Doctoral Degrees (Horticulture)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/557

Browse

Browsing Doctoral Degrees (Horticulture) by browse.metadata.advisor "Dzikiti, Sebonasi"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Sensitivity of apple rootstocks to water stress
    (Stellenbosch : Stellenbosch University, 2023-03) Muchena, Lindsay; Midgley, S. J. E.; Dzikiti, Sebonasi; Lotze, Elmi; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The widespread occurrence of water deficits is increasingly one of the most prevalent limiting factors in apple production, which will likely worsen due to population growth and increased rate of climate change. Since apple production in South Africa is done under irrigation, increased periods of water deficits require, amongst other technologies, the use of apple rootstock selections that support high orchard water productivity under limited water supply. This would create resilience specifically in areas at risk of having their water allocation cut because of insufficient irrigation water availability. The aim of this study was to quantify whole tree water use, gas exchange and growth dynamics of mature, bearing (in-field) and non- bearing (potted) ‘Rosy Glow’ apple trees on a range of rootstocks, which varied from semi-vigorous to dwarfing, before, during and after periods of water deficit during the 2017/2018 and 2018/2019 growing seasons. In the field trial, mean basal crop coefficient (Kcb), orchard transpiration per unit ground area (Ec) and transpiration per unit leaf area (EL) were higher in well-watered trees on the more vigorous rootstocks compared to more dwarfing rootstocks. An analysis of the tree hydraulic characteristics showed that well- watered trees on more dwarfing rootstocks had higher stem (Rx), canopy (Rc), and soil to stem (Rsx) hydraulic resistances compared to trees on the more vigorous rootstocks, which could possibly explain their lower water use. The Penman-Monteith equation accurately predicted water use of trees on different rootstocks, despite differences in tree vigour and hydraulic resistances. Measured and projected water productivity (i.e., calculated using the South African standard industry spacings for each rootstock) under well-watered conditions were both found to be higher in trees on more dwarfing MM.109/M.9 (MM.109 with an M.9 EMLA interstem) and M.9 RN.29 rootstocks compared to trees on more vigorous rootstocks. This means that these rootstocks could be a viable option to maximise productivity in high density planting systems. In the field trial EL was not reduced in water deficit relative to well-watered trees for any rootstock during both water deficit cycles. Trees on M.793 and M.9 RN.29 displayed more water use, relative to pre- stress values, indicating improved water uptake after re-watering. Trees on more vigorous rootstocks had significantly higher net CO2 assimilation rate (A), stomatal conductance (gs) and transpiration rate (E), and significantly lower intrinsic water use efficiency (WUEi) compared to trees on more dwarfing rootstocks on day 18 of water deficit cycle 1. These responses were closely associated with higher maximum rate of carboxylation (Vcmax) and higher leaf chlorophyll a (chla), chlorophyll b (chlb) and carotenoids concentrations. In the pot trial with non-bearing trees, there was a significant reduction in water use in the water deficit treatment relative to the well-watered treatment during both water deficit cycles. Like the field trial, trees on M.793 and M.9 RN.29 displayed enhanced water use after the two water deficit cycles indicating enhanced water uptake. Except for trees on G.222, trees on more dwarfing rootstocks displayed higher Rx, Rc and Rsx than more vigorous rootstocks, as observed in the field trial. Water deficit treatment decreased A, gs, E, chla, chlb, and chla/chlb ratio and improved WUEi, for all rootstocks. Trees on G.222, M.7 and M.9 RN.29 had higher Vcmax, Jmax and triose phosphate utilization (TPU) than trees on M.793 and G.202 during water deficit cycle 2, but this did not lead to significant differences in A amongst the different rootstocks. Partitioning of biomass towards root development differed significantly amongst trees on different rootstocks, with trees on more vigorous rootstocks having significantly higher increase in root biomass compared to trees on more dwarfing rootstocks. This study sheds light on the water requirements and stress responses of apple trees on rootstocks with different vigour-inducing capacities, which is instrumental in selecting suitable rootstock options to maximise water productivity. Trees on the dwarfing M.9 RN.29 rootstock were not more sensitive to water deficit in the field and in pot trials relative to more vigorous rootstocks. The use of M.9 RN.29 in high-density plantings could result in a significant saving in irrigation water.