Influence of crop based water and nutrient strategies on physiological aspects of apple trees ‘Brookfield Gala’
Thesis (PhD(Agric) (Horticulture))--Stellenbosch University, 2008.
It is a common practise in the Western Cape to use micro sprinklers as the standard irrigation system for apple trees. Over the past forty years much effort has been put into the optimisation of the tree canopy. Less attention has been given to root proliferation, and the question as to whether root stimulation and proliferation, through intensive water and nutrient management, can contribute towards improved tree efficiency and more efficient water use. This is addressed in this study. ‘Brookfield Gala’ apple trees were studied in the Genadendal area near Greyton, in the Western Cape Province, South Africa. The trees were planted in Dundee soil (well aerated sandy loam soil) during winter 2003. Both horticultural aspects (tree growth, shoot growth, fruit yield and quality, trunk circumference and root growth) and gas exchange were studied from 2004/5 until 2007/8 under three different water application strategies, namely micro sprinkler irrigation, daily drip and pulsing drip irrigation and using two different rootstocks: M793 and M7. Irrigation under micro sprinkler irrigation was applied once to three times weekly, daily drip irrigation once daily/twice daily, and pulsing drip irrigation one to six times daily. Water use for bearing apple trees was calculated using long-term evaporation data (for Villiersdorp and Caledon) and existing crop factors for apples. Annual nutrient requirements were adapted from literature and divided percentage-wise into the requirements for five different phenological stages. Soil sensors were used to keep plant available soil water between 100% and 50%. A computer software program was used to incorporate all the above mentioned information and calculate the exact amounts of water and nutrients, and the application times. In general, drip irrigation systems used ±26% less water than micro sprinkler irrigation system. Significantly higher fruit yields were obtained with trees under daily or pulsing drip irrigation than those under micro irrigation during 2005/6 and 2007/8. During 2006/7 the crop load was low due to unfavourable weather conditions during flowering, resulting in poor fruit set and no differences in yield. There was a significantly higher number of thin plus medium roots (3mm and less in diameter) in the 0─400mm rooting zone and total root mass at 0─800mm rooting zone under drip irrigation systems than under micro sprinkler irrigation. Brookfield Gala’ apple trees grown under daily drip irrigation and pulsing drip irrigation performed better compared to those grown under micro sprinkler irrigation with respect to CO2 assimilation rate (A), stomatal conductance (gs), water use efficiency (WUE) and leaf water potential. None of the three irrigation systems affected the biochemical efficiency of the leaf significantly, except on a few occasions during the pre-harvest period. This implied that the changes in leaf biochemical efficiency were as a result of both stomatal and non-stomatal effects (temperature and vapour pressure deficit). The removal of fruit at harvest had a great influence on leaf photosynthetic capacity under micro irrigation but less so under drip irrigation systems. Higher chlorophyll a and chlorophyll b concentrations were observed under drip irrigation systems than under micro sprinkler irrigation, implying efficient biochemical efficiency under these systems compared to micro sprinkler irrigation during the post-harvest period. Use of daily drip irrigation and pulsing drip irrigation delayed the process of leaf ageing. This study demonstrated the benefits of more intensive water and nutrient application for apple trees. Improved root proliferation, increased fruit yield and photosynthetic efficiency have been found under drip irrigation system than under micro sprinkler irrigation.