Living cells need to be more or less saturated with water to function normally, but they are usually incomplete in this desirable condition. The two basic parameters which describe the degree of unsaturation, i.e. the plant water deficit are (i) the water content and (ii) the energy status of the water in the cell. The water content is usually expressed as relative to that at full saturation, i.e. the relative water content or water saturation deficit, and the energy status of the water is usually expressed as the total water protential. Although the two parameters are linked in such a way that the total water potential decreases as the water content decreases, the relationship between the two, variously known as the moisture release curve, water potential isotherm or water retention characteristic, is not unique but varies with species, growth conditions and stress history (Slatyer, 1960; Jarvis and Jarvis, 1963; Altmann and Dittmer, 1966; Noy Meir and Ginzburg, 1969; Ludlow, 1976; Jones and Turner, 1978). Thus for completeness, both the water content and energy status of the water in plant tissue need to be measured. The total water potential (7') at any point in the plant can be partitioned into its components: the osmotic potential (re), turgor pressure (P), matric potential (z) and gravitational potential. As the gravitational component of the total water potential is only 0.01 MPa m- 1 (0.1 MPa = 1 bar), it can be neglected, except in very tall trees (Conner et al., 1977). For cells in equilibrium with their surroundings the total water potential is the same throughout the system, i.e. in the wall, cytoplasm, organelles and vacuole. However, the components of the total water potential may be quite different: in the vacuole the total water potential arises largely from osmotic and turgor forces, whereas in the wall, it arises largely from matric forces and to a small degree from osmotic forces. Thus the total water potential of a plant cell is given

Techniques and experimental approaches for the measurement of plant water status

�Summary Because plant wilting has been described as a consequence of cadmium (Cd 2þ ) toxicity, we investigate Cd 2þ effects on plant water losses, gas exchanges and stomatal behaviour in Arabidopsis thaliana L. Effects of 1week Cd 2þ application in hydroponic condition (CdCl2 10–100mM) were analyzed. A 10-lM Cd 2þ concentration had no significant effect on the plant–water relationship and carbon assimilation. At higher Cd 2þ concentrations, a Cd 2þ -dependent decrease in leaf conductance and CO2 uptake was observed despite the photosynthetic apparatus appeared not to be affected as probed by fluorescence measurements. In epidermal strip bioassays, nanomolar Cd2þ concentrations reduced stomatal opening under light in A. thaliana, Vicia faba and Commelina communis. Application of 5mM ABA limited the root-to-shoot translocation of cadmium. However, the Cd 2þ -induced stomatal closure was likely ABA-independent, since a 5-day treatment with 50 lM Cd 2þ did not affect the plant relative water content. Additionally, a similar Cd 2þ induced stomatal closure was observed in the ABA insensitive mutant abi1-1. Interestingly, this mutant displayed a higher transpiration rate than the wild type but did not accumulate more Cd2þ , arguing that Cd 2þ uptake is not dependent only on the transpiration flow. Application of putative calcium channels inhibitors suppressed the inhibitory effect of Cd 2þ in epidermal strip experiments, suggesting that Cd 2þ could enter the guard cell through calcium channels. Patch-clamp studies with V. faba guard cell protoplasts showed that plasma membrane K þ channels were insensitive to external Cd 2þ application whereas Ca 2þ channels were found permeable to Cd 2þ . In conclusion, we propose that Cd 2þ affects guard cell regulation in an ABA-independent manner by entering the cytosol via Ca 2þ channels. Abbreviations: ABA, Abscissic acid; DMSO, dimethyl sulfoxide; DW, dry weight; FW, fresh weight; NPPB, 5-nitro-2,3-phenylpropylaminobenzoic acid; RWC, relative water content.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.2002.01442.x

Heavy metal toxicity: cadmium permeates through calcium channels and disturbs the plant water status.

Relative water content may be accurately estimated using the ratio of tissue fresh weight to tissue turgid weight, termed here relative tissue weight That relative water content and relative tissue weight are linearly related is demonstrated algebraically The mean value of r(2) for grapevine (Vitis vinifera L cv Shiraz) leaf tissue over eight separate sampling occasions was 0993 Similarly high values were obtained for maize (Zea mays cv Cornell M-3) (0998) and apple (Malus sylvestris cv Northern Spy) (0997) using a range of leaf ages The proposal by Downey and Miller (1971 Rapid measurements of relative turgidity in maize (Zea mays L) New Phytol 70: 555-560) that relative water content in maize may be estimated from water uptake was also investigated for grapevine leaves; this was found to be a less reliable estimate than that obtained with relative tissue weight With either method, there is a need for calibration, although this could be achieved for relative tissue weight at least with only a few subsamples

Rapid Estimates of Relative Water Content

Summary
1 The impact of sample preparation, rehydration procedure and time of collection on the determination of specific leaf area (SLA, the ratio of leaf area to leaf dry mass) and leaf dry matter content (LDMC, the ratio of leaf dry mass to fresh mass) of mature leaves was studied in three wild species growing in the field, chosen for their contrasting SLA and LDMC.
2 Complete rehydration was achieved 6 h after samples were placed into water, but neither of the procedures tested – preparation of samples before rehydration or temperature applied during rehydration – had a significant effect on the final values of SLA or LDMC.
3 As expected, water-saturated leaves had a lower LDMC than non-rehydrated leaves; more surprisingly, their SLA was also higher. The impact of rehydration on SLA was especially important when the SLA of the species was high.
4 There was no significant effect of time of sampling on either trait in any species over the time period covered (09·00–16·30 h).
5 These results suggest that SLA and LDMC obtained on water-saturated leaves (SLASAT and LDMCSAT) can be used for species comparisons. We propose a standardized protocol for the measurement of these traits. This would allow for better consistency in data collection, a prerequisite for the constitution of large databases of functional traits.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.0269-8463.2001.00563.x

A standardized protocol for the determination of specific leaf area and leaf dry matter content

Cytokinins (CKs) regulate plant growth and development via a complex network of CK signaling. Here, we perform functional analyses with CK-deficient plants to provide direct evidence that CKs negatively regulate salt and drought stress signaling. All CK-deficient plants with reduced levels of various CKs exhibited a strong stress-tolerant phenotype that was associated with increased cell membrane integrity and abscisic acid (ABA) hypersensitivity rather than stomatal density and ABA-mediated stomatal closure. Expression of the Arabidopsis thaliana ISOPENTENYL-TRANSFERASE genes involved in the biosynthesis of bioactive CKs and the majority of the Arabidopsis CYTOKININ OXIDASES/DEHYDROGENASES genes was repressed by stress and ABA treatments, leading to a decrease in biologically active CK contents. These results demonstrate a novel mechanism for survival under abiotic stress conditions via the homeostatic regulation of steady state CK levels. Additionally, under normal conditions, although CK deficiency increased the sensitivity of plants to exogenous ABA, it caused a downregulation of key ABA biosynthetic genes, leading to a significant reduction in endogenous ABA levels in CK-deficient plants relative to the wild type. Taken together, this study provides direct evidence that mutual regulation mechanisms exist between the CK and ABA metabolism and signals underlying different processes regulating plant adaptation to stressors as well as plant growth and development.

/pdf/analysis-of-cytokinin-mutants-and-regulation-of-cytokinin-3wr6zsoist.pdf

Analysis of Cytokinin Mutants and Regulation of Cytokinin Metabolic Genes Reveals Important Regulatory Roles of Cytokinins in Drought, Salt and Abscisic Acid Responses, and Abscisic Acid Biosynthesis

The relative turgidity technique consists in comparing the initial and turgid water contents, on a percentage basis, of disks punched from leaves, the turgid water content being obtained by floating the disks on water.

/pdf/a-re-examination-of-the-relative-turgidity-technique-for-j8kgr5ofyv.pdf

HD Barrs

Papers

A Re-Examination of the Relative Turgidity Technique for Estimating Water Deficits in Leaves