Xylem structure and function in three grapevine varieties
Xylem vessels are responsible for conducting water, in a metastable state, to the transpiring leaves. As tension increases, hydraulic failure may occur. At any given tension, the resistance of the xylem vessels to cavitation depends on their morphology, of which vessels diameter has been pointed out as relevant. Grapevines (Vitis vinifera L.) are often cultivated in Mediterranean climates and mostly under controlled water deficit conditions. Besides, a high variability in the stomatal sensitivity exists in different varieties. ‘Cabernet Sauvignon’ (CS) is recognized as isohydric, ‘Syrah’ (S) as anisohydric, and no clear information exist for ‘Carménère’ (C), an important grapevine variety for the Chilean wine industry. In the present study, xylem morpho-anatomical traits from stems in CS, S, and C, growing in a single vineyard, were analyzed, inferring their specific theoretical hydraulic conductivity. Also, the maximal hydraulic conductivity and vulnerability to cavitation, the latter expressed as the xylem water potential reducing a 50% loss in hydraulic conductivity (PLC50) were assessed in stems collected from the field. Higher vessels diameter was found in S and C, followed by CS, in general ranging from 21 to 120 μm, resulting in an expected proportional theoretical hydraulic conductivity of nearly 40 to 43 kg s-1 m-1 MPa-1 in C and S, and merely 23 kg s-1 m-1 MPa-1 in C. This values were similar -but not proportionally- to maximal actual hydraulic conductivity measured in stems with median values, in average, of 28, 17, and 13 kg s-1 m-1 MPa-1 in S, C, and CS, respectively. Even though wider xylem vessels have been correlated with higher xylem vulnerability to cavitation, PLC50 was significantly higher in S (-2.3 MPa) compared to C (-1.1 MPa), and even though xylem vessels diameter were similar between S and C, C was as vulnerable as CS. We found no such tradeoff between hydraulic efficiency and vulnerability.