Masters Degrees (Horticulture)

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Browsing Masters Degrees (Horticulture) by Author "Cameron, Anouska"

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    Calcium uptake and distribution in relation to periods of active white root growth in young, potted apple trees (Malus domestica Borkh. cv. ‘Golden Delicious’) in the Western Cape
    (Stellenbosch : Stellenbosch University, 2019-02) Cameron, Anouska; Lotze, Elmi; Hoffman, L. (Lynn) (Horticulturalist); Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Two-year-old ‘Golden Delicious’/M7 apple trees were chosen as the experimental material for this study. Potted trees were fertigated with a low calcium (Ca), balanced nutrient solution over two consecutive seasons (2015/16 and 2016/17). In the first season, trees were either left to drop their leaves naturally (NLD) during autumn and winter or were completely defoliated by hand earlier in autumn (AD), during April 2016. At the beginning of the second root flush, in winter (May 2016), calcium nitrate (Ca(NO3)2) was applied to the soil at three different rates: no additional Ca (control), moderate Ca (1X) and high Ca (2X). At the end of winter (August 2016), all trees were destructively sampled for macro mineral analysis of the different plant organs. Although an increase in soil Ca supply did not have a significant impact on white root numbers during winter (determined by minirhizotrons), early leaf loss via AD caused a drastic decline in white root numbers during winter. Despite this, Ca uptake and reserve accumulation in the roots and stems of the 2X(AD) treatment was not compromised, indicating possible active uptake by older, brown roots. Sap flow rates in leafless trees (determined by dendrometers) also remained above zero during winter, suggesting that leaf transpiration-driven sap flow is not essential for root Ca uptake and translocation in the xylem. In the NLD trees, an increase in autumn soil Ca supply resulted in a significant increase in leaf Ca concentration following the extended leaf drop period. At the same time, scanning electron microscopy and wavelengthdispersive x-ray spectroscopy (SEM-WDS) results indicated rapid uptake of soil-applied Ca by apple white root tips during winter. As 50 % leaf drop was reached late in winter (July), a substantial fraction of total Ca uptake in the 2X(NLD) treatment was lost via leaf abscission at the expense of Ca allocation to the reserve tissues during winter. The preferential allocation of Ca to the leaves was possibly the result of relative high rates of leaf transpiration-driven sap flow prior to 50 % leaf drop. Calcium partitioning in relation to periods of active white root growth, after harvest in the second season, were also investigated. Treatments comprising Ca(NO3)2 soil applications included a summer-only (2X) treatment, an autumn-only (2X) treatment and a summer (1X)/autumn (1X) treatment. Although fruit Ca concentrations were satisfactory at harvest, no significant differences were found between treatments. Whilst confirming primary xylem transport to the more dominant leaf and shoot sinks, the lack of response to summer applications (current season, 2016/17) indicates the predominant role of Ca reserve accumulation in the trees in supporting new growth the following season. In contrast to the summer/autumn treatment, a significantly higher % of total Ca content was found in the fruit of the autumn-only treatment. These results suggest that under local conditions of insufficient winter chilling, relatively high rates of soil-applied Ca(NO3)2, both in summer and autumn during active white root growth, may benefit the next season’s crop through remobilization of stored Ca in the roots and reserve tissues of the stems.