Relative Effects of Growth Retardant (Paclobutrazol) and Water Stress on Tree Growth and Photosynthesis in Ponkan (Citrus reticulata Blanco).

Abstract

The effects of water stress and/or chemical growth retardant, paclobutrazol, applied either as foliar spray or soil drench, on photosynthesis, tree vegetative and reproductive growth in young ponkan trees (Citrus reticulata Blanco cv. Yoshida) were evaluated. Paclobutrazol was applied when spring shoots were about 5 mm in length, either as an 840 ppm 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 trees before re-watering. Water stress improved fruit set, whereas paclobutrazol reduced it. The proportion of abnormal flowers was increased by applying paclobutrazol without water stress. Abnormal flowers were reduced when paclobutrazol was applied in combination with water stress. Tree height increased about 14% for soil drench and 60% for foliar applied paclobutrazol based on initial heights at beginning of the experiment. Water stress caused up to 20% reduction in leaf expansion, reduced photosynthesis, and moderately reduced tree growth. Application of paclobutrazol ameliorated photosynthetic reductions associated with water stress, by maintaining higher photosynthetic and transpiration rates during water stress and ensuring quicker recovery after re-watering.