Effects of Concentration and Application Time of GA_3 and Urea on yield, Fruit Size Distribution and Crop Value of Clementine Mandarin in California

摘要

GA_3 is routinely used in the production of seedless mandarins to increase both fruit set and fruit size. GA_3 concentrations and application times effective for achieving these production goals in Spain, Morocco and South Africa have not proven reliable in California. The efficacy of eight combinations of GA_3 concentrations and application times and two concentrations of urea as a winter prebloom foliar application to maximize yield and yield of commercially valuable large size fruit of 'Nules' Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard in California's San Joaquin Valley. Foliar applications of 10 mg L~(-1) GA_3 at 60% full bloom, 75% petal fall and in early July or 25 mg L~(-1) at 60% and 90% full bloom,75% petal fall and 10 days after 75% petal fall reduced total yield (kg) relative to the untreated control and trees receiving 1% low-biuret urea prebloom (December) (P = 0.0849). The two GA_3 treatments significantly reduced the kilograms and number offruit per tree in the combined pool of large + jumbo + mammoth (57.16-76.20 mm transverse diameter) (P < 0.0001). Application of GA_3 (15 or 25 mg L~(-1)) at 60% and 90% full bloom, 75% petal fall and 10 days after 75% petal fall resulted in retention of significantly more fruit (kilograms and number per tree) of packing carton sizes tiny and small (fruit < 44.45 and 44.45-50.80 mm transverse diameter, respectively) compared to untreated control trees (P = 0.0001 and 0.0238, respectively). Trees treated with GA_3 at 10 mg L~(-1) at these same times also produced significantly more fruit of packing carton size small (kilograms and number per tree) (P = 0.0238), but not tiny, compared to the control. GA_3 strategies that increased retention of packing carton size small resulted in significantly lower yields (kilograms and number) of commercially valuable fruit of packing carton sizes large, jumbo and mammoth (57.16-63.50, 63.51-69.85, and 69.86-76.20 mm transverse diameter, respectively) per tree compared to control trees and trees treated with 1% low-biuret urea prebloom. The greater yields (kilograms and number per tree) of commercially valuable fruit in the combined pool of large + jumbo + mammoth were achieved by delaying the first application ofGA_3 (10 mg L~(-1)) to 90% full bloom with only two additional applications at 10 days after 75% petal fall and in early July, by application of low-biuret urea (1% or 3%) prebloom, and by the untreated control. In California, the interaction between GA_3 concentration and application time has a critical effect on the value of the Clementine mandarin crop.