Evaluating the Effects of Controlled Release Fertilizer Application Rate and Watering Regime on Photosynthesis and Growth of Potted Grapefruit Foliage

Few previous studies have focused on the effects that horticultural practices have on the vegetative growth of young, non-bearing grapefruit trees. The results of this study describe the interacting effects of the amount of controlled release fertilizer applied and watering regime on the photosynthetic metabolism and growth of foliage in potted grapefruit (Citrus paradisi Macf. cv. Ray Ruby). Individual plants were assigned to one of three amounts of a dry, controlled release fertilizer [no fertilizer added (NF), low fertilizer volume (LF, 15 mL), or high fertilizer volume (HF, 30 mL)] and to one of two watering regimes [high water volume input (HW, 1,200 mL/week) or low water volume input (LW, 600 mL/week)]. There were no significant differences in mean, instantaneous leaf net photosynthetic rate among treatments. However, treatments LFLW and HFLW had significantly larger mean values of instantaneous, leaf water-use efficiency (WUE), relative to the NFHW treatment. All of the other treatments had intermediate mean values of WUE, compared to the LFLW and HFLW and NFHW treatments. For the LW watering regime, regardless of the amount of fertilizer applied, values of leaf WUE were related linearly to the total leaf dry weight, number of leaves produced, stem length, and stem diameter. For the HW watering regime, regardless of the amount of fertilizer applied, values of leaf WUE were related linearly only to total leaf dry weight and number of leaves produced. In conclusion, watering regime had a greater influence on leaf WUE than did the amount of fertilizer applied. Increasing values of leaf WUE were related to greater leaf dry weight, number of leaves produced, stem length, and stem diameter in potted grapefruit plants subjected to a restricted watering schedule.