Response of Navel orange trees to timed partial root zone drying in northern Zimbabwe
Dzingai, Hellen G.
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Navel orange trees (Citrus sinensis (L.)Osbeck) were drip irrigated in an 8 hectare commercial orchard at Mazowe Citrus Estate (Mazowe Citrus Estate: block U5A) in the 2009/2010 season. We investigated the effects of partial root zone drying (PRD) applied at different periods of fruit growth on fruit growth rates, fruit yield, fruit quality and water use efficiency. This was done in order to determine the importance of the timing of PRD application on physiological and agronomic citrus response under semi arid conditions. Partial root zone drying (PRD) is an irrigation system which allows a reduction of seasonal irrigation volume near the 50 percent of a well watered crop, and improves fruit quality parameters, without significant yield reduction. In the PRD technique only half of the root system is irrigated, the other half is maintained in drying soil. After 10-15 days, the wet and the dry root zones are inverted. This system of watering would allow separating some positive effects of water deficit (better fruit quality, reduction of transpiration losses) from the negative effects such as fruit yield reduction. The first part of the project entailed monitoring of sap flow rates over a period of seven days (DOY 135-DOY 141). This exercise gave a guide to the typical sap flow rates attained under different PRD and non PRD treatments. This information was useful in timing of PRD. Yields attained under the different treatments were analysed and related to the sap flow rates. The PRD treatment gave lower sap flow rates and higher yields. The PRD treatment used water more efficiently compared to the other two treatments. In the second part of the project, six irrigation treatments were applied: Control which applied water one side of tree irrigating according to ETo data from the automatic weather station and, the growers practice, which is the convectional drip irrigation, PRD which applied 50 percent water as that of the control, PRD 50% stage 2, PRD 50 % stage 3, PRD 75% stage 2, 50% stage 3.The treatments received different amounts of irrigation water due to the different irrigation durations in each treatment. Fruit growth rates during the experimental period were not significantly affected by timing PRD treatments. There were no significant treatments effects on fruit yield and fruit quality was significantly affected by timing PRD. Water use efficiency was lowered when timing of PRD was introduced but favourable results were obtained. Fruit size was significantly affected by crop load.