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After the imposition of soil drying treatments, an elevation of xylem sap pH is one of the earliest observable responses in many herbaceous model plant species. It is theorized that alkalization of sap results in a concurrent elevation in abscisic acid (ABA) concentration delivered to transpiring tissues by preventing Henderson–Hasselbalch-regulated partitioning between the apoplast and symplast. However, here it is demonstrated that the sap alkalzation response to soil drying is far from universal in higher plant species. Tests were conducted to determine how universal the pH response to drying soil was in a range of perennial species from a diverse range of plant families. The response was not found in the majority of the 22 species tested. Four species exhibited significant increases in pH, but the majority showed no significant change in xylem sap pH. There was no evolutionary relationship between the species that showed alkalization under drought stress. However, the species that alkalized sap also exhibited good control over internal water status and were the most isohydric species of those tested. None of the species exhibiting anisohydric responses alkalized xylem sap under drought stress. Regardless of alkalization response, plants still retain the ability to respond to changes in xylem sap pH when manipulated by alkaline buffer foliar sprays. This finding indicates that plants have conserved the ability to respond to changes in xylem pH and redistribute ABA, even if they do not currently utilize the mechanism when exposed to drought stress. It was found in Buddleja davidii, Euonymus fortunei, and Hydrangea serrata that the xylem sap pH response to water deficits mirrored the natural pH changes that occur as sap is transported to the leaves, indicating that plants need to be able to have naturally occurring alkalization processes in place for them to be up-regulated under drought stress. [ABSTRACT FROM PUBLISHER]

Abstract There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal. [ABSTRACT FROM AUTHOR]

Three-month-old Cedrella odorata were exposed to a soil-drying treatment. During this period, xylem sap was periodically collected from the plant by applying pneumatic pressure to the roots. This also allowed whole-plant water status to be measured by recording the balancing pressure applied. The concentration of ABA in xylem sap (C) was related to the whole-plant transpiration rate (V) which was measured witha sap flow gauge. The analysis of these paired measurements centred on how the reciprocal of C (R) varied with respect to V. This revealed that (1) the observed increases in C could not be explained by the reductions in V alone, (2) initially, decreases in V were associated with proportional increases in the whole-plant ABA flux (M), and (3) this relationship broke down at low values of V since zero flow was associated with a finite value for C estimated to be 41 pmol ABA mmol-1 H2O. A simple static model is developed from the observations that is able to explain the data well, and the results are discussed in terms of the effects of ABA on stomatal conductance (gsw). [ABSTRACT FROM AUTHOR]

It is now clear that drying of the soil does not always result in an early change in shoot water status. This may be because stomata close and leaf growth slows to reduce water loss. When this is the case, it is necessary to ask how the change in soil water status has been 'sensed' by the shoot. The current view is that soil drying results in some type of chemical signalling between roots and shoots. The sensitivity of the response and experiments involving the manipulation of small parts of root systems suggest that the signalling involves more than a simple change in root activity in response to soil drying. In this paper, we consider the evidence for chemical signalling between roots and shoots and discuss the possible candidates for such signals. In some plants, root-sourced ABA can apparently influence shoot physiology and growth in the absence of any perturbation of shoot water relations. The ABA produced is quantitatively sufficient to account for the responses observed. Applied ABA can mimic many of the effects of soil drying on plants, including effects at the plasma membrane and on gene expression. Perhaps uniquely, ABA seems to be involved in signalling between different plant organs, and in signalling at the transmembrane and genome levels. We review the effects of ABA on leaf cells with a view to gaining some understanding of how soil drying may influence plant development. [ABSTRACT FROM AUTHOR]

Maize plants were grown in l-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than wellwatered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants. Water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0.25 g cm[SUP-3] (a bulk soil water potential of between -0.2 and -0.3MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying. [ABSTRACT FROM AUTHOR]

Experiments with isolated roots of wheat plants suggested that when water uptake rates are low, low concentrations of abscisic acid (ABA) may increase the flux of water into roots. This increase was recorded despite an ABA-stimulated reduction in the hydraulic conductance of the whole root system. Hydraulic conductances were measured under steady state conditions. A system is described where the stomatal behaviour and water movement through roots of a single intact plant may be concurrently monitored. Experiments with intact plants confirmed that application of ABA could increase the rate of water movement into roots when uptake rates were low. No such increase was observed at high flux rates. Application of ABA to roots caused partial stomatal closure and caused conductance to oscillate around a reduced mean value. An ABA-stimulated increase in the turgor sensitivity of stomata is postulated and the significance of this effect is discussed. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) fed in pulses to the petioles of detached cherry leaves in enclosed leaf chambers, caused a reduction in leaf conductance. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regression analysis of the data showed both variables to have a significant effect on leaf conductance. A hypothetical maximum ABA concentration occurring in the leaf apoplast was calculated for each pulse from a simple model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half-life of 36 min for the initial rate of decay. This figure is similar to that derived from the model, the importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot communication by ABA in the xylem stream. [ABSTRACT FROM AUTHOR]

Background and Aims Bioenergy crops are central to climate mitigation strategies that utilize biogenic carbon, such as BECCS (bioenergy with carbon capture and storage), alongside the use of biomass for heat, power, liquid fuels and, in the future, biorefining to chemicals. Several promising lignocellulosic crops are emerging that have no food role – fast-growing trees and grasses – but are well suited as bioenergy feedstocks, including Populus , Salix , Arundo , Miscanthus , Panicum and Sorghum. Scope These promising crops remain largely undomesticated and, until recently, have had limited germplasm resources. In order to avoid competition with food crops for land and nature conservation, it is likely that future bioenergy crops will be grown on marginal land that is not needed for food production and is of poor quality and subject to drought stress. Thus, here we define an ideotype for drought tolerance that will enable biomass production to be maintained in the face of moderate drought stress. This includes traits that can readily be measured in wide populations of several hundred unique genotypes for genome-wide association studies, alongside traits that are informative but can only easily be assessed in limited numbers or training populations that may be more suitable for genomic selection. Phenotyping, not genotyping, is now the major bottleneck for progress, since in all lignocellulosic crops studied extensive use has been made of next-generation sequencing such that several thousand markers are now available and populations are emerging that will enable rapid progress for drought-tolerance breeding. The emergence of novel technologies for targeted genotyping by sequencing are particularly welcome. Genome editing has already been demonstrated for Populus and offers significant potential for rapid deployment of drought-tolerant crops through manipulation of ABA receptors, as demonstrated in Arabidopsis , with other gene targets yet to be tested. Conclusions Bioenergy is predicted to be the fastest-developing renewable energy over the coming decade and significant investment over the past decade has been made in developing genomic resources and in collecting wild germplasm from within the natural ranges of several tree and grass crops. Harnessing these resources for climate-resilient crops for the future remains a challenge but one that is likely to be successful. [ABSTRACT FROM AUTHOR]

Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots in unlikely. [ABSTRACT FROM AUTHOR]