WATER STRESS, GAS EXCHANGE AND FRUIT SET OF 'OLINDA' VALENCIA ORANGE TREES IN EASTERN TRANSVAAL AREA OF SOUTH AFRICA

Therefore, irrigation is essential to reduce water stress and improve fruit set and yields. Three irrigation regimes were compared for 28-year-old 'Olinda' valencia orange (Citrus sinensis L. [Osb.]) trees on Cairn rough lemon (C. limon Burm. F.) rootstock based on soil tensiometer levels, <20 kPa, 45 kPa, and >70kPa. Net CO₂ assimilation of leaves was measured at full bloom, petal fall, and at the end of initial fruit set. In addition, percent fruit set was measured weekly until the end of stage 1 of fruit development. In a separate study, net CO₂ assimilation was monitored over the same time period for leafy and leafless blooms from well-watered trees (<20kPa). The >70 kPa treatment significantly delayed bloom, retarded flower opening, and decreased fruit set compared to the other treatments. Net CO₂ assimilation was low and variable for all treatments but tended to be consistently lower for the >70 kPa treatment. The coefficient of variability for net CO₂ assimilation was nearly 50% for all treatments. Net CO₂ assimilation of leafy and leafless blooms was low (<4 μmol m^-2s^-1) but similar during all developmental periods. Therefore, under the high temperature and VPD conditions of the Eastern Transvaal the previously reported differences in net CO₂ assimilation between these types of blooms does not occur and may not be related to fruit set as is the case under less severe field or greenhouse conditions.

Download Adobe Acrobat Reader (free software to read PDF files)