Stomatal and leaf growth responses to partial drying of root tips in willow

Root hydraulic conductance and aquaporin abundance respond rapidly to partial root-zone drying events in a riparian Melaleuca species

• Drying a portion of a root system (partial root-zone drying (PRD)) can induce partial stomatal closure, but this response is not always observed. We hypothesized that some of the variation in PRD response reflects adaptations to the native environment, where plants subjected to frequent PRD events may display a greater degree of root-level compensation. • Here, we examined PRD responses of Melaleuca argentea, a tree native to intermittent waterways in which PRD events are common. Seedlings were grown with part of their root system in soil and part in an aquatic compartment, mimicking conditions often observed in the field. • The aquatic roots initially provided two-thirds of total water uptake, but draining the aquatic compartment had no effect on stomatal conductance, so long as soil moisture remained c. 80% of field capacity. Water uptake from the soil compartment increased threefold within 24 h, with a corresponding transient threefold increase in root hydraulic conductance (L(p)), an increase in plasma membrane intrinsic protein 1 (PIP1) aquaporins at 24 h, and a decrease in PIP2 aquaporins by 48 h. • Our results demonstrate that PRD can induce rapid changes in L(p) and aquaporin expression in roots, which may play a role in short-term water uptake adjustments, particularly in species adapted to heterogeneous water availability.

Augmentation of abscisic acid (ABA) levels by drought does not induce short-term stomatal sensitivity to CO2 in two divergent conifer species

The stomata of conifers display very little short-term response to changes in atmospheric CO(2) concentration (C(a)), whereas the stomatal responses of angiosperms to C(a) increase in response to water stress. This behaviour of angiosperm stomata appears to be dependent on foliar levels of abscisic acid (ABA(f)). Here two alternative explanations for the stomatal insensitivity of conifers to C(a) are tested: that conifers have either low ABA(f) or a higher or absent threshold for ABA-induced sensitivity. The responsiveness of stomatal conductance (g(s)) to a sequence of transitions in C(a) (386, 100, and 600 μmol mol(-1)) was recorded over a range of ABA(f) in an angiosperm and two divergent conifer species. The different ABA levels were induced by a mild drought cycle. Although the angiosperm and conifer species showed similar proportional increases in ABA(f) following drought, conifer stomata remained insensitive to changes in C(a) whereas angiosperm stomata showed enhanced sensitivity with increasing ABA(f). The conifers, however, had much higher ABA(f) prior to drought than the angiosperm species, suggesting that non-sensitivity to C(a) in these conifers was due to an absent or inactive response/signalling pathway rather than insufficient ABA(f).

Control of leaf growth by abscisic acid: hydraulic or non-hydraulic processes?

Abscisic acid (ABA) affects plant metabolism and water transfers via multiple mechanisms at cell, organ and whole plant levels. These mechanisms translate into contradictory effects on leaf growth, so the literature reports positive, null or negative effects of ABA on leaf growth upon water deficit. We review evidences based on genetic manipulations of ABA biosynthesis, feeding the plant with artificial ABA or partial root drying and provide elements to avoid confusions of effects. We propose that ABA has mainly three effects on growth. (i) Via its controlling effect on stomatal aperture and transpiration rate, an increased concentration of ABA tends to buffer the day-night alternations of leaf growth rate and the negative effect of evaporative demand. (ii) ABA tends to improve leaf growth via an increase in the conductance to water transfer in the plant as a result of increased tissue hydraulic conductivity. (iii) ABA has also a modest non-hydraulic effect which is negative in plants subjected to water deficit, either manipulated for ABA synthesis or fed with artificial ABA, but can be positive in well watered plants deficient of ABA. The overall effect of increasing ABA biosynthesis depends on the relative weight of each of these effects under different environmental scenarios.

Influence of a drying cycle on post-drought xylem sap abscisic acid and stomatal responses in young temperate deciduous angiosperms

•   Post-drought patterns of water relations and gas exchange were studied in relation to xylem sap abscisic acid (ABA) concentration during recovery for young plants of five woody species. The potential role of xylem sap [ABA] in these responses was the object of study. •   Potted plants were allowed to deplete soil water and then were rewatered. At peak drought and during recovery, predawn and midday leaf water potential (Ψ_l ), stomatal conductance (g_s ), and xylem sap [ABA] were measured. •   Water potentials recovered rapidly after rewatering but stomatal re-opening was delayed. Xylem sap [ABA] was elevated early in recovery and might have affected stomatal opening, but after 1 d at high soil water content [ABA] in recovering plants was equal to or lower than in control plants. Stomata appeared to be more sensitive to xylem sap [ABA] in recovering than droughted plants. •   Xylem sap [ABA] may play some role in delayed recovery of stomatal opening after drought, but may not completely explain the responses.