The aim of this study was to determine the effect of soil temperature between flowering and veraison on growth, nonstructural carbohydrates, hormones and leaf function in Shiraz vines. Previous studies considered the period from dormancy to flowering. This project focused on the period between flowering and veraison when fruit set and early berry development occurs.
For the trial, three year old potted Shiraz vines were used in controlled glasshouse conditions. Soil temperatures were regulated using a recirculating water system. There were four treatments:
- Cool-cool – soil temperature was kept at 13°C throughout the growing season;
- Cool-warm – soil temperature was kept at 13°C until flowering, then warmed to 23°C;
- Warm-warm – soil temperature was kept at 23°C all season;
- Warm-cool – soil temperature was kept at 23°C until flowering, then cooled to 13°C;
Temperatures in each pot was recorded at 5 minute intervals, at 20cm depth. Various measurements were taken to quantify fruit set, biomass production, physiology and hormones in xylem sap.
- Soil temperature before flowering did not affect primary shoot length nor total leaf number and leaf area;
- Reduction in shoot length was greatest in the warm-cool treatment;
- Total dry biomass was the same across treatments however, in cooler soils, starch storage in roots and wood was stimulated, leading to increased root biomass compared to shoot biomass. In warmer soils, increased shoot and lateral growth from utilization of starch reserves led to a decrease in root biomass;
- Vines that were cooled prior to flowering had higher fruit biomass at veraison, possibly because they had more root reserves that could support early berry development;
- Where soil temperatures were cooler during flowering, fruit set was reduced and this was associated with increased root carbohydrates;
- Photo assimilation, transpiration and stomatal conductance were higher throughout the day in vines grown in warmer soil;
- The results show that grapevines respond to soil temperature by favouring shoot and fruit development under warmer conditions, whereas carbohydrate accumulation in roots are preferred under cooler coil temperatures;
- Soil temperature prior to veraison will alter the carbohydrate reserves with which vines enter the post veraison stage.
Significance of the study:
It is well understood that ambient temperature influences grapevines in terms of phenology and ripening. It is also a key consideration when deciding which cultivars to plant in a specific site. Soil temperatures are however also of interest, especially in the zone where grapevine roots are located. This research proved that soil temperature significantly affects grapevine growth due to the utilization and restoration of non-structural carbohydrates from roots and trunks relating to changes in biomass of different plant organs. To a certain extent, soil temperatures can be manipulated in the short to medium term through certain management practices such as mulching and plastic sheeting and there is merit to giving it more attention.
Stewart K. Field, Jason. P. Smith, Erin N. Morrison, R. J. Neil Emery, Bruno P. Holzapfel. Soil temperature prior to veraison alters grapevine carbon partitioning, xylem sap hormones and fruit set. Am J Enol Vitic. October 2019: ajev.2019.19038. https://www.ajevonline.org/content/early/2019/10/09/ajev.2019.19038.article-info