Photosynthetic capacity and diurnal gas exchange of 'Brookfield Gala' apple leaves under three irrigation systems
Water and nutrient application using three irrigation systems, namely daily drip irrigation, pulsing drip irrigation and micro irrigation were studied with respect to photosynthetic efficiency, water use efficiency and leaf water relations in 'Brookfield Gala' apple trees during 2004/5 and 2005/6 in the Western Cape Province, South Africa. The amount of water used per irrigation strategy was 5254 m3 ha-1 yr-1 and 5661 m 3 ha-1 yr-1 for the micro irrigation; 3429 m3 ha-1 yr-1 and 39260 m3 ha -1 yr-1 for the daily drip and 3429 m3 ha -1 yr-1 and 4047 m3 ha-1 yr -1 for the pulsing drip for 2004/5 and 2005/6 seasons respectively. Trees were budded on either M793 or M7 rootstocks and planted in blocks using a split-plot experimental design with irrigation as the main effect and rootstock as the sub-plot. Maximum light-saturated net CO2 assimilation rate (Amax) was consistently higher under pulsing drip irrigation (this system applies water several times during the day creating an even shorter frequency cycle than the normal daily drip) compared to micro irrigation (applies water once to twice a week and uses sprinklers that spray water in a circle resulting in wetting a continuous strip within the drip row) but, occasionally, significantly higher than daily drip irrigation (applies water on a daily basis creating a shorter cycle compared to micro sprinkler system). The maximum rate of electron transport (Jmax) showed similar trends to Amax during 2004/5, but not during 2005/6. Amax peaked during midmorning (10h00) with a steady decline thereafter in all treatments, but with significantly lower rates under micro irrigation. Stomatal conductance (gs) declined steadily throughout the morning, with higher g s under the drip based systems compared to the micro system. The implications are that, irrigation application should be given early in the morning to sustain stomatal conductance and maximise CO2 assimilation rates during the period between 08h00 to 12h00. Results indicate higher photosynthetic capacity and water use efficiency under both drip-based irrigation systems compared to micro irrigation. Differences in photosynthetic capacity were related to both stomatal and non-stomatal responses.