Sap flow, trunk diameter and plant-water relations parameters as stress indicators of apricot trees

Abstract

The aim of this paper was to study continuous and discrete parameters of plant-water relations in mature apricot trees, in order to analyse the physiological basis of these plant-based sensors as well to determine the most sensitive indicator of plant water deficits. The experiment was performed during 1998-2001 in twelve-year-old apricot trees (Prunus armeniaca L., cv. Búlida), growing under field conditions in Murcia, Spain (semi-arid climate, with 1500 mm average annual evaporation and 350 mm rainfall) in a clay loam texture soil (128 mm m-1 available soil water content and 1.45 Mg m-3 bulk density). Trees were submitted to two drip irrigated treatments: a control treatment irrigated at 100 % of seasonal ETc and a water deficit treatment (WD) irrigated at 50 % of the control treatment. The diurnal courses of leaf water potentials, leaf conductance, net photosynthesis, leaf temperature, sap flow and trunk diameter fluctuations were studied throughout one growing season. The deficit treatment showed stomatal regulation as an adaptive mechanisms to drought, which allows plants to regulate water loss more effectively. Leaf temperature values were higher in WD than in the control treatment, and acted as a good stress indicator. Net photosynthesis showed a good correlation with leaf conductance values. In deficit irrigation conditions, osmotic adjustment was not observed. Both leaf water potential and gas exchange parameters were well correlated with evaporative demand of the atmosphere. Trunk diameters oscillate over a 24-h cycle, reaching a maximum value just before sunrise and a minimum sometime in the afternoon. The difference between maximum and minimum values is termed maximum daily shrinkage (MDS). This parameter increased in WD plants during the first period of the growing season, however, when the water stress was accumulated (in the last period of the growing season), MDS shown an important decrease in the water stressed plants respect to the irrigated ones. The sap flow values were lower in the deficit than in the control treatment throughout the growing season, although the greatest differences were observed in spring. Trunk diameter and sap flow showed moderately good correlations with the mean daily ETo in adult apricot trees grown under non-limiting soil water conditions.