Minimizing the effects of thermal stress by foliar nutrition, irrigation and kaolin applications in 'Gala' apple trees

Apple production under Mediterranean conditions faces new challenges in response to climate changes. Heat waves are becoming increasingly more common, requiring growers to adjust production techniques in order to minimize their adverse effects. An orchard of Malus domestica Borkh ‘Gala Brookfield’, grafted on MM106 rootstock, with 1481 trees ha^‑1 (4.50×1.50), located in Alcobaça (39.549 N, 8.957 W), was used to test the effects of foliar nutrition, alone (FN) and combined with supplementary irrigation (3.14 L tree^‑1 day^‑1 – FNI), kaolin (K), supplementary irrigation alone (4.57 L tree^‑1 day^‑1 – I2) against the control (1.71 L tree^‑1 day^‑1 – C) in the reduction of thermal stress. Assessments included determinations of photosynthetic rate (A), intrinsic water use efficiency (WUE_i), instantaneous water use efficiency (WUE_inst), stomatal conductance (gs), performance index (PIabs), maximum quantum yield of the PSII photochemistry (Fv/Fm), SPAD units, relative water content (RWC), specific leaf area (SLA), leaf and fruit temperature, fruit growth and quality. Trials began on July 10 and were extended until harvest (August 30, 2019). Average A was higher in FN (10.71±1.10 µmol m^‑2 s^‑1) and lower in K (7.94±0.99 µmol m² s^‑1), with FN, FNI and I2 having the best performance in all the periods of the day. FN and FNI had the best WUE_i and WUE_inst and I2 the best gs (0.27±0.04 mmol m^‑2 s^‑1). SPAD ended higher in FNI (53.17±1.27). In the beginning of the trials SLA was lower in C but ended better in I2 (97.28±1.59 cm² g^‑1). RWC was always lower in K and higher in I2 (72.1%) or FNI (66.1%). Average temperature was higher in exposed fruits (42.17±0.75°C) and leaves (32.06±0.56°C) from C and lowest in exposed fruits from K (40.79±0.57°C) and exposed leaves from I2 (30.55±0.49°C), with an ambient temperature around 30°C. Plant vitality, expressed by PIabs, showed higher values in FN, I2 and FNI and Fv/Fm had constant values of approximately 0.82. During 2019, this region did not experience abnormal temperatures, so trials should be repeated and trends confirmed. However, it was possible to perceive the effects of these treatments in conditions of moderate summer temperatures, providing important information for technical management.