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University of KwaZulu-Natal (1994)

Ecophysiological studies and tree manipulation for maximisation of yield potential in avocado (Persea americana Mill.)

Whiley, Anthony W.

Titre : Ecophysiological studies and tree manipulation for maximisation of yield potential in avocado (Persea americana Mill.)

Auteur : Whiley, Anthony W.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy (PhD) 1994

Résumé
Tree fruit crops generally consist of scion and rootstock components, which through interactive synergism affect tree performance. Coupled with tree architecture, sink/source relationships (both spatial and temporal), genotypic responses to environments, and carry-over seasonal effects present a high level of complexity which often confounds research results. The development, description and use of pheno/physiological models as research and crop management tools is a new holistic approach to reduce complexity and improve understanding of the critical factors which influence crop productivity. A pheno/physiological model is described for cv. Hass avocado growing in a cool, mesic subtropical environment in S.E. Queensland, Australia. Seasonal shoot and root growth had bimodal periodicity with root growth offset and delayed with respect to shoot growth. The priority sink strength of developing shoots compared with roots was confirmed with 14(C) studies. Root growth in summer extended through until late winter when there was a substantial decline following anthesis - a critical time in fruit development with competition between reproductive and vegetative sinks for limited resources. Delayed harvesting of fruit over several seasons resulted in alternate bearing patterns, while removal of fruit at the minimum legal maturity of 21 to 24% dry matter sustained successive high yields. With cv. Hass, production was directly related to starch concentrations in trunks or shoots in July (midwinter) immediately prior to anthesis. However, seasonal starch concentration fluxes in trunks were much lower in coastal subtropical Australia compared with those previously reported from interior areas in more southerly latitudes (7.5% vs. 18% maximum). Current assimilate from over-wintered leaves was necessary to bridge the gap in early spring between the depletion of starch reserves by new reproductive and vegetative shoot growth, and the sink/source transition of the spring shoot growth. Net CO2 assimilation of summer grown leaves reached ca. 17 µmol CO2 m(-2) s(-1), approximately twice as high as previously reported rates on container-grown plants or trees in minimum temperatures were < 10⁰C for 50 days, this being the first report of this phenomenon in field-grown avocado trees. Partial recovery occurred prior to senescence of previous season’s leaves in spring after minimum temperatures increased above 10⁰C.

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