The drought, the temperature, and the time: drivers of osmotic adjustment?

Abstract

Context and purpose of the study. Leaf osmotic adjustment (i.e., active accumulation of osmolytes in the cells) has been reported in grapevines in response to drought and as a natural process throughout the growing season (seasonal osmotic adjustment). However, the seasonal progression and associated temperature changes were not separately considered in previous studies. Therefore, we investigated the capacity of Pinot noir leaves to modulate their osmotic potential in response to (i) time (seasonal osmoregulation), (ii) growing temperatures, and (iii) drought events. Additionally, we analyzed the contribution of key compounds acting as osmolytes (e.g., sugars and cations).

Material and methods. Trials were conducted under semi-controlled field conditions and in greenhouse chambers set to three distinct temperature regimes: cool (20/15 °C), warm (25/20 °C), and hot (30/25 °C) day/night conditions. Potted Vitis vinifera plants, cultivar Pinot Noir grafted onto SO4, were subjected to two water treatments for at least 30 days: well-watered (WW) and water deficit (WD). Leaf osmotic adjustment was assessed using an osmometer, while sugars and cations were quantified through ion chromatography (HPIC).

Results. Seasonal osmoregulation was observed throughout the growing season, with osmotic potential at full turgor decreasing by an average of -0.30 MPa in 70 days. Temperatures affected the intensity of osmotic potential at full turgor but not its seasonal trend.  Surprisingly, osmoregulation wasn’t triggered by the lower water potentials in WD vines. The absence of osmotic adjustment in response to drought observed in this cultivar suggests Vitis genotypes exhibit a wide range of drought responses, with drought-coping strategies varying significantly by cultivar. The total contributions of sugars and cations covered up to 60% of the measured osmotic potential. Glucose and fructose were the predominant sugars, while cations were the main contributors to the seasonal osmotic adjustment.