Abstract

Controversies exist regarding the iso/anisohydric continuum for classifying plant water-use strategies. Isohydricity has been argued to result from plant-environment interaction rather than it being an intrinsic property of the plant itself. Discrepancies remain regarding the degree of isohydricity (sigma) of plants and their threshold for physiological responses and resistance to drought. Thus, the aim of this study was to evaluate the isohydricity of the grapevine varieties Syrah and Carmenere under a non-lethal water deficit progression from veraison from two different locations, the Cachapoal Valley (CV) and Maipo Valley (MV), in central Chile and with different rootstock only in Syrah. For this purpose, the midday stem water potential (psi(mds)) regulation and stomatal responses to drought, leaf traits related to pressure-volume curves, stomatal sensitivity to ABA, cavitation threshold, and photosynthetic responses were assessed. A higher atmospheric water demand was observed in the CV compared to the MV, with lower psi(mds) values in the former for both varieties. Also, the sigma values in Carmenere were 1.11 +/- 0.14 MPa MPa-1 and 0.68 +/- 0.18 MPa MPa-1 in the CV and MV, respectively, and in Syrah they were 1.10 +/- 0.07 MPa MPa-1 in the CV and 0.60 +/- 0.10 MPa MPa-1 in the MV. Even though similar variations in sigma between locations in both varieties were evident, Carmenere plants showed a conserved stomatal response to psi(mds) in both study sites, while those of Syrah resulted in a higher stomatal sensitivity to psi(mds) in the site of lower sigma. Besides the differences in seasonal weather conditions, it is likely that the different rootstock and clonal variability of each season in Syrah were able to induce coordinated changes in sigma, psi(gs12), and osmotic potential at full turgor (pi(0)). On the other hand, irrespective of the sigma, and given the similarity between the pi(0) and psi(gs12) in leaves before drought, it seems that pi(0) could be a convenient tool for assessing the psi(mds) threshold values posing a risk to the plants in order to aid the irrigation decision making in grapevines under controlled water deficit. Finally, water deficits in vineyards might irreversibly compromise the photosynthetic capacity of leaves.