COMPARISON OF TWO TECHNIQUES FOR MEASURING LEAF WATER POTENTIAL IN Vitis vinifera VAR. ALBARIÑO COMPARAÇÃO DE DUAS TÉCNICAS DE MEDIÇÃO DO POTENCIAL DA ÁGUA NA FOLHA EM Vitis vinifera VAR. ALVARINHO

The need to determine the variables that help characterize the real water status of vineyards calls for further research aimed at testing current techniques for water status quantification, and particularly for the determination of leaf water potential (LWP). The Scholander Pressure Chamber (SPC) has been widely used and considered as the reference technique. Yet, the frequent use of thermocouple psychrometers and, more specifically, of water activity meters (WAMs) demands a comparative analysis of the performance of SPC and WAMs and the applicability of WAMs to plant samples instead of soil samples, which has seldom been studied. This paper presents a comparison of two techniques for the determination of leaf water potential in Vitis vinifera var. Albariño in Galicia (NW Spain), WAMs (two models: WP4 and WP4-T) and SPC (model 600-PMS). In addition, this paper presents an assessment of the time required to perform determinations with the three instruments tested. The performances of the three instruments were assessed during two seasons: in 2011, we assessed SPC performance against WAM performance; in 2010, we assessed the performances of the two models of WAM. The comparison between SPC and WP4 was carried out by randomly selecting 22 vines under two irrigation treatments [rainfed (R) and irrigation] and with two irrigation systems [surface drip irrigation (DI) and subsurface drip irrigation (SDI)]. LWP readings were taken simultaneously with SPC and WAM model WP4. The performance of the two WAM models was assessed in terms of calibration protocol and measurement protocol. To this end, five vines were randomly selected and the following physiological indicators were determined: predawn and midday LWP, predawn and midday osmotic LWP and stem water potential. The time required for measurements was computed for the three instruments. Results reveal a strong correlation between LWP values measured with SPC and WAM (WP4 model), with coefficients of determination above 0.84. According to the results, WAMs are more versatile than SPC, but SPC measurements require less time. Among WAMs, WP4 produces faster measurements than WP4-T and requires fewer calibrations per sample. The use of a WP4 WAM to take real measurements produced reliable results and allowed for the determination of plant water status according to different irrigation treatments showing high sensitivity to plant water status variations among treatments.