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University of Pretoria (2020)

Seasonal water use dynamics of Macadamia (F. Muell) orchards

Smit, Theunis Gerhardus

Titre : Seasonal water use dynamics of Macadamia (F. Muell) orchards

Auteur : Smit, Theunis Gerhardus

Université de soutenance : University of Pretoria

Grade : Doctor of Philosophy (PhD) 2020

Résumé partiel
Increased demand for sustainably produced, healthy, and nutritious food has seen certain segments of the world agricultural sector flourish in the past few decades. The macadamia nut industry in particular has expanded at a tremendous rate, with more than 10 000 hectares of trees being planted annually across a range of environments. The greatest portion of these expansions occur in semi-arid areas, which are characterized by highly variable rainfall patterns, and are as a result irrigated to minimize the risk of yield, quality and income losses, brought about by water stress. The recently commercialized nature of the crop, in combination with lack of water use research specific to macadamia, has created great uncertainty amongst producers. This study has therefore firstly aimed at gaining a fundamental understanding of leaf gas exchange and macadamia transpiration (Ec) in response to a range of environmental and physiological variables, in an attempt to identify the driving variables of transpiration. Secondly, the study aimed to identify crop water use models that best incorporate the driving variables of Ec, in order to transfer results obtained from this study, to a range of growing environments. Measurements of leaf gas exchange, hydraulic conductance, canopy dimensions, weather, and Ec were made over an approximate three year period, in a fully irrigated commercial mature bearing (MB) and immature bearing (IB) macadamia orchard in the Mpumalanga province of South Africa. Leaf gas exchange measurements, included, but were not limited to net CO2 assimilation rate (A) and stomatal conductance (gs). Transpiration measurements were obtained using sap flow measurements using the heat ratio method of the heat pulse velocity technique. Macadamia A was found to be slightly lower than that of other evergreen subtropical crops, which is largely attributed to substantial stomatal and non-stomatal limitations to A. Non-stomatal limitations to A were linked to an internal light limitation resulting from the sclerophyllous nature of leaves. Stomatal limitations stem from the predominantly isohydric nature of macadamias, where gs is carefully controlled in order to maintain midday leaf water potential within certain safety margins. Isohydric behaviour suggested an underlying hydraulic limitation, which was found to exist within the stem to leaf interface of macadamias. Responses of gs to leaf vapour pressure deficit (VPDleaf) showed that gs declined as VPDleaf exceeded 2.5 kPa. The response of gs to VPDleaf, however, varied substantially throughout the season, being significantly higher during fruiting periods compared to non-fruiting periods, implying isohydrodynamic behaviour and emphasizing the influence of phenology on leaf gas exchange

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