Effects of cropping, nutrition and water supply on accumulation and distribution of biomass and nutrients for apple trees on'M9'root systems

The effects of cropping, nutrition and water supply on accumulation and distribution of biomass and nutrients for apple trees on'M9'root systems were examined using trees growing in lysimeters. For 3-year-old 'Golden Delicious' trees, cropping significantly increased total biomass, although leaf and root biomass were reduced. For 3-year-old'Golden Delicious', 'Coxs Orange' and 'Gloster' trees, cropping only in the third year reduced root biomass by 39 to 45%. Cropping did not affect water use by trees of any cultivar early in the season, but increased water use later in the season for 'Gloster' trees. For trees with fruit, total non-structural carbohydrate (TNC) contents at the end of the season were highest in fruit (up to 1 400 g plant ⁻¹). For trees without fruit, TNC contents were generally highest in the roots (up to 110 g planr ⁻¹). Supplying trees with nutrient solution diluted to 10% of the standard composition increased the root biomass for 'Golden Delicious' trees only, but decreased biomass of above-ground tissues for all cultivars. Reducing the nutrient supply decreased water and nutrient uptake, and reduced nutrient contents within vegetative parts of the tree more than within roots. For 'Golden Delicious' trees, restricting the water supply to 50% or 25% of that consumed by control trees significantly reduced above-ground biomass, but root biomass was not significantly affected. The N, P and K. contents for the trees were also reduced by water stress, due to reduced contents within above-ground organs. Water stress reduced the TNC and starch contents of all tissues, except the roots. These results are discussed in relation to the efficiency of root systems for water and nutrient uptake.     

Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and thus often occurs in saline areas fringing permanent wetlands and in ephemeral swamps. The dominance of this tree in natural groundwater discharge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal changes in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m^–1; 0.3–1.2 m deep) allowed the patterns of M. halmaturorum root water uptake to be followed over a 15 month period. Tree water sources were examined using the naturally occurring stable isotopes of water, while new root growth was examined using a field root observation window and from soil cores. The presence of isotopic fractionation of ²H under conditions of soil salinity and waterlogging was tested in a glasshouse experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in taking up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was concentrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at the end of summer. When groundwater rose, at the end of winter, roots responded by extracting water from near the soil surface (0–0.1 m), at the new watertable. This pattern of water uptake in response to groundwater fluctuations and salt accumulation in the surface soil was also reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water potential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summer and a combination of rainfall and groundwater from the surface soils in winter. The ability to take up water from saline substrates through the maintenance of low leaf water potential, combined with this ability to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.     

Water stress effects on leaf elongation,leaf water potential,transpiration, and nutrient uptake of rice,maize, and soybean

A pot experiment was conducted in the greenhouse to determine and compare the responses of rice (Oryza sativa L. var, IR 36), maize (Zea mays L. var. DMR-2), and soybean (Glycine max [L.] Merr. var. Clark 63) to soil water stress. Leaf elongation, dawn leaf water potential, transpiration rate, and nutrient uptake in stressed rice declined earlier than in maize and soybean. Maize and soybean, compared with rice, maintained high dawn leaf water potential for a longer period of water stress before leaf water potential. Nutrient uptake under water stress conditions was influenced more by the capacity of the roots to absorb nutrients than by transpiration. Transport of nutrients to the shoots may occur even at reduced transpiration rate It is concluded that the ability of maize and soybean to grow better than rice under water stress conditions may be due to their ability to maintain turgor as a result of the slow decline in leaf water potential brought about by low, transpiration rate and continued uptake of nutrient, especially K, which must have allowed osmotic adjustment to occur.     

Root distribution and seasonal production in the northwestern Sonoran Desert for a C3 subshrub, a C4 bunchgrass, and a CAM leaf succulent

To investigate root distribution with depth, which can affect competition for water, surface areas of young and old roots were determined in 4-cm-thick soil layers for the C3 subshrub Encelia farinosa Torrey and A. Gray, the C4 bunchgrass Pleuraphis rigida Thurber, and the CAM (crassulacean acid metabolism) leaf succulent Agave deserti Engelm. At a site in the northwestern Sonoran Desert these codominant perennials had mean rooting depths of only 9-10 cm for isolated plants. Young roots had mean depths of 5-6 cm after a winter wet period, but 11-13 cm after a summer wet period. Young roots were most profuse in the winter for E. farinosa, which has the lowest optimum temperature for root growth, and in the summer for P. rigida, which has the highest optimum temperature. Roots for interspecific pairs in close proximity averaged 2-3 cm shallower for A. deserti and a similar distance deeper for the other two species compared with isolated plants, suggesting partial spatial separation of their root niches when the plants are in a competitive situation. For plants with a similar root surface area, the twofold greater leaf area and twofold higher maximal transpiration rate of E. farinosa were consistent with its higher root hydraulic conductivity, leading to a fourfold higher estimated maximal water uptake rate than for P. rigida. Continuous water uptake accounted for the shoot water loss by A. deserti, which has a high shoot water-storage capacity. A lower minimum leaf water potential for P. rigida than for A. deserti indicates greater ability to extract water from a drying soil, suggesting that temporal niche separation for water uptake also occurs.     

Root morphology and water transport of Pistacia lentiscus seedlings under contrasting water supply: A test of the pipe stem theory

Restoration of degraded Mediterranean areas often requires the reintroduction of key-stone woody species but the establishment of seedlings of native species is frequently poor. This is partly due to insufficient knowledge of the ecology of these species at the seedling stage. Fast rooting and efficient water supply under water limiting conditions may be crucial to withstand summer drought and ensure establishment. However, knowledge of the relationship between root morphology and the water transport capacity of Mediterranean woody species in response to drought is still scarce. We evaluated the effect of low water availability on biomass allocation, root morphology and transpiration of a common Mediterranean shrub species, Pistacia lentiscus L. Seedlings of this species were grown in pots filled with soil under glasshouse conditions for 6 months, and irrigated either weekly (W+) or monthly (W?). Low water availability strongly reduced all fractions of biomass, and decreased relative biomass allocation belowground. Average diameter of fine roots colonising the soil was higher in W+ plants, but this resulted in only marginal effects on specific root length. Water limitation did not affect the topology of secondary roots colonising the soil. Surprisingly, the ratio of leaf area to coloniser roots surface area was higher in W? seedlings. Sapwood area was strongly correlated with leaf area, secondary roots cross-sectional area, and surface area of fine roots colonising the soil when all seedlings were pooled. In agreement with the pipe stem theory, the ratio of sapwood area to leaf area was not affected by watering regime. Plant water loss when soils were taken to field capacity was significantly correlated with leaf area, sapwood area, secondary roots cross-section area and coloniser roots surface area. Water loss at high water availability was greatly reduced in W? plants, as leaf area decreased and transpiration rates on a leaf area basis were similar in W+ and W? seedlings. P. lentiscus showed limited capacity to acclimate to low water availability by modifying biomass allocation and root morphology. Thus, parallel to what has been observed aboveground, this species can benefit from periods of high water availability by showing relatively high root growth rates, but may respond poorly to water scarcity.     

High sensitivity of Lobelia dortmanna to sediment oxygen depletion following organic enrichment

? Lobelia dortmanna thrives in oligotrophic, softwater lakes thanks to O(2) and CO(2) exchange across roots and uptake of sediment nutrients. We hypothesize that low gas permeability of leaves constrains Lobelia to pristine habitats because plants go anoxic in the dark if O(2) vanishes from sediments. ? We added organic matter to sediments and followed O(2) dynamics in plants and sediments using microelectrodes. To investigate plant stress, nutrient content and photosynthetic capacity of leaves were measured. ? Small additions of organic matter triggered O(2) depletion and accumulation of NH(4)(+), Fe(2+) and CO(2) in sediments. O(2) in leaf lacunae fluctuated from above air saturation in the light to anoxia late in the dark in natural sediments, but organic enrichment prolonged anoxia because of higher O(2) consumption and restricted uptake from the water. Leaf N and P dropped below minimum thresholds for cell function in enriched sediments and was accompanied by critically low chlorophyll and photosynthesis. ? We propose that anoxic stress restricts ATP formation and constrains transfer of nutrients to leaves. Brief anoxia in sediments and leaf lacunae late at night is a recurring summer phenomenon in Lobelia populations, but increased input of organic matter prolongs anoxia and reduces survival.     

STRANGLER FIG ROOTING HABITS AND NUTRIENT RELATIONS IN THE LLANOS OF VENEZUELA

The strangler figs, Ficus pertusa and F. trigonata, are abundant in the seasonally flooded palm savanna (llanos intermedio) near Calabozo, Venezuela. The most common host tree for the hemiepiphytic figs is the palm Copernicia tectorum; nearly half of the palms support either an epiphytic or a ground-rooted fig. During their epiphytic stage the figs are rooted behind the palms' marcescent leaf bases. Material trapped behind the leaf bases is higher in organic matter, nitrogen, magnesium, and potassium than soil from the ground near the palms. The suggestion that nutrient availability to epiphytes is high is supported by the observation that concentrations of several nutrients, including N, P, and K, are significantly higher in epiphytic leaves than in tree leaves. Figs retain access to the epiphytic medium by producing upwardly growing (apogeotropic) roots that remain attached in the host palm's crown long after the fig has become firmly rooted in the ground. Although upward growing roots are expected to be more important in nutrient than water uptake, there are no obvious differences in the xylem anatomy of upward and downward growing fig roots. Terrestrial roots of fig trees are generally infected with vesicular-arbuscular mycorrhizae, but the epiphytic roots of the same individuals are not infected.     

植物内皮层的分化及其屏障功能研究进展

植物根系最主要的作用之一是从土壤中获取养分并将其运输至地上部。水和营养物质径向穿过根的表皮、皮层、内皮层等所有外部细胞层,才能到达中柱,以供地上部代谢所需。其中,内皮层细胞在发育过程中会经历两个特殊的分化阶段,分别形成凯氏带和木栓层两种扩散屏障,二者在控制养分获取与流失方面起着重要的作用。该文就近年来国内外有关植物内皮层分化过程及其屏障功能方面的研究进展进行了综述,以期对深入探索内皮层屏障在植物生长发育和逆境适应中的作用提供参考,为植物育种工作开辟新的思路。     

Mechanisms for enhancing nutrient uptake in plants,with particular reference to mediterranean South Africa and Western Australia

The major constraints to nutrient uptake by vascular plants in mediterranean South Africa and Western Australia are: very infertile soils, relatively low temperatures when water availability is high, and hot, dry summers. These constraints are partly overcome through increased efficiency of uptake, tapping novel sources of nutrients, and prolonging water uptake. Absorptive area per unit “cost” may be enlarged directly through increased fineness of the root system and proliferation of long root hairs. This reaches its greatest development in the root clusters of the Proteaceae (proteoid roots), Restionaceae (“capillaroid” roots) and Cyperaceae (dauciform roots). Absorptive area is increased indirectly through fungal hyphae which extend from hairless rootlets into the soil. Two major groups can be recognised: general (VA mycorrhizas) and host-specific (ericoid, orchid and sheathing mycorrhizas). Mycorrhizas are the most widespread specialised modes of nutrition and are probably universal in such major taxa here asPodocarpus, Acacia, Fabaceae, Poaceae, Asteraceae, Rutaceae, terrestrial orchids, Ericales and Myrtaceae. General mycorrhizas are the least drought-adapted of mechanisms for maximising absorptive area. All have been implicated in enhancing P uptake through increasing access to inorganic P, solubilisation and shortening the diffusion path. However, selective uptake of other nutrients, especially N, by host-specific mycorrhizas may be equally important. Included under novel sources of nutrients are free N₂ (utilised by N₂-fixing nodules), small-animal prey (carnivorous leaves) and persistent leaf bases (aerial roots ofKingia australis). Both legume and non-legume N₂-fixing species are well-represented in these two regions, with stands of individual species in southwestern Australia estimated to contribute 2–19 kg N/ha/yr to the ecosystem. Free nitrogen fixation requires additional nutrients, especially Mo and Co, but is enhanced following fires and by supplementary uptake mechanisms, especially VA mycorrhizas. Southwestern Australia is particularly rich in carnivorous species. Nitrogen, P, K and S are important nutrients absorbed, with digestion aided by enzymes provided by bacteria and the glands. Parasitic plants both tap novel sources of nutrients and capitalise on any efficient water and nutrient uptake mechanisms of the hosts. Root parasites are better represented than stem parasites in mediterranean South Africa and Western Australia. Phosphorus and K in particular are absorbed preferentially by the haustoria, but much remains to be known about their modes of operation. Maximum activity of all uptake mechanisms, except those attached to some deep-rooted plants, is restricted to winter-spring. Most new seasons’s rootlets and specialised roots are confined to the uppermost 15 cm of soil, especially in or near the decomposing litter zone. Nutrient uptake is further enhanced by the tendency for the rootlets to cluster, trapping water by capillary action and prolonging nutrient release. As an early product of decomposition, N tends to be available as NH₄ (rather than NO₃) and it is absorbed preferentially by almost all specialised modes of nutrition. Microorganisms are required in the formation and/or functioning of all these structures, except haustoria. Uptake mechanisms which are optional to the plant reach their peak contribution to the root system at soil nutrient levels well below those required for greatest plant growth, when they may be absent altogether. It is only over the narrow range of nutrient availability, where shoot content of a nutrient is greater in the presence of the mechanism than in its absence (other factors remaining constant), that specialised modes can be termed nutrient-uptake “strategies.” For all specialised modes of nutrition, the component genera are better represented in these two regions than in the surrounding more fertile, arid to subtropical regions of much greater area. Endemism of species with each mode exceeds that for the two floras overall (75%). This is taken as preliminary evidence that specialised modes of nutrition are best represented in nutrient-poor soils. While they serve to limit nutrient loss from the ecosystem, their proliferation is therefore not necessarily a response to increasing “leaks” in the system. A hierarchical scheme of the functional/structural relationships between the various mechanisms is presented, starting with the rootless, VA-mycorrhizal plant as the most primitive condition. Taxa with many of the specialised modes of nutrition at present in southwestern South Africa and Western Australia have been evident in the pollen record since the early Tertiary Period. The absence of ectomycorrhizal forests in mediterranean South Africa, in marked contrast to Western Australia, can be traced to differences in their paleohistory. In both regions, the combination of fluctuating, but essentially diminishing, nutrient and water availability that began with the first mediterranean climate < 5 million years ago resulted in decimation of the less-tolerant rainforest ancestors on the one hand, and remarkable rates of speciation of the pre-adapted sclerophyll nucleus on the other.     

Transpiration by an emergent macrophyte: source of water and implications for nutrient supply

The contribution of sediment interstitial water and the water column to the transpiration stream of Myriophyllum aquaticum (Vellozo) Verdcourt was determined to estimate the significance of mass flow in supply of sediment nutrients for plant growth. Sediment interstitial water accounted for about 2% of the water transpired over a 37 day period. Because of the small volume of water that originated in the sediment we concluded that mass flow did not significantly enhance nutrient supply to the roots of M. aquaticum. Relative growth rate (RGR) of adventitious, water roots was greater than whole plant RGR, and RGR of sediment roots was not significantly different from zero, indicating a shift in the biomass allocation after emergence of the apical meristem into the air. Water use, measured by the transpiration coefficient, averaged 260 ml H₂O mg DW^-1, which is similar to C-4 terrestrial plants. M. aquaticum has leaf characteristics commonly associated with xerophytic habitats. These characteristics may be necessary if a high transpiration rate and a mechanical requirement for high cell turgor pressure, required by a reliance upon hydrostatic pressure for support of the aerial stems, are mutually exclusive because of morphological constraints on hydraulic conductivity.     

Drought survival,summer dormancy and dehydrin accumulation in contrasting cultivars of Dactylis glomerata

To study survival under prolonged and severe drought in the perennial grass Dactylis glomerata we compared dormant, resistant and sensitive cultivars (cvs.) in both field and glasshouse experiments. Water status, membrane stability and expression of dehydrins were assessed in the immature leaf bases, which are the last surviving organs. Analysis of leaf elongation and senescence of aerial tissues showed that dormancy was exhibited by the potentially dormant cultivar (cv.) only in the field. This cultivar exhibited a high survival rate, similar levels of dehydration and expression of a low-molecular weight (22–24 kDa) dehydrin in both drought and irrigated plants, whether fully dormant or not. At the same level of soil water deficit, there were no differences between the non-dormant drought resistant and drought sensitive cultivars in plant water status and membrane stability. However, the accumulation of dehydrins as drought progressed was markedly different between these cultivars and was associated with their contrasting survival. The possible role of the major low-molecular dehydrins in maintenance of cell integrity under dehydration is discussed with reference to both summer dormancy and survival under severe drought.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Effects of drought stress on phosphorus and potassium uptake dynamics in summer maize (Zea mays) throughout the growth cycle

The effects of drought stress on the phosphorus (P) and potassium (K) uptake dynamics of summer maize (Zea mays L.) throughout the growth cycle were studied. Field trials were conducted under a completely randomized design with three field water capacity (FC) regimes: 75?% FC was well watered and considered to be the control, 55?% FC represented moderate stress (MS), and 35?% FC represented severe stress (SS). The water regimes were applied from the third leaf stage until maturity. Drought stress induced sharp decreases in total K and P uptake of maize organs at different developmental stages and, in particular, detrimentally affected the nutrient uptake capability of roots. SS caused more deleterious effect than MS on both total K and P uptake by plant organs. The results suggested maize plants differ in their ability to maintain nutrient uptake under drought stress, and it is highly dependent on the intensity and duration of drought stress and the developmental stage. The decrease in total K and P uptake caused by both MS and SS was accompanied by reduction in biomass production in drought-stressed tissues. The biomass allocation patterns in response to drought stress fluctuated strong mostly because of competitive changes in the shoot and roots at different stages, thus the root:shoot ratio increased at some stages and decreased at other stages. SS induced a dramatic reduction in the harvest index (HI), whereas MS slightly decreased HI. Thus, water limitation caused lower K and P uptake and HI.     

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.     

Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream

1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams.
2. Four submerged macrophyte species were tested, Elodea canadensis , Callitriche cophocarpa , Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants.
3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa , indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment.
4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.     

Water Uptake by Different Regions of the Barley Root. Pathways of Radial Flow in Relation to Development of the Endodermis

The water uptake by different lengths of lateral roots and 1.0cm or 5.0 cm lengths of the seminal axes from differentregionsof the root were measured in potometers with the shoot in airat two humidity regimes. A model of the contribution by thesedifferent regions of the root to the total water absorptionby the plant agreed well with measurements of water uptake bythe whole root system. According to this model, about one halfof the water taken up by the main axis came from the older suberizedregions further than 10 cm from the tip, and together with itsassociated lateral roots this region provided 75% of the totalwater transpired. The development of State III endodermal cellswas correlated with decreases in both the water uptake by theolder regions of the root and the translocation of calcium.Thus in the younger regions of the root where water uptake ismaximal, the flow of water is principally apoplastic althoughthere is also likely to be flow via the symplast Despite a 43%difference in transpirational demand between the two humiditytreatments, the leaf water potentials remained constant, implyinga change in root resistance. This change in resistance mightbe explained if there were an apoplastic pathway within thesuberin lamellae of State III endodermal cells. The responseto the increased transpirational demand is met by the olderregions of the root, in particular by the zone of lateral emergencewhere an apoplastic pathway is known to exist as the Casparianband in the endodermis breaks down with the emergence of thelateral roots. Key words: Endodermis, Pathways, Water Uptake     

Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.     

Differential uptake of summer precipitation among co-occurring trees and shrubs in a pinyon-juniper woodland

Measurements of the ratio of deuterium to hydrogen (D/H) in stem xylem water were used to determine the relative uptake of summer precipitation by four co-occurring plant species in southern Utah. The species compared included two trees, Juniperus osteosperma and Pinus edulis, and two shrubs, Artemisia tridentata and Chrysothamnus nauseousus. There were significant differences among species in the relative use of summer precipitation. Chrysothamnus nauseosus had stem water D/H ratios in May through August 1990 that were not significantly different from that of groundwater. In contrast, the other three species had stem water D/H ratios that were intermediate between the groundwater value and summer precipitation values, indicating that a mixture of both precipitation and groundwater was being used by these species. The two tree species generally had higher D/H values than did A. tridentata indicating a higher average uptake of summer precipitation, although the roots of J. osteosperma and P. edulis may not be as responsive to small precipitation events as A. tridentata. There was a strong negative correlation between stem water D/H ratios and predawn water potential, which suggests a relationship between plant rooting pattern and water source use. In addition, water-use efficiency during photosynthetic gas exchange, calculated from leaf carbon isotope composition, differed among species and was strongly correlated with differences in the relative uptake of summer precipitation.     

The periodic wetting of leaves enhances water relations and growth of the long‐lived conifer Araucaria angustifolia

The importance of foliar absorption of water and atmospheric solutes in conifers was recognised in the 1970s, and the importance of fog as a water source in forest environments has been recently demonstrated. Araucaria angustifolia (Araucariaceae) is an emergent tree species that grows in montane forests of southern Brazil, where rainfall and fog are frequent events, leading to frequent wetting of the leaves. Despite anatomical evidence in favour of leaf water absorption, there is no information on the existence and physiological significance of a such process. In this study, we test the hypothesis that the use of atmospheric water by leaves takes place and is physiologically relevant for the species, by comparing growth, water relations and nutritional status between plants grown under two conditions of soil water (well‐watered and water‐stressed plants) and three types of leaf spraying (none, water and nutrient solution spray). Leaf spraying had a greater effect in improving plant water relations when plants were under water stress. Plant growth was more responsive to water available to the leaves than to the roots, and was equally increased by both types of leaf spraying, with no interaction with soil water status. Spraying leaves with nutrient solution increased shoot ramification and raised the concentrations of N, P, K, Zn, Cu and Fe in the roots. Our results provide strong indications that water and nutrients are indeed absorbed by leaves of A. angustifolia, and that this process might be as important as water uptake by its roots.     

Root growth and functioning under atmospheric CO2 enrichment

This paper examines the extent to which atmospheric CO₂ enrichment may influence growth of plant roots and function in terms of uptake of water and nutrients, and carbon allocation towards symbionts. It is concluded that changes in dry matter allocation greatly depend on the experimental conditions during the experiment, the growth phase of the plant, and its morphological characteristics. Under non-limiting conditions of water and nutrients for growth, dry matter partitioning to the root is not changed by CO₂ enrichment. The increase in root/shoot ratio, frequently observed under limiting conditions of water and/or nutrients, enables the plant to explore a greater soil volume, and hence acquire more water and nutrients. However, more data on changes in dry matter allocation within the root due to atmospheric CO₂ are needed. It is concluded that nitrogen fixation is favored by CO₂ enrichment since nodule mass is increased, concomitant with an increase in root length. The papers available so far on the influence of CO₂ enrichment on mycorrhizal functioning suggest that carbon allocation to the roots might be increased, but also here more experiments are needed.Abbreviations LAR leaf area ratio - LWR leaf weight ratio - SWR stem weight ratio - RGR relative growth rate - R/S root/shoot - RWR root weight ratio