This is the comprehensive version of the same article published in the Jun/Jul'22 edition of the SA Fruit Journal.
Water resources are generally limited in most grape-growing regions, and inconsistent rainfall causes periodic droughts. By Carolyn Howell and Philip Myburgh (ARC Infruitec-Nietvoorbij)
The shortage of water in many grape growing areas may worsen if climate change reduces rainfall, and increases air temperature. Therefore, growers must use irrigation water more efficiently by means of sound irrigation scheduling practices.
The table grape industry also needs to reduce its "water footprint" to convince consumers that scarce water resources are being used responsibly. Calibration of instruments used for scheduling is not necessarily correct or accurate, because calibrations can differ between soils and/or different soil layers. Refill points, i.e. when irrigation is required, are often selected haphazardly.
Consequently, table grape vineyards are over-irrigated in many cases. Instruments can be calibrated against soil water content or plant water status. However, soil calibrations are tedious and require specialised skills and equipment. On the other hand, it is fairly simple to measure grapevine water status by means of the pressure chamber technique to measure stem water potential (S).
Read MoreIn this regard, it has been proven that instruments currently used for irrigation scheduling of commercial vineyards can easily be calibrated against grapevine water status. The objective of this study was to develop guidelines to use this approach for table grape irrigation by setting irrigation refill lines according to midday S thresholds and determining how different table grape cultivars responded to midday S thresholds.
The project was carried out in commercial table grape vineyards in the Noorder-Paarl area of the Berg River Valley region, for three seasons (2018/19, 2019/20 and 2020/21). Ten of the more popular white and red cultivars, based on SATI statistics of the past few years, were included (Table 1).
The vineyards were located within 5 km of each other. For each cultivar, there were two experiment plots adjacent to each other. The irrigation systems were adapted to allow separate irrigation of the first experiment plot. The second plot, i.e. a reference plot, was irrigated with the rest of the block according to the growers' schedules.
After bud break, soil water content (SWC) and midday S were measured concurrently in both plots to determine the relationship between S and SWC (Fig. 2). The soil in the experiment plot was allowed to dry out until midday. S in the grapevines reached -0.8 MPa in the pre-harvest period and -1.2 MPa in the postharvest period. These thresholds were based on previous research data that indicated that at this level of S, berry mass and yield of table grapes are not negatively affected.
Once the refill points were established for each cultivar, grapevines in the experiment plots were irrigated when the SWC was depleted to the refill point. All other vineyard management practices and bunch manipulation were carried out according to the growers' standard methods. Berry yield and its components, as well as juice characteristics at harvest were determined. Grapes were packed and evaluated after six weeks in cold storage.
Analysis of the data showed that less irrigation water was applied where grapevines were irrigated according to midday S compared to the growers' irrigation schedules. A comparison between the seasonal water use of the experiment plots and reference blocks showed that grapevines in experiment plots used approximately 550 mm of water and those in the reference blocks used approximately 750 mm of water (Fig. 2).
Therefore, grapevines in the experiment plot used approximately 28% less water than those in the reference block. When separating the pre- and postharvest irrigation, experiment grapevines received on average 7% less water in the pre-harvest period compared to the reference blocks (Fig. 3). However, irrigation of grapevines in the experiment plots was drastically reduced in the postharvest period, i.e. 57% less compared to the grower's irrigation strategies.
Irrigation scheduling based on midday S did not have negative effects on berry mass and diameter (Fig. 4), yield and its components, as well as juice TSS, TA and pH at harvest, and grape quality after cold storage compared to the growers’ irrigation schedules.
Conclusion
Midday ΨS correlated well with the SWC and growers can calibrate any SWC probe against ΨS, as long as the probe detects SWC correctly. Irrigation according to a pre-harvest S threshold of -0.8 MPa had no negative effects compared to the reference blocks, irrespective of cultivar.
Water savings in the postharvest period did not have any carry-over effects in the following season. Results showed that irrigation scheduling of table grapes based on midday ΨS could save substantial amounts of water, particularly in the postharvest period.
Such water savings will reduce the water footprint, particularly the blue water footprint and convince consumers that scarce water resources are being used responsibly. Water savings might not be possible when rainfall is low in the postharvest period or in the summer rainfall regions.
Acknowledgments
- SATI for partial funding of the research project.
- The Agricultural Research Council (ARC) for funding, research resources and infrastructure.
- JDK Pty Ltd; Mr. J. Basson and Hoekstra Fruit Farms for permission to work in their vineyards, as well as providing technical assistance.
- Netafim for sponsoring irrigation the equipment.
- Colleagues at ARC Infruitec-Nietvoorbij, in particular Mrs Karen Freitag and Mr Francos Baron, for dedicated technical support.
For further information, please contact Carolyn Howell at howellc@arc.agric.za.
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