cladodes and new roots is considered to be an avoidance strategy to water scarcity, which in combination with net CO₂ uptake responses to temperature and relative humidity at night, and perhaps association with arbuscular-mycorrhizal fungi, were the most important adaptations for O. robusta to the prolonged drought conditions that prevailed in its rocky environment. Such adaptations of O. robusta are in addition to the classical physiological and structural modifications of CAM plants to tolerate prolonged drought.     

Reverse flow of sap in tree roots and downward siphoning of water by Grevillea robusta

1. Constant-power heat-balance sap flow gauges were used to compare sap flow in vertical and lateral roots of Grevillea robusta trees growing without access to ground water at a semiarid site in Kenya.
2. Reversal of sap flow occurred when root systems crossed gradients in soil water potential. Measurement of changes in the direction of flow was possible because of the symmetrical construction of the sap flow gauges; gradients in temperature across the gauges, and thus computed rates of sap flow, changed sign when reverse flow occurred.
3. Reverse flow in roots descending vertically from the base of the tree occurred, while uptake by lateral roots continued, when the top of the soil profile was wetter than the subsoil. The transfer of water downwards by root systems, from high to low soil water potential, was termed 'downward siphoning'; this is the reverse of hydraulic lift.
4. Downward siphoning was induced by the first rain at the end of the dry season and by irrigation of the soil surface during a dry period.
5. Downward siphoning may be an important component of the soil water balance where there are large gradients in water potential across root systems, from a wet soil surface downwards. By transferring water beyond the reach of shallow-rooted neighbours, downward siphoning may enhance the competitiveness of deep-rooted perennials.     

Post-fire seedlings of Nothofagus alpina in Southern Chile show strong dominance of a single ectomycorrhizal fungus and a vertical shift in root architecture

We investigated belowground responses of Nothofagus alpina seedlings to post-fire conditions during natural regeneration after a wildfire in Chile, focusing on mycorrhizal community and root architecture. The complete root systems of 2-year-old N. alpina seedlings were extracted from a post-fire site with natural regeneration and compared to roots of seedlings from undisturbed forest nearby. Mycorrhizal morphotype richness was determined in each seedling. Morphometric parameters of tertiary root structure and dry biomass of whole root systems were determined in 5 cm vertical intervals and in four lateral root classes. With 43.5% of colonized vital mycorrhizal root tips, the Basidiomycete Descolea antarctica was the most abundant fungal symbiont on post-fire seedlings. Tertiary root morphology of these seedlings was distinct from control plants and characterized by a deep-reaching tap root with rather evenly distributed lateral branches whereas seedlings from the undisturbed site had shallower root systems with most lateral roots concentrated in the upper soil layers. Post-fire seedlings had more mycorrhizal rootlets and mycorrhiza-bearing third order lateral roots than control plants which was expressed in a 34% higher total root number but only a 10% higher total root biomass, although both values were not statistically significant. A major part of these fine roots in seedlings from burnt forest was found in deeper soil horizons, compared to the seedlings from undisturbed forest. According to our results, post-fire conditions clearly favour Descolea antarctica as an early ectomycorrhizal colonizer of Nothofagus seedlings at the studied site. As no significant changes in soil chemistry could be observed at the burnt site, the deep-reaching tertiary root architecture of these seedlings may be interpreted as a response to other abiotic factors like reduced moisture in surface soil.     

Ecophysiology of ectomycorrhizal fungi associated with Pinus spp. in low rainfall areas of Western Australia

As a potential means of monitoring functional properties of plantations of Pinus pinaster and Pinus radiata established as part of land rehabilitation in the wheatbelt of Western Australia, we examined aspects of the ecophysiology of ectomycorrhizal fungi associated with tree roots. A single species of ectomycorrhizal fungi, Rhizopogon roseolus, dominated the mycorrhizal flora. Sporocarps of Rhizopogon roseolus appeared with the onset of winter rains in May, increased in number and total biomass to peak in September, and decreased to negligible levels at the beginning of the summer drought in December. A greater number of sporocarps, and consequently a greater biomass of sporocarp tissue, was associated with roots of P. radiata than P. pinaster. A similar seasonal pattern of mycorrhizal root infection was determined by counts of individual ectomycorrhizal root tips from bimonthly collection of soil core samples. At the low rainfall (380 mm annually) site, greater numbers of live root tips were more strongly correlated with soil moisture than organic matter content of soil. In contrast, in wetter areas closer to Perth (800 mm annually), highest numbers of active root tips and greatest amounts of organic matter were both within 0–10 cm depths. Results suggest an overriding importance of soil moisture rather than nutrient status of the soil as the key determinant of spatial and temporal distribution of the fungus. Results from a range of assays determining enzyme activity of soil (protease, phosphomonoesterase, cellulase, L-asparaginase, L-glutaminase and β-glucosidase) surrounding mycorrhizal roots indicated seasonal patterns to be similar to those described for reproductive activity of mycorrhizal fungi. Factors responsible for patterns of seasonal activity and distribution of ectomycorrhizal roots are discussed in terms of managing systems in order to maximise tree growth and form while effectively restoring soil water balance.     

Growth and root responses of woody species to rocky substrate: implications for gully rehabilitation

Gullies formed in the Velhas River basin in Brazil have been filled with urban construction waste for physical stabilisation purposes. Aimed at rehabilitating gullies, we selected woody species from the Brazilian Cerrado that can grow on rocky substrates under greenhouse conditions. An assessment was made regarding plant growth in both rocky and natural soil substrates by analysing the height, diameter, fresh and dry weights of shoots and roots, plant water content, root occupation and architecture. Principal component analysis and Chi-squared tests segregated rock-tolerant species based on the specific influence on root dry and fresh weights. Fast-growing species reduced the emergence of their lateral roots under rocks, compromising their growth in height and biomass production. In contrast, slow-growing woody species were particularly suitable for gully rehabilitation because these species exhibited a genetic pattern of low lateral root emergence that prevented damage to their roots. Most slow-growing species demonstrated a similar growth pattern in both substrates, and some of them, such as Copaifera langsdorffii, achieved better growth in height and biomass production on rocks than on soil, a finding attributed to the root plastic response involving primary root elongation and lateral root emergence. Therefore, slow-growing species are recommended for gully rehabilitation procedures.     

土壤-根系统水分再分配:土壤-植物-大气连续体中的一个小通路

吸收和传导水分一直被视为植物根系最主要的功能之一,而人们对根系在某些情况下还可以向土壤释放水分的事实及其对植物生长和生态系统功能的影响了解得还很不充分,尽管这样的证据由来已久。土壤-根系统水分再分配(Hydraulic redistribution, HR)是近20年间被发现和证实的,指水分从土壤中较湿的部分经由植物的根系传导而运动到土壤中较干的部分,通常发生在蒸腾减弱的夜间,可以沿水势梯度下降的方向而在不同土层间向上向下或侧向运动。HR研究揭示了土壤-植物-大气连续体中有时会存在土壤-根-土壤的水流小通路,细化了土壤-根系统中水分储存和运输的时空动态和机制。土壤水分状况的连续监测、根木质部液流测量、稳定性同位素技术的使用构成了HR实验研究的三大手段。当土壤中深层水分充足的时候,HR可以提高根系吸收和传导水分的效率,有利于植物充分利用资源,延长了浅层土壤的水分可利用期,有利于维持植物组织的生理活性和水流传导;旱季后降水来临的时候,HR可以将一部分降水转移到深层土壤,增加了可利用性水分的总量。对于干旱半干旱的沙地和草原、季节性干旱的森林等类型,HR过程可能对生态系统水分循环产生重要影响。有必要在国内针对这些生态系统展开深入的实验研究,同时探索将HR过程适当结合到生态系统模型和水文模型中,从而更准确地研究和预测群落内植物水分关系和生态系统水分动态。此外,结合农林设计、植被恢复、生态需水量估算和农业节水等方面进行的HR研究也值得深入探索。     

Hydraulic redistribution in three Amazonian trees

About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.     

The influence of shade and clouds on soil water potential: The buffered behavior of hydraulic lift

In the sagebrush/bunchgrass steppe of the North American Great Basin soil water potential has been shown to exhibit diel fluctuations with water potential increasing during the night as a result of water loss from roots in relatively dry soil layers. We hypothesized that environmental conditions promoting low transpiration rates (shading, cloudiness) would cause a net increase in soil water potential as a result of reduced soil water depletion during the day and continuing water efflux from roots during the night. We examined the response of soil water potential to artificial shading in sagebrush/bunchgrass plantings and used a simple model to predict how soil water potential should respond to reduced transpiration. Field measurements of soil water potential indicated that shading reduced daytime soil water depletion, but that the magnitude of the soil water potential increase during the night was related to the magnitude of the soil water potential decline during the preceding day. As a result, shading had little net effect on soil water potential. This behavior was consistent with model results and appears to result from the fact that soil water depletion during the day is largely responsible for creating the water potential gradients that drive nocturnal recharge of the shallow soil layers. The overall effect of such behavior is to buffer the seasonal course of soil water depletion in the rooting zone against day-to-day fluctuations in evapotranspiration. Despite the buffered behavior of soil water potential change, reduced evapotranspiration during light summer rains, and resulting soil water redistribution in the rooting zone, may enhance plant water status to a greater extent that would be expected on the basis of the rainfall received.     

Clonal Variation in Morphology of Populus Root Systems Following Irrigation with Landfill Leachate or Water during 2 Years of Establishment

Increased municipal solid waste generation in North America has prompted the use of Populus for phytoremediation of waste waters including landfill leachate. Populus species and hybrids are ideal for such applications because of their high water usage rates, fast growth, and extensive root systems. Adventitious rooting (i.e., lateral rooting from primordia and basal rooting from callus) of Populus is important for phytotechnologies to ensure successful plantation establishment with genotypes that thrive when irrigated with highly variable or specific contaminants. We evaluated differences in root system morphology following establishment with high-salinity municipal solid waste landfill leachate or uncontaminated well water (control). Populus clones (NC13460, NC14018, NC14104, NC14106, DM115, DN5, NM2, and NM6) were irrigated during 2005 and 2006 in northern Wisconsin, USA and tested for differences in morphology of lateral and basal root types, as well as fine (0–2 mm diameter), small (2–5 mm), and coarse (>5 mm) roots. Across treatments and clones, trees averaged five roots per root type. Leachate-irrigated trees had 87% (lateral) and 105% (basal) as many roots as those irrigated with water. Leachate-irrigated trees had 96% as many fine roots as watering with irrigation water, whereas trees with leachate had 112% (small) and 88% (coarse) as many roots versus water. Despite root necrosis and regrowth in 23% of the trees, leachate irrigation did not negatively affect root diameter or dry mass. Given that adequate rooting is necessary for plantation establishment, leachate and similar waste waters are viable irrigation and fertilization sources of Populus crops used as feedstocks for biofuels, bioenergy, and bioproducts.     

Phenology of five tree species of a tropical dry forest in Yucatan, Mexico: effects of environmental and physiological factors

In order to relate phenological responses of trees to environmental variables we recorded the phenological patterns and select morphological and physiological traits (wood density and water potentials) of five tree species (Acacia gaumeri, Apoplanesia paniculata, Bursera simaruba, Gymnopodium floribundum, and Diospyros cuneata) in the tropical dry deciduous forest of the National Park of Dzibilchaltún, Yucatan, Mexico, over a period of 2 years (2004 and 2005). We chose two sites: one close to a permanent water source, locally known as ‘cenote’ (the CC site, ground water table was found at 2 m) and the other far from the cenote (FC site ground water table was at a depth of 10 m). Sites mainly differed in soil depth (FC site having greater soil depth) and soil nutrient characteristics (FC site more potassium, CC site more phosphorous). Our results indicated significant differences in phenology within species between sites and years, with leaf, flower and fruit production tending to be higher at the CC site and in the year 2004. Wood density and xylem water potentials were negatively related to each other, and midday water potentials were higher at the CC site. Differences in phenology found among years suggest that the timing of rainfall as well as the duration of periods without rain may play a more important role in phenology than total annual precipitation. Also differences inter-sites suggest a strong effect of site on tree phenology. Proximity to superficial bodies of water, such as cenotes, coupled with a greater concentration of available phosphorus in the soil may modify the effect of drought in this tropical dry deciduous forest.     

The anchorage mechanics of deep rooted larch, Larix europea L. japonica

The anchorage of deep rooted 16-year-old larch trees, Larixeuropea japonica, has been studied by combining winching testswith analyses of strain around the base of the trunk and rootsystem and mechanical tests on individual roots. These showedthat anchorage is provided by the laterals which emerge fromaround the stem base, sinker roots which emerge along theirlength, and tap roots positioned directly underneath the bole.During anchorage failure the leeward laterals are bent and eventuallybreak close to their base, whilst the windward laterals arepulled out of the ground, with their sinker roots intact. Afterinitially being confined by the soil and bending, the tap rootrotates in the soil. Anchorage failure is similar when the soilis dry as when it is wet, but failure occurs closer to the trunk.Strain measurements along the lateral roots revealed that thestresses were highest close to the trunk and that these regionsof the roots contribute most to tree stability. The two major components of anchorage were found to be the resistanceof leeward laterals to bending and the resistance of tap rootsand windward sinkers to uprooting. Bending tests on leewardlaterals revealed that they provide around 25% of tree anchorage.Almost 75% of the anchorage strength must, therefore, be providedby the windward sinkers and tap roots. Anchorage strength ofroots was positively correlated to their cross-sectional area.The vertical orientation of the sinkers makes the anchoragesystem of larch more efficient than the plate system formedby Sitka spruce on waterlogged soils and means that no root-soilplate is formed. Key words: Anchorage, root architecture, sinker roots, root bending strength, windthrow     

Repercussions of Simulated Climate Change on the Diversity of Woody-Recruit Bank in a Mediterranean-type Ecosystem

Extreme climatic events have the potential to affect plant communities around the world, and especially in the Mediterranean basin, where the frequency of milder and drier summers is expected to be altered under a global-change scenario. We experimentally investigated the effect of three contrasting climatic scenarios on the diversity and abundance of the natural woody-recruit bank among three characteristic habitats in a Mediterranean-type ecosystem: forest, shrubland, and bare soil. The climatic scenarios were dry summers (30% summer rainfall reduction), wet summers (simulating summer storms), and current climatic conditions (control). Seedling emergence and survival after the first summer was recorded during 4 consecutive years. The wet summer boosted abundance and diversity at emergence and summer survival, rendering the highest Shannon H??index. By contrast, the dry summer had no effect on emergence, although survival tended to decline. Nonetheless, the habitat had a key role, bare soil showing almost null recruitment whatever the climatic scenario, and forest keeping the highest diversity in all of them. Our results show that recruit-bank density and diversity depends heavily on extreme climatic events. Community dynamics will depend not only on increased drought but also on the balance between dry and wet years.     

Water relations in competitive interactions of Mediterranean grasses and shrubs

In Mediterranean ecosystems, competition between opportunistic grasses and slower-growing woody species may affect the speed and path of ecosystem recovery and the success of restoration plantings after natural or human-induced disturbance. In this experiment, competitive interactions between Mediterranean annual and perennial grass species (Avena fatua and Brachypodium retusum, respectively) and an important Mediterranean shrub (Rosmarinus offlcinalis) were examined under semi-controlled conditions simulating wet and dry Mediterranean rainfall regimes. The identity of the grass competitor and the level of water availability in the plots interacted to produce differing rates of R. offlcinalis growth but similar levels of mortality. In particular, competition with the perennial grass resulted in very low rates of R. offlcinalis growth at both irrigation levels. Measurements of soil water content showed that both grasses reduced soil moisture to low levels, though this effect was temporary in the case of the winter annual grass. Resistance to hydraulic flow in roots was highest in the perennial grass, smaller but of similar magnitude in the shrub, and much lower in the annual grass. Transpirational response to decreasing leaf water potential was a quick, sharp drop in conductance in R. offlcinalis, in contrast to a moderated decline from much lower initial transpiration rates in B. retusum. The annual grass largely maintained both leaf water potential and transpiration through leaf-tip senescence and death. Quantification of the rate of hydric recuperation of leaves after irrigation of drought-stressed plants showed that the perennial grass recovered at a rate four times that of R. offlcinalis, suggesting a strategy for making quick use of rare summer rains that may contribute to its competitive success. The appropriateness of planting or suppressing grasses in restoration of disturbed sites in Mediterranean Spain is discussed.     

胡杨根系水力提升作用的证据及其生态学意义

生长在塔里木河流域的荒漠河岸林植被虽长期忍受着高温和干旱的威胁, 然而它们却能够一直延续并保存至今。除了植物深根系吸水作用外, 另一个更主要的原因可能就是荒漠河岸林植被存在水力提升的效应。该文采用HRM热比率法茎流仪对3株胡杨(Populus euphratica)主根和侧根的液流速率分别进行了为期4 d的连续监测; 利用自动气象站对微气象因子：风速、空气相对湿度、叶面温度和地表温度进行连续监测; 同时采用了烘干法对不同深度土层在不同时刻的土壤含水率进行了取样分析。试验结果表明：胡杨主根液流在白天和夜间均表现为正值, 相反的, 胡杨侧根液流速率则出现了明显的夜间负向流动。胡杨根系0~120 cm土层土壤水分含量具有下湿上干的变化趋势; 胡杨侧根在夜间发生负向流动后, 土壤含水率显著升高, 尤其在60~120 cm土层中, 4:00土壤含水率上升幅度达到4:00时刻土壤含水率的22%~26%。影响胡杨侧根液流速率的主要气象因子主要是叶面水汽压亏缺。     

Influence of summer rainfall on root and shoot growth of a cold-winter desert shrub,Atriplex confertifolia

Summary The influence of irrigation and nitrogen fertilization in early summer on root and shoot growth of Atriplex confertifolia, a C₄ shrub species, was examined in a cold-winter desert community in northern Utah. Soil water and xylem pressure potentials were monitored during the summer period.At the time of watering the surface soil (0–30 cm) was dry but there were turgid fine roots in this horizon. Watering of the soil reduced plant water stress from-30 to-15 bars (dawn values) indicating that roots near the surface were capable of absorbing water, and induced root growth in the 0–30 cm zone. The addition of N to the water treatment did not further increase root production. However, watering and watering +N fertilizer failed to stimulate shoot elongation or any dry weight increase of shoots. This shoot dormancy during summer is not typical of C₄ plants and is probably associated with adaptation to the cool arid environment.This work was carried out while the senior author was on study leave from CSIRO