Optimum plot size and number of replications for determining watermelon yield, fruit size, fruit firmness, and soluble solids

Abstract

Plot size and number of replications were evaluated for watermelon [Citrullus lanatus (Thunb.)] over a 3-year period. Four different methods were used, including plotting basic unit plots against changes in CVs, Hatheway's method for detecting a true difference as a percent of the mean with a 20% threshold, Bartlett's homogeneity of variance test, and computed least significant differences (LSDs). An initial evaluation was done in a trial with several varieties. Plotting CV against number of basic units using plots with different watermelon varieties suggests a plot size of ≈7 basic units (1 basic unit = 3.34 m2). With a single variety and basic units of 6.69 m2 for inside rows, the plot size was ≈4 basic units and for the outside row 5 basic units. With plot sizes of 2.23 m2, the number of basic units per plot was estimated at ≈5 basic units. Bartlett's test suggests larger basic unit plot sizes of 14 to 20 with a 3.34-m2 basic unit size with multiple varieties. With a single variety, 4 to 8 basic unit plot sizes are required with 6.69-m2 basic unit size. Results were unreliable with 2.23-m2 plot sizes using Bartlett's test. Computed LSDs, which were 5% of the mean or less, could be achieved with plots sizes of 10 basic units and five replications with 3.34-m2 basic unit plots and using multiple varieties. Other combinations meeting these criteria included 14 basic unit plots and two or three replications. With a 6.69-m2 basic unit size, plots of 6 basic units and four replications would meet the 5% criteria. Finally, with the 2.23-m2 basic unit size, a plot size of 8 basic units and three replications would result in an LSD of 5% of the mean. Results with Hatheway's method were similar to plots of basic units against CV. Hatheway's method also has an estimate of number of replications and with 3.34-m2 basic unit, the 20% threshold of detecting true differences occurred with 10 to 14 basic units and four replications. For fruit size, firmness, and soluble solids, the basic unit plot sizes ranged from 5 to 7. Plot size estimates were larger with 6.69 m2 compared with 2.23 m2 for fruit characteristics.