Using sap flow sensors to study the influence of rootstock and mid-summer water deficit on transpiration of apple trees in South Africa

South African apple growers require information on water-use of apple cultivars and rootstocks with the aim of increasing resilience to water deficits. This study aimed to quantify the water-use of mature full-bearing and young non-bearing apple trees on different rootstocks under water deficits. Field-grown mature 'Rosy Glow' trees on rootstocks RN29, MM109/M9 (MM109 with an M9 interstem), G222 and M793 (in order of increasing vigour) were exposed to 2 two- to three-week water deficit cycles from January to March 2018 (mid- to late-summer). The trial was repeated in 2019 using potted one-year-old 'Rosy Glow' trees, with the addition of M7 and replacement of MM109/M9 by G202. Transpiration was estimated by measuring sap flow using the heat ratio method, and the stem heat balance method for mature and potted trees, respectively. Mature control trees transpired 995, 725, 715 and 375 mm on rootstocks M793, G222, MM109/M9 and RN29, respectively, during the full growing season. During the deficit period of both cycles, mean water use (mm day^‑1) of mature trees (deficit treatment “WD”) was not reduced relative to the well-watered treatment (“WW”) for any rootstock. This was unexpected given that soil water content was clearly reduced. During the cycle 1 recovery period, WW and WD trees (RN29, MM109/M9, G222) had similar daily water use relative to pre-deficit values, and M793-WD had periods of higher water use than M793-WW relative to pre-deficit values. During the cycle 2 recovery period, M793-WD and RN29-WD both had period of greater water use compared to M793-WW and RN29-WW relative to pre-deficit values. Potted trees subjected to water deficit showed significant reductions in mean daily water use during both deficit cycles compared to the control treatment. At the end of both cycles, G202, G222 and M7 only partially recovered. M793 and RN29 generally fared better than G202, G222 and M7 in both cycles, recovered rapidly and showed a possible ability to slightly increase transpiration relative to WW trees during the recovery period. The results suggest that differential sensitivities to summer water deficit and recovery abilities between different apple rootstocks can be quantified using sap flow techniques. Potential physiological response mechanisms are still under investigation.