Incubator Initiative - How does phenological stage influence grapevine water requirements for Shiraz and Chardonnay in the Riverland?
Abstract
Under an increasingly water scarce environment, optimising irrigation scheduling in vineyards is critical to conserving freshwater, controlling vine size, and achieving target yields. We used a whole-canopy gas exchange approach over two growing seasons, 2019-20 and 2020-21, in the Riverland viticultural region of South Australia to quantify grapevine (Vitis vinifera L. cvs. Shiraz and Chardonnay) water use in order to inform irrigation practices in the region for the improvement of crop water use efficiency (yield per unit of water applied [t ML-1]). We quantified vine water use at key phenological stages of grapevine development (flowering, bunch closure, véraison, pre-harvest, and post-harvest. We also compared the whole canopy approach with micrometeorological (evapotranspiration (ET)) and sap flow approaches. Cultivar-specific crop factors (Kc) were determined at each phenological stage for the estimation of crop ET (ETc). Canopy water use ranged from 26-39 L vine-1 day-1 in Chardonnay and 24-33 L vine-1 day-1 in Shiraz in 2020-21, with higher values observed in Chardonnay early in the season and Shiraz late in the season. Post-harvest water use dropped by 42% (compared to harvest) in Shiraz and by 28% in Chardonnay. Daily dry matter production exceeded 200 g vine-1 at véraison with correspondingly highest seasonal canopy water use efficiencies observed during this time. Both ETc and sap flow estimates of canopy transpiration were lower than whole-canopy estimates. Canopy water use increased by up to 14% during heatwaves and the additional irrigation applied during the heatwave period was adequate to support this increased transpiration rate. Quantitative information of grapevine water use throughout the season would allow growers to optimise irrigation applications while achieving specific targets of yield or grape composition.
Summary
Climate change in South Australian viticultural regions has resulted in higher growing season temperatures leading to water scarcities for irrigators during the growing season. In this environment, optimising irrigation applications in vineyards, where grapevine water needs are matched with irrigation, is essential for sustainable viticulture. A mismatch between irrigation water applied and actual crop water use is one of the main reasons for low water use efficiency (Yunusa et al. 2005).
In the present study, we quantified the seasonal water requirements of productive Chardonnay and Shiraz grapevines in the warm, inland region of Riverland, South Australia over two growing seasons, 2019-20 and 2020-21. Three approaches were used for this purpose: (i) whole canopy gas exchange in which canopy transpiration, dry matter production, and water use efficiency were quantified; (ii) crop evapotranspiration (ETc); and, (iii) sap flow sensors to estimate canopy transpiration (2020-21 season only). These measurements were taken at key phenological stages of the growing season – flowering, bunch closure, véraison, harvest, and post-harvest on a diurnal (24 h) basis.
2019-20 was a slightly cooler season based on the growing degree days (accumulated heat units) compared to 2020-21, which, in addition to the higher soil moisture levels in the second season, may have contributed to the higher canopy transpiration rates quantified using the whole-canopy chambers. Daily water consumption by the vines ranges from 12-20 L vine-1 (2.2-3.6 mm) in the first season and 24-39 L vine-1 (4.3-7.1 mm) in the second season. Chardonnay tended to use more water early in the season, while Shiraz had higher water consumption during the veraison and post-veraison periods. On a leaf area basis, however, both cultivars were similar in transpiration rates throughout the season. Irrigation applied over the season was lower than the water requirements of Chardonnay, but matched that of Shiraz. There were no indications of water stress on any of the vines, based on measurements of stem water potential.
Dry matter production for Chardonnay was lower than Shiraz, reaching a peak of 238 g vine-1 day-1, while Shiraz peaked around 284 g vine-1 day-1. On a leaf area basis, Chardonnay tended to have higher rates than Shiraz during flowering and bunch closure stages, while Shiraz was higher during the véraison and pre-harvest stages. Whole canopy water use efficiencies (WUE) were highest early in the day between 0800-1000 h. On a seasonal basis, canopy WUE was highest around véraison in Shiraz and around flowering in Chardonnay.
Various methods to estimate vine water use were evaluated in the present study including whole-canopy chambers (Ecanopy), crop evapotranspiration (ETc), and, additionally in the second season, sap flow (Esf). For the estimation of ETc, crop factors (Kc) were measured throughout the season using the ‘Paso Panel’ (light interception) method. Results overall indicate a good agreement of all three methods with ETc only slightly underestimating water requirements compared to the other two methods. It was determined that the sap flow sensors require in-field calibration with an independent and robust method to provide reliable estimates of canopy transpiration rates.
In the second season, 2020-21, we used the calibrated sap flow sensors to quantify vine water use before, during and after two heatwaves received during the post-veraison period in both cultivars. Canopy water use (Esf) increased between 2-14% during heatwaves, representing an additional water use of between 1.0-7.5 L vine-1 day-1. Additional irrigation provided during the heatwaves (~1.7 mm day-1) would have likely alleviated the effects of heat stress on the vines. Our calculations suggest that not all this additional water may have been used for transpirational cooling although an overall benefit to vine microclimate would have occurred alleviating physiological heat stress.