Stand Establishment and Water Productivity of Grafted Watermelons: Effects Planting Density and Deficit Irrigation

The effects of deficit irrigation and plant population density on stand establishment and water productivity of grafted watermelons (Citrullus lanatus; cv Tri-X 313) were investigated. Replicated plots of grafted and non-grafted watermelons were established on a sandy soil with a known history of soil-borne diseases. Seedlings were planted at four densities by varying in-row spacing (0.46, 0.91, 1.4, and 1.8m), and were subjected to two irrigation regimes (100% and 75% replacement of crop evapotranspiration, ETc). Stand establishment, estimated as percent survival and vine elongation rates was significantly higher in grafted plants compared to non-grafted plants. Leaf physiological parameters including leaf water potential (ψ_l), stomatal conductance, transpiration, and photosynthesis were generally high in grafted than in non-grafted plants, and declined with deficit irrigation, but generally increased with plant population density. Fruit yields were also significantly higher in grafted plants subjected to 100 ETc than those subjected to 75% ETc.  Average marketable fruit yields increased with plant density at ~3.5‰ plant density.  Average number of fruits per plant declined with increasing plant density; however, average fruit size did not differ significantly among treatments.  Water productivity (tons/acre-inch) of grafted plants was ~23% higher than that of non-grafted plants and increased significantly with plant density at about ~0.43‰ plant density, but did not differ between irrigation regimes. The data suggest that increased planting density can increase net returns with the same amount of irrigation input.  Alternatively, the planting density of grafted plants can be decreased without significantly affecting overall productivity.