Grafted Watermelon Root Length Density and Distribution under Different Soil Moisture Treatments

Delineating the depth and extent of the watermelon [Citrullus lanatus (Thumb.) Matsum. & Nak.] root zone assists with proper irrigation management and minimizes nutrient leaching. The objective of this 3-year field study was to measure root distribution and root length density of watermelon (cv. Wrigley) grafted on two different rootstocks (Lagenaria siceraria cv. ‘FR Strong’ and Cucurbita moschata 3 Cucurbita maxima cv. Chilsung Shintoza) and grown under three soil moisture treatments. Irrigation treatments tested were: no irrigation (NI), briefly irrigated for fertigation and earlyseason plant establishment; minimally irrigated (MI), irrigated when soil moisture in top 0.30 m of soil fell below 50% available water capacity (AWC); well irrigated (WI), irrigated when soil moisture in top 0.30 m of soil fell below 15% (AWC). Root length density (RLD) was measured from 75-cm-deep soil cores at two locations three times per growing season and a third location at the end of the season. Cores 1 and 2 sample locations were 15 cm to the side of each plant: Core 1 on the same side as the drip tape and Core 2 on the opposite side. At the end of the season, Core 3 was taken 15 cm outside of the bed in bare ground. RLD was significantly greater in the 0to 30-cm soil depth and dropped dramatically below 30 cm; it was not significantly affected by irrigation treatment or rootstock. Core 1, next to the drip tape, had greater RLD than Core 2, 30 cm from drip tape, but only at the later sampling dates. Roots were found in Core 3 at all depths, but the RLD was significantly less than that measured in Cores 1 and 2. These findings suggest that the effective root zone depth for watermelon is 0 to 30 cm and that the particular scion/rootstock combinations tested in this study do not differ in root system size or location. Plant roots play a vital role in the acquisition of belowground resources, yet we have a limited understanding of how they function in natural soil environments (Smit et al., 2000). Both agricultural and non-agricultural water use is increasing globally as are concerns about groundwater contamination from irrigated field leachate (Clothier and Green, 1994). Appropriate irrigation application and the reduction of root zone drainage must be based on accurate delineation of the extent and depth of crop root zones. The effective root zone (ERZ) depth is the depth of soil used by the main body of the root system for water and nutrient uptake under proper irrigation (Ross and Hardy, 1997). Several studies have highlighted the dominant role of surface roots in extracting soil water. Gardner (1983) determined that soil water extraction dropped off markedly away from the soil surface in a variety of crops. Csizinsky (1979) stated that long drip irrigation durations do not necessarily provide improved lateral movement of water beyond 25 cm and can result in leaching of fertilizer below the ERZ. Research has shown that other vegetable crop root systems are relatively shallow when transplanted and grown with drip irrigation under polyethylene mulch. Research in tomato provides some of the most relevant examples. Oliveira and Calado (1996) found that 88% to 96% of total tomato root length was present in the top 40 cm and that RLD decreased rapidly with depth. Bar-Yosef (1977) found 87% of tomato roots were within a 25-cm lateral and downward distance from the drip emitters on sandy soils. Singh et al. (1989) reported that 74% of drip-irrigated tomato roots were confined to the top 15 cm of soil, whereas Machado and Oliveira (2005) found 63% to 78% of tomato root length in the top 10 cm. There is little published information on the root characteristics of watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] grown under drip irrigation with polyethylene film mulch. An early root study was conducted on the diploid cultivar Kleckly Sweet on bare ground with no irrigation (Weaver and Bruner, 1927). In this case watermelon roots extended 1.2 m deep, although little root development was found beneath the first foot of soil. More recent studies compared root characteristics of transplanted vs. direct-seeded watermelons (Elmstrom, 1973; NeSmith, 1999). In both cases, transplanted watermelons lacked a taproot and had significantly greater root length than direct-seeded melons, particularly at shallow soil depths. It is often stated that cucurbit rootstocks have larger root systems than scion varieties (Davis et al., 2008), but specific data quantifying the extent and depth of grafted watermelon root systems are not available. In contrast, many physiological differences between grafted and self-rooted plants have been demonstrated. Pulgar et al. (2000) found that watermelon scions grafted on pumpkin rootstocks showed a higher capacity for nitrogen (N) assimilation, resulting in greater growth in grafted plants. More efficient N assimilation in grafted Oriental melon and cucumber has been reported (Heo, 1991), and grafted cucumber appears to absorb other nutrients more efficiently than self-rooted plants (Ahn et al., 1999; Gomi and Masuda, 1981; Kim and Lee, 1989). The concentration of cytokinins, which are primarily synthesized in roots, is also higher in the xylem sap of some grafted eggplants (Kato and Lou, 1989). These research findings infer conceivable root system differences in self-rooted and grafted watermelon plants, which need to be delineated to allow for more appropriate irrigation management. The objectives of this research were (1) to estimate the ERZ for grafted and ungrafted triploid watermelons (Citrullus lanatus cv. Wrigley) grown under polyethylene mulch; and (2) to quantify root distribution and root HORTSCIENCE VOL. 48(8) AUGUST 2013 1021 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION