Cane Water Content and Yield Responses of Vitis vinifera L. cv. Sultanina to Overhead Irrigation During the Dormant Period P.A. Myburgh* and L.D. van der Walt ARC Infruitec-Nietvoorbij, Private Bag X5026, 7599 Stellenbosch, South Africa Submitted for publication: September 2003 Accepted for publication: April 2005 Key words: Cane, donnant, irrigation, overhead, Suitanina, yield The possibility to increase grapevine cane water content during the dormant period by applying overhead irrigation, and thereby increasing yield, was investigated in a semi-arid summer rainfall climate. A field trial was carried out with Sultanina grapevines in the Lower Orange River region over two seasons. During the 2000/2001 season control grapevines (TI) that received no overhead irrigation in winter were compared to ones receiving overhead irrigation applied over a ca. 3O-day period, starting either in July (T2), mid-July (T3), August (T4) or mid-August (T5). Overhead irrigation was applied as ten-minute pulses on the hour from 10:00 until 16:00. Due to the lack of winter rainfall, all treatments received normal, under-vine irrigation in winter to avoid severe water deficits. All treahoents, except T2, were repeated during the 2001/2002 season. Two additional treatments which received no irrigation dur- ing winter were included during the second season. Of these two, T6 received overhead irrigation in August, where- as T7 received no overhead irrigation. Compared to the TI control, overhead irrigation applied during August 2000 (T4) not only increased cane water content measured before bud break, i.e. early September, but also increased yield. The other overhead irrigation treahoents did not a1Tect cane water content or yield. During the second season over- head irrigation started in mid-July (T3) or in the beginning of August (T4) induced higher yields compared to grapevines that received overhead irrigation in the period hefore bud break (T5). Where available soil water deple- tion of 90% occurred, overhead irrigation (T6) seemed to negate the adverse effects of dry soil on yield. In contrast, cane water content and yield were considerably lower where neither normal nor overhead irrigation was applied (TI), compared to T3 and T4. These results confirmed that overhead irrigation can increase cane water content and yield, and that soil water deficits during the dormant period should be avoided. Overhead irrigation andlor water deficits during winter did not affect cane mass, as measured at the end of the following growing season. Seasonal yield variation of Sultanina grapevines is a common phenomenon in the semi-arid Lower Orange River region of South Africa (Smit, 1970; Myburgh, 2003a), and also in regions such as the Murray Valley of Australia (May & Antcliff, 1963). Since most grapevines are grown under Mediterranean conditions (dry summers/wet winters), knowledge on the effects of water deficits during winter, or dormant period, in summer rainfall regions (wet summers/dry winters) is limited. Efforts to study water deficits during the post-harvest period in a field trial at Robertson under winter rainfall conditions failed due to untimely rain (Van Zyl, 1984). Field observations in the Lower Orange River region showed that too dry conditions during dormancy can be detrimental to growth at the beginning of the season (Goosen, 1956). The opposite was found in California, where bud break in Perlette grapevines occurred earlier, when irrigation was cut-off early in the post-harvest period (15 September) compared to the normal, later cut off date on I December (Williams et al.. 1991). However, the earlier irrigation cut-off had no effect on the num- ber of active buds at the end of the bud break period. On the other hand, over-irrigation of Suitanina during the post-harvest period, in combination with high temperatures at that stage, can induce *Corresponding aurhor: E-mail excessive vegetative growth (Goosen, 1956). Although this could limit reserve accumulation and induce stunted spring growth, reduced irrigation during the post-harvest period had no signifi- cant effect on cane starch content of Sultanina at pruning, irre- spective of the occurrence of vigorous shoot growth after the grapes had been removed (Myburgh, 2003b). Evaporative cooling during the post-harvest period is used to achieve earlier and more uniform bud break in many warm grape- growing areas, where chilling is minimal or non-existent (Williams et aI., 1994 and references therein). Some table grape growers in the warmer, western part of the Lower Orange River region use evaporative cooling to induce dormancy. This is obtained by applying overhead irrigation in the post-harvest peri- od during four to five relatively cool days, i.e. when the weather forecast predicts maximum temperatures lower than 16°C (J.1. Burnett, personal communication). When the temperature reach- es I DoC on these days, overhead irrigation is switched on and applied for five minutes at 20-minute intervals until sunset. By following this approach, maximum temperature can be reduced by ca. 4°C. For early cultivars, evaporative cooling would usual- ly start mid-May and for late cultivars early in July. Acknowledgements: Technical assistance of Ihe Suil Science staff at ARC lnfruitec-Nietvoorhij and C. Jackson of the SADORfarm. S. Afr. J. Enol. Vitic., Vol. 26, No.1, 2005 /