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The typical isohydric plant response to low relative humidity involves stomatal closure, followed by long-term responses like adjustment of shoot-to-root ratios. Little information is available on the early responses of the root system to exposure of shoots to low humidity, nor is it clear to what extent responses of Arabidopsis thaliana conform to the isohydric model. In this study, A. thaliana plants grown hydroponically at high humidity were exposed to two constant relative humidities, 17% and 77%, while the root system remained in aerated nutrient solution. Leaf conductance (gs), transpiration, water potential (Ψl), osmotic potential, and whole plant hydraulic conductance (K) were determined for the following time intervals: 0–10, 10–20, and 20–40 min, and 0–5, 5–10, and 24–29 h. At low relative humidity, no change in gs was detected. Ψl decreased by 0.28 MPa during the first 5 h and then remained stable. During the first hour, leaf-specific K averaged 1.6×10−5 kg MPa−1 m−2 s−1 at high humidity. At low humidity it increased >3-fold to 5.8×10−5 kg MPa−1 m−2 s−1. Similar significant differences in K were observed during all time periods. Low concentration mercury amendments in the hydroponic solution (5 μM and 10 μM HgCl2) had no discernible influence, but pre-exposure to 50 μM HgCl2 reduced K differences between humidity treatments. As HgCl2 is known to be a potent inhibitor of aquaporin function, this suggests that aquaporins may have played a role in the fast hydraulic response of plants transferred to low humidity. The rapid hydraulic response and the influence of mercury raise the possibility that an alternative response to atmospheric dryness is increased K modulated by aquaporins. [ABSTRACT FROM PUBLISHER]

Although plant shoots can be ‘hardened’ by abiotic stresses, little is known about such changes in roots. In order to investigate possible induction of root-hardening in response to short water-stress episodes, barley seedlings (Hordeum vulgare L) hydroponically grown under a controlled environment were moderately water-stressed by addition of a non-penetrating osmoticum, polyethylene glycol (PEG) 6000 at −0.4 MPa water potential, to the aerated nutrient solution. Seedlings were then hydrated in dilute nutrient solution without PEG before excision and assay of the seminal roots. Previous water stress treatments for 72 h, 12 h, or even 6 h induced an apparent root-hardening process. Thus, root radial shrinkage during subsequent exposure to strongly dehydrating conditions was remarkably decreased. The root hardening was related to biophysical adjustments: turgor-pressure increased while osmotic potential decreased from −0.45 ± 0.02 MPa to −0.60 ± 0.02 MPa. Moreover, the maximum bulk volumetric modulus of elasticity, ɛmax determined by pressure–volume analysis, increased from 2.1 ± 0.4 MPa to 3.7 ± 0.4 MPa, i.e. root elasticity was decreased. Root hardening in response to episodes of water stress may have ecological significance for barley plants in regions where intermittent drought episodes are frequent. [ABSTRACT FROM AUTHOR]

Abstract: The creation of a water-stress environment usually starts with a reduction in air relative humidity (RH) while soil water potential still reflects favorable conditions. Arabidopsis plants subjected to low RH (17%) exhibited a significant increase in leaf specific hydraulic conductance, reducing the water potential. However, no detectable effects on stomatal performance or osmotic leaf adjustment were noted relative to plants exposed to high RH. In the present study, we profiled gene expression in roots 2.5 and 5h after shoot exposure to low RH. Multiple genes with various putative biological roles were identified as differentially expressed under these conditions; among them were aquaporins, some of whose expression was induced under low RH. Transcription of two aquaporin was localized to the roots, especially to cells around the vascular system and to cells of the differentiation zone, and to leaf trichomes. Our results suggest that plant roots perceive the low RH stimulus from shoots through a sensing mechanism(s), leading to distinct plant transcriptional responses, potentially reflecting activation of various biological processes. This activation might be a prelude to the expected forthcoming drought conditions, providing the plant with enhanced resistance to future water-stress situations. [Copyright &y& Elsevier]