RELATIVE EFFECTS OF GROWTH RETARDANT (PACLOBUTRAZOL) AND WATER-STRESSON TREE GROWTH AND PHOTOSYNTHESIS IN PONKAN (CITRUS-RETICULATA BLANCO)

The effects of water stress and/or chemical growth retardant, paclobut razol, applied either as foliar spray or soil drench, on photosynthesi s, tree vegetative and reproductive growth in young ponkan trees (Citr us reticulata Bianco cv. Yoshida) were evaluated. Paclobutrazol was ap plied when spring shoots were about 5 mm in length, either as an 840 p pm foliar spray or as a soil drench (0.5 g active ingredient per tree) . Soil water stress of up to pF 2.95 was gradually imposed on the tree s before re-watering. Water stress improved fruit set, whereas paclobu trazol reduced it. The proportion of abnormal flowers was increased by applying paclobutrazol without water stress. Abnormal flowers were re duced when paclobutrazol was applied in combination with water stress. Tree height increased about 14% for soil drench and 60% for foliar ap plied paclobutrazol based on initial heights at beginning of the exper iment. Water stress caused up to 20% reduction in leaf expansion, redu ced photosynthesis, and moderately reduced tree growth. Application of paclobutrazol ameliorated photosynthetic reductions associated with w ater stress, by maintaining higher photosynthetic and transpiration ra tes during water stress and ensuring quicker recovery after re-waterin g.