PARTIAL ROOTZONE DRYING OF DRIP IRRIGATED 'NAVEL' ORANGE TREES [CITRUS SINENSIS (L.) OSBECK] UNDER SEMI-ARID TROPICAL CONDITIONS

Abstract

The diurnal transpiration of 'Navel' orange trees [Citrus sinensis (L.) Osbeck], inferred from sap flow measurements, followed periodic oscillations due to the cyclic opening and closing of stomata round the day. As a consequence of this cycling, stomatal control of transpirational losses was tight under optimal soil water conditions. A curvilinear relationship between the daily total transpiration and the daily radiation sum occurred indicating a suppression of transpirational losses at high irradiances. The application of the partial root zone drying (PRD) irrigation method to these low transpiring species was evaluated over two growing seasons in a commercial citrus orchard in northern Zimbabwe. Three treatments, namely a single-line control (100% ETc replacement), PRD100 and a well-watered treatment (200% ETc replacement), were applied during the 2004/05 fruit growing season. Two additional treatments, namely a PRD50 and a regulated deficit (RD150), were introduced during 2005/06 wherein trees received 50% of the volume of water applied to the control treatment. All the treatments comprised a single replicate. Leaf water potential and stomatal conductance were monitored at hourly intervals on selected days during two growing seasons. Stomatal conductance in the PRD100, PRD50 and RD150 treatments was lower than in the well-watered treatment, while the leaf water potential in the PRD50 remained comparable with the well-watered treatment. The leaf water potential in the RD150 treatment was lower than in the PRD50 and the well-watered treatments for the same periods. Average yield per tree in the three treatments in 2004/05 was comparable although a yield reduction occurred in the PRD100 and in the control treatment used by the local growers in 2005/06.