Physiological and growth reaction of Shiraz/101-14 Mgt to row orientation and soil water status

Abstract

Context and purpose of the study – Advanced knowledge on grapevine row orientation is required to improve establishment, management and outcomes of vineyards on terroirs with different environmental conditions (climate, soil, topography) and in view of a future change to more extreme climatic conditions. The purpose of this study was to determine the combined effect of row orientation, plant water status and ripeness level on the physiological and viticultural reaction of Shiraz/101-14 Mgt.

Material and methods – The project is carried out in the Breede River Valley, Robertson, South Africa. Shiraz(clone SH 9C)/101-14 Mgt vines were planted during 2003 to a VSP trellis and four row orientations (NS, EW, NE-SW, NW-SE), replicated five times on a flat terroir with uniform clayey loam soil. Vines are spaced to a fixed distance of 1.8 x 2.7 m and pruned to two buds per spur. Since 2014, two water deficit levels are applied to each row orientation treatment, i.e. 75 % and 50 % of the control (reference), the latter receiving the full soil water adjustment per week (based on ET0 values and standard seasonal crop factors). Grapes are harvested at two ripeness levels, i.e. targeting 23 °B and 25 °B.

Results – Total relative evapotranspiration (ETo) fluctuated prominently according to Tmax, RHmin and especially Radiation changes. Photosynthetic activity of the 50 % irrigation treatment tended to be lower across row orientations. Photosynthetic activity of primary leaves decreased during the season. Stem water potential generally decreased with higher water deficit. Clear diurnal and nocturnal profiles of photosynthesis and water potential occurred. The collective physiological response of vines to various environmental factors (light, water availability, temperature, humidity) is complicated and requires understanding at whole plant level. Results on vegetative and reproductive growth characteristics as well as grape composition were variable, but trends are surfacing. Primary and secondary leaf area as well as total leaf area/vine seemed reduced by water deficit treatment. This led to a general increase in primary:secondary leaf area ratio for water deficit treatments. This ratio is an indication of the presence of young leaves in the canopy and is an important measure of canopy capacity to support the obtainment of full grape ripeness. Canopies of control vines thus seemed better suited for supporting complete grape ripening, confirming the importance of judicious vine management to increase the presence of younger leaves. Mass and volume parameters of berries and bunches as well as yields at the two ripeness levels were mostly reduced by water deficit treatments. Differences between the 100% and 75% irrigation treatments were not consistent. Yields generally showed large losses from the first to the second ripeness level.