Osmotic adjustment in cowpea plants: Interference of methods for estimating osmotic potential at full turgor.

Osmotic adjustment is a persisting controversy in studies on the effect of salt and water stress in cowpea crops. Our hypothesis is that the osmotic potential determination method interferes with the osmotic adjustment calculation. The objective of this study was to consider the osmotic adjustment comparing results obtained by pressure-volume (P-V) curves and osmometry. The experiment was conducted in a randomized block design, with six salt water concentrations 0, 20, 40, 60, 80, and 100 mmol L-1 of NaCl in a Fluvisol. The osmotic adjustment found through osmometry were lower than those found through P-V curves. The apoplastic water fraction of the cowpea had a dilution effect, denoting overestimation of the osmotic potential by the methodology based on osmometry. This may be the source of the different interpretations of osmotic adjustment in cowpea plants. Thus, osmotic adjustment should be calculated preferably using the osmotic potential determined by method of pressure-volume curves.

Pressure-volume (P-V) curves in Atriplex nummularia Lindl. for evaluation of osmotic adjustment and water status under saline conditions.

The survival of Atriplex nummularia plants in saline environments is possible mainly due to the presence of salt-accumulating epidermal vesicles. Commonly, destructive methods, such as plant material maceration and subsequent reading in osmometers, are employed in studies on water relations and osmotic adjustment and are inconvenient due to their underestimation of the total water potential inside the cells, which can cause overestimation of an osmotic adjustment that is not present. As a result, methods that preserve leaf structure, such as pressure-volume (P-V) curves, which take into consideration only the salts that compose the symplastic solution, are more adequate. Thus, the main objectives of this study were to evaluate the effect of determination methods of osmotic potential (Ψo) in Atriplex nummularia through destructive and leaf structure-preserving techniques and to determine the water relations of the species under increasing NaCl concentrations. Plants were subjected to daily irrigations, maintaining soil moisture at 80% of field capacity, with solutions of increasing NaCl concentration (0, 0.05, 0.1, 0.2, 0.25 and 0.3 M) for 84 days. Water potential, osmotic potential and osmotic adjustment were determined. In addition, P-V curves were constructed using pressure chambers. Water and osmotic potentials decreased linearly with increasing NaCl concentration in the irrigation solution. The main discrepancies observed were related to the osmotic adjustments determined through maceration and P-V curves. Based on the present research, it was possible to conclude that in studies with species that have salt-accumulating vesicles in the epidermis, such as the plants in the genus Atriplex, constructing P-V curves is more adequate than destructive methods.