The impact of water stress at different phenological stages on the yield and quality of Macadamia (F. Muell)

Dissertation (MSc (Agric.) Horticultural Science)--University of Pretoria, 2021.

Irrigated agriculture is currently the single largest consumer of water on the planet, accounting for a near 22% of total freshwater consumption. Efficient water use strategies in the agricultural sector are therefore fundamental. Current macadamia irrigation quantities are, however, based on the physiology, phenology and morphology of other horticultural crops, which could be less tolerant to water stress and deficit irrigation. A lack of crop specific information with regards to water stress tolerances results in a large amount of uncertainty with regards to efficient water management practices, a problem that is exacerbated in macadamia given the inconsistent responses to irrigation reported previously. This study, therefore, attempted to determine the effect of water stress at different phenological stages on the yield and quality of macadamias. Seven water deficit treatments were imposed on 63 randomly selected, mature bearing macadamia trees (cv. HAES 695, ‘Beaumont’, M. tetraphylla x M. integrifolia, grafted) and respective yield, quality and physiological responses were compared to a well-watered control over a three year period. Mild water deficits were imposed at various phenological periods and was analyzed alongside longer term water deficit treatments. Tree based measurements such as stomatal conductance (gs), light saturated net photosynthetic assimilation rate (Amax), pre-dawn leaf (Ψpd) and midday stem (Ψx) water potential, were used in conjunction with soil water potential measurements to establish different macadamia water deficit thresholds under orchard conditions. Results from this study suggest that macadamias are less sensitive to water deficit conditions than most other horticultural crops. Withholding water during key phenological stages had no impact on yield for one season of measurements relative to the control. Irrigating half the volume of the control or relying solely on rainfall also had no impact on yield over two seasons. Water deficits during flowering and nut set, and shell hardening phenological stages were detrimental to macadamia quality, significantly reducing the percentage total kernel recovery relative to the control. Severe water deficits at any phenological stage will likely have a negative impact on both yield and quality, however, in deep soils and under well distributed rainfall conditions, reaching such level of stress would be unlikely. Results from this study suggest that both Ψpd and Ψx can be used as indicators of water stress in macadamia orchards, with Ψpd <-0.6 MPa and Ψx <-0.9 MPa probably indicating the onset of stress that could impact both yield and quality.

Water Research Commission (WRC), South African Macadamia Association (SAMAC), Agricultural Research Council (ARC), the University of Pretoria and the University of Kwazulu-Natal

Plant Production and Soil Science

MSc (Agric.) Horticultural Science

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