Foliar pathogenesis and plant water relations: a review

As the world population grows, there is a pressing need to improve productivity from water use in irrigated and rain-fed agriculture. Foliar diseases have been reported to decrease crop water-use efficiency (WUE) substantially, yet the effects of plant pathogens are seldom considered when methods to improve WUE are debated. We review the effects of foliar pathogens on plant water relations and the consequences for WUE. The effects reported vary between host and pathogen species and between host genotypes. Some general patterns emerge however. Higher fungi and oomycetes cause physical disruption to the cuticle and stomata, and also cause impairment of stomatal closing in the dark. Higher fungi and viruses are associated with impairment of stomatal opening in the light. A number of toxins produced by bacteria and higher fungi have been identified that impair stomatal function. Deleterious effects are not limited to compatible plant-pathogen interactions. Resistant and non-host interactions have been shown to result in stomatal impairment in light and dark conditions. Mitigation of these effects through selection of favourable resistance responses could be an important breeding target in the future. The challenges for researchers are to understand how the effects reported from work under controlled conditions translate to crops in the field, and to elucidate underlying mechanisms.     

Intra-annual radial growth and water relations of trees: implications towards a growth mechanism

There is a missing link between tree physiological and wood-anatomical knowledge which makes it impossible mechanistically to explain and predict the radial growth of individual trees from climate data. Empirical data of microclimatic factors, intra-annual growth rates, and tree-specific ratios between actual and potential transpiration (T PET(-1)) of trees of three species (Quercus pubescens, Pinus sylvestris, and Picea abies) at two dry sites in the central Wallis, Switzerland, were recorded from 2002 to 2004 at a 10 min resolution. This included the exceptionally hot and dry summer of 2003. These data were analysed in terms of direct (current conditions) and indirect impacts (predispositions of the past year) on growth. Rain was found to be the only factor which, to a large extent, consistently explained the radial increment for all three tree species at both sites and in the short term as well. Other factors had some explanatory power on the seasonal time-scale only. Quercus pubescens built up much of its tree ring before bud break. Pinus sylvestris and Picea abies started radial growth 1-2 weeks after Quercus pubescens and this was despite the fact that they had a high T PET(-1) before budburst and radial growth started. A high T PET(-1) was assumed to be related to open stomata, a very high net CO2 assimilation rate, and thus a potential carbon (C)-income for the tree. The main period of radial growth covered about 30-70% of the productive days of a year. In terms of C-allocation, these results mean that Quercus pubescens depended entirely on internal C-stores in the early phase of radial growth and that for all three species there was a long time period of C-assimilation which was not used for radial growth in above-ground wood. The results further suggest a strong dependence of radial growth on the current tree water relations and only secondarily on the C-balance. A concept is discussed which links radial growth over a feedback loop to actual tree water-relations and long-term affected C-storage to microclimate.     

The canopy water relations of old-growth Douglas-fir trees

 We investigated whole tree water relations in 56–65 m tall, old-growth Pseudotsuga menziesii trees within the Wind River Canopy Crane site, Carson, Washington, USA. We measured at predawn and solar noon the vertical gradients in xylem pressure potential using a pressure chamber. On an Abies amabilis sapling located in the understory at the base of one of the study trees, predawn and solar noon xylem pressure potentials were also measured. Xylem pressure potential data were measured from late June through early September 1996 on foliage sampled from 1 to 64.5 m. Over this height gradient, predawn water potentials ranged from –0.23 to –1.10 MPa. Solar noon values showed an even greater range (from –0.44 to –2.51 MPa). At predawn, the water potential gradient approached the theoretical hydrostatic gradient (–0.0105 vs –0.010 MPa m^–1). The gradient at solar noon was steeper (–0.0331 MPa m^–1). Instantaneous stomatal conductances were not greatly different between young, sapling-sized and old-growth trees [0.094±0.033 (SD) vs 0.086±0.045 cm s^–1, respectively]. Stomata of both size classes of trees appeared very sensitive to increasing vapor pressure deficits. A comparison of stable carbon isotope values from the old-growth and sapling-sized trees indicated lower stomatal conductances in the old-growth. This study provides sound documentation regarding the utility of the cohesion theory in the interpretation of water potential gradients. This study also emphasizes inherent differences between sapling-sized and tall, old-growth trees. Received: 10 January 1998 / Accepted: 12 October 1998     

Comparative water relations of co-occurring trees in a mixed podocarp-broadleaf forest

Aims As extreme climatic events including droughts and heat waves become more common in a changing climate, tree mortality has increased across the globe. In order to determine whether certain species have a competitive advantage over others, we explored the water-relations and leaf-gas exchange of four co-occurring species in a forest in northern Aotearoa-New Zealand. We studied the ecologically and culturally significant foundation species,Agathis australis(a conifer), two additional conifers,Phyllocladus trichomanoidesandPodocarpus totaraand the angiospermKnightia excelsa.     

Phloem loading strategies and water relations in trees and herbaceous plants

Most herbaceous plants employ thermodynamically active mechanisms of phloem loading, whereas in many trees, the mechanism is passive, by diffusion. Considering the different water transport characteristics of herbs and trees, we hypothesized that water relations play a role in the adoption of phloem loading strategies. We measured whole-plant hydraulic conductance (K(p)), osmolality, concentrations of polar metabolites, and key inorganic ions in recently mature leaves of 45 dicotyledonous species at midafternoon. Trees, and the few herbs that load passively, have low K(p), high osmolality, and high concentrations of transport sugars and total polar metabolites. In contrast, herbs that actively load sucrose alone have high K(p), low osmolality, and low concentrations of sugars and total polar metabolites. Solute levels are higher in sugar alcohol-transporting species, both herbs and trees, allowing them to operate at lower leaf water potentials. Polar metabolites are largely responsible for leaf osmolality above a baseline level (approximately 300 mm) contributed by ions. The results suggest that trees must offset low K(p) with high concentrations of foliar transport sugars, providing the motivating force for sugar diffusion and rendering active phloem loading unnecessary. In contrast, the high K(p) of most herbaceous plants allows them to lower sugar concentrations in leaves. This reduces inventory costs and significantly increases growth potential but necessitates active phloem loading. Viewed from this perspective, the elevation of hydraulic conductance marks a major milestone in the evolution of the herbaceous habit, not only by facilitating water transport but also by maximizing carbon use efficiency and growth.     

Nutrient availability constrains the hydraulic architecture and water relations of savannah trees

Leaf and whole plant-level functional traits were studied in five dominant woody savannah species from Central Brazil (Cerrado) to determine whether reduction of nutrient limitations in oligotrophic Cerrado soils affects carbon allocation, water relations and hydraulic architecture. Four treatments were used: control, N additions, P additions and N plus P additions. Fertilizers were applied twice yearly, from October 1998 to March 2004. Sixty-three months after the first nutrient addition, the total leaf area increment was significantly greater across all species in the N- and the N + P-fertilized plots than in the control and in the P-fertilized plots. Nitrogen fertilization significantly altered several components of hydraulic architecture: specific conductivity of terminal stems increased with N additions, whereas leaf-specific conductivity and wood density decreased in most cases. Average daily sap flow per individual was consistently higher with N and N + P additions compared to the control, but its relative increase was not as great as that of leaf area. Long-term additions of N and N + P caused midday PsiL to decline significantly by a mean of 0.6 MPa across all species because N-induced relative reductions in soil-to-leaf hydraulic conductance were greater than those of stomatal conductance and transpiration on a leaf area basis. Phosphorus-fertilized trees did not exhibit significant changes in midday PsiL. Analysis of xylem vulnerability curves indicated that N-fertilized trees were significantly less vulnerable to embolism than trees in control and P-fertilized plots. Thus, N-induced decreases in midday PsiL appeared to be almost entirely compensated by increases in resistance to embolism. Leaf tissue water relations characteristics also changed as a result of N-induced declines in minimum PsiL: osmotic potential at full turgor decreased and symplastic solute content on a dry matter basis increased linearly with declining midday PsiL across species and treatments. Despite being adapted to chronic nutrient limitations, Cerrado woody species apparently have the capacity to exploit increases in nutrient availability by allocating resources to maximize carbon gain and enhance growth. The cost of increased allocation to leaf area relative to water transport capacity involved increased total water loss per plant and a decrease in minimum leaf water potentials. However, the risk of increased embolism and turgor loss was relatively low as xylem vulnerability to embolism and leaf osmotic characteristics changed in parallel with changes in plant water status induced by N fertilization.     

Daily variations in water relations of apricot trees under different irrigation regimes

Mature apricot (Prunus armeniaca L. cv. Búlida) trees, growing under field conditions, were submitted to two drip irrigation treatments: a control (T1), irrigated to 100 % of seasonal crop evapotranspiration (ETc), and a continuous deficit (T2), irrigated to 50 % of the control throughout the year. The behaviour of leaf water potential and its components, leaf conductance and net photosynthesis were studied at three different times during the growing season, when they revealed a diurnal and seasonal pattern in response to water stress, evaporative demand of the atmosphere and leaf age. The deficit-irrigated trees showed, among other effects, a pronounced decrease in leaf water potential (ψ_w), decreased in leaf conductance (g_s) and no osmotic adjustment. For this reason, g_l and ψ_w can be considered good indicators of mature apricot tree water status and can therefore be used for irrigation scheduling.     

Drought resistance of 2-year-old saplings of Mediterranean forest trees in the field: relations between water relations,hydraulics and productivity

Drought resistance was examined for 2-year-old saplings of Ceratonia siliqua L., Olea oleaster Hoffmgg. et Link., Quercus suber L. and Q. pubescens Willd. growing in the field in Sicily, with the aim of testing their possible use in the reforestation of degraded areas. To this purpose, leaf conductance to water vapour (g _L), transpiration rate (E _L), relative water content (RWC) and water potential (_L) were measured between pre-dawn and sunset, monthly from May to November. Parallel measurements of loss of hydraulic conductance of twigs of the current year (PLC) were made together with an estimate of whole-plant hydraulic conductance (K _PLANT) on the basis of the ratio of maximum E _L to (_PD-_MIN), where _PD is pre-dawn _L and _MIN is the minimum diurnal _L. C. siliqua saplings maintained high g _L throughout the study period with high RWC (over 90%) and _L. They grew rapidly and increased their foliage area (A _L) by over 60% from May to winter rest. This was accompanied by low twig PLC (about 20% in September) resulting in high K _PLANT all through the study period. In contrast, O. oleaster saplings underwent distinct dehydration in July, i.e. they showed stomatal closure due to a drop in RWC (to 75%) and _L (to the turgor-loss point). This was apparently due to twig cavitation, resulting in a strong decrease of K _PLANT. Plants, however, tolerated summer drought, and showed growth (A _L increased by 15% from May to November). Saplings of Q. suber and Q. pubescens were much more vulnerable to twig cavitation (PLC was 35–48% from June to November) than the other two species, and their average K _PLANT was lower. Saplings of Q. suber and Q. pubescens grew only during the wet spring period, and no new foliage was produced thereafter. Changes in twig hydraulic conductance played a dominant role in determining changes in K _PLANT in that the two variables were well correlated to each other (r=0.68 for P=0.001). We conclude that C. siliqua is an ideal candidate for reforestation of Sicilian degraded areas as is O. oleaster that, however, requires some additional water supply in the summer, at least during the early years after plantation. In wetter locations of Sicily, to an altitude between 0 and 500 m, Q. suber and Q. pubescens can be used for reforestation with expected higher competitiveness of the latter over the former species.     

Plant water relations and the effects of elevated CO2: a review and suggestions for future research

Increased ambient carbon dioxide (CO₂) has been found to ameliorate water stress in the majority of species studied. The results of many studies indicate that lower evaporative flux density is associated with high CO₂-induced stomatal closure. As a result of decreases in evaporative flux density and increases in net photosynthesis, also found to occur in high CO₂ environments, plants have often been shown to maintain higher water use efficiencies when grown at high CO₂ than when grown in normal, ambient air. Plants grown at high CO₂ have also been found to maintain higher total water potentials, to increase biomass production, have larger root-to-shoot ratios, and to be generally more drought resistant (through avoidance mechanisms) than those grown at ambient CO₂ levels. High CO₂-induced changes in plant structure (i.e., vessel or tracheid anatomy, leaf specific conductivity) may be associated with changes in vulnerability to xylem cavitation or in environmental conditions in which runaway embolism is likely to occur. Further study is needed to resolve these important issues. Methodology and other CO₂ effects on plant water relations are discussed.Abbreviations A net photosynthesis - C_a ambient [CO₂] - C_i internal [CO₂] - E evaporative flux density - g₁ leaf conductance - g_s stomatal conductance - LSC leaf specific conductivity - IRGA infrared gas analyzer - LAI leaf area index - PAR photosynthetically active radiation - total plant water potential - _soil soil water potential - _s solute potential - _pt turgor pressure potential - _px xylem pressure potential - RH relative humidity - R : S root to shoot ratio - RWC relative water content - SLA specific leaf area - SLW specific leaf weight - SPAC soil-plant-atmosphere-continuum - SWC soil water content - VPD vapor pressure deficit - WUE water use efficiency     

Allocation to leaf area and sapwood area affects water relations of co-occurring savanna and forest trees

Water availability is a principal factor limiting the distribution of closed-canopy forest in the seasonal tropics, suggesting that forest tree species may not be well adapted to cope with seasonal drought. We studied 11 congeneric species pairs, each containing one forest and one savanna species, to test the hypothesis that forest trees have a lower capacity to maintain seasonal homeostasis in water relations relative to savanna species. To quantify this, we measured sap flow, leaf water potential (Ψ_L), stomatal conductance (g _s), wood density, and Huber value (sapwood area:leaf area) of the 22 study species. We found significant differences in the water relations of these two species types. Leaf area specific hydraulic conductance of the soil/root/leaf pathway (G _t) was greater for savanna species than forest species. The lower G _t of forest trees resulted in significantly lower Ψ_L and g _s in the late dry season relative to savanna trees. The differences in G _t can be explained by differences in biomass allocation of savanna and forest trees. Savanna species had higher Huber values relative to forest species, conferring greater transport capacity on a leaf area basis. Forest trees have a lower capacity to maintain homeostasis in Ψ_L due to greater allocation to leaf area relative to savanna species. Despite significant differences in water relations, relationships between traits such as wood density and minimum Ψ_L were indistinguishable for the two species groups, indicating that forest and savanna share a common axis of water-use strategies involving multiple traits.     

Ectomycorrhizas from a Lower Eocene angiosperm forest

The development of mycorrhizal associations is considered a key innovation that enabled vascular plants to extensively colonize terrestrial habitats. Here, we present the first known fossil ectomycorrhizas from an angiosperm forest. Our fossils are preserved in a 52 million-yr-old piece of amber from the Tadkeshwar Lignite Mine of Gujarat State, western India. The amber was produced by representatives of Dipterocarpaceae in an early tropical broadleaf forest. The ectomycorrhizas were investigated using light microscopy and field emission scanning electron microscopy. Dissolving the amber surrounding one of the fossils allowed ultrastructural analyses and Raman spectroscopy. Approx. 20 unramified, cruciform and monopodial-pinnate ectomycorrhizas are fossilized adjacent to rootlets, and different developmental stages of the fossil mycorrhizas are delicately preserved in the ancient resin. Compounds of melanins were detectable in the dark hyphae. The mycobiont, Eomelanomyces cenococcoides gen. et spec. nov., is considered to be an ascomycete; the host is most likely a dipterocarp representative. An early ectomycorrhizal association may have conferred an evolutionary advantage on dipterocarps. Our find indicates that ectomycorrhizas occurred contemporaneously within both gymnosperms (Pinaceae) and angiosperms (Dipterocarpaceae) by the Lower Eocene.     

Termites and trees: a review of recent advances in termite phylogenetics

Summary: Modern termite phylogenetics is critically reviewed, with an emphasis on tree topologies as phylogenetic hypotheses. Studies have especially concentrated on (1) the position of Isoptera among the Dictyoptera and (2) the family group relationships within the Isoptera. The first of these problems is still controversial; although the weight of evidence now suggests that termites are nested within the cockroaches, thus making "Blattaria" as presently constituted paraphyletic. The exact position of termites within the cockroaches is uncertain, although Cryptocercus is the most plausible sister group.¶Family groups relationships are rather better resolved. Mastotermitidae is now generally accepted to be the most basal termite group. Termopsidae, Hodotermitidae and Kalotermitidae are all basal to (Termitidae + Serritermitidae + Rhinotermitidae), although their relative positions within that part of the tree are disputed. Most recent studies support a sister group relationship for Serritermitidae and (Termitidae + Rhinotermitidae). However, no study has yet unambiguously found the Rhinotermitidae monophyletic. The Termitidae are well established as monophyletic and as the most apical termite family. However, within the Termitidae the monophyly of none of the subfamilies is well established, making subfamily level analyses unreliable.¶A number of problem areas are identified: (1) poor taxon sampling is a universal problem, (2) higher taxonomic groupings are often assumed to be monophyletic a priori without adequate support, (3) datasets are collected from different taxa and character systems without consideration of the overall international effort.     

Non-native gall-inducing insects on forest trees: a global review

Gall-inducing insects cause the development of specialised plant tissues (galls) that provide them with nutrition and some measure of protection from physical and biotic stresses. Their interaction with the plant is the most intimate metabolically of any herbivore group and is often associated with high host specificity. We survey the gall inducers that have become invasive pests of forest trees, most of which belong to just four insect families in three orders: Hemiptera (Adelgidae), Diptera (Cecidomyiidae) and Hymenoptera (Cynipidae and Eulophidae). Most are associated with introduction of plants on which they are specialists, but some have also shifted from introduced to native plant hosts. No formal comparative analysis of traits associated with success of establishment and subsequent range expansion has yet been made, and it is often hard to identify why one species has become a major range-expanding pest, while closely related and biologically very similar species have not. We provide an overview of biological traits likely to facilitate gall inducer range expansion, and highlight the importance of natural enemies in community impacts and biological control. Increasing global trade is likely to result in further range expansions by economically damaging species. The effects of climate change on the direction, frequency, and impact of gall inducer range expansions are likely to be complex and probably species-specific.     

The water relations of Verna Lemon trees from flowering to the end of rapid fruit growth

Lemon plants (Citrus limonum L. cv. Verna) were grown in the field under two different flood irrigation treatments. The dry treatment received four irrigations per year (March, July, September and November) and the wet treatment one monthly. The amounts of water applied per year for dry and wet treatments were 340.0 mm and 1020.0 mm, respectively. The effects of the two treatments on certain aspects of the plant water relations during the period between flowering and the end of rapid fruit growth (critical period) were studied. Soil matric potential (ψm) and leaf water potential (ψi) values in the dry treatment revealed development of water stress during the experimental period. The water supply in the wet treatment seems sufficient to achieve the crop water requirements. The g₁ values in July were higher in the wet than dry treatments. Pronounced oscillations in g₁ from sunrise to afternoon were found especially in the dry treatment.     

Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.