Comparing Minirhizotron and Soil Core Methods for Measuring Root Growth of Melons under Deficit Irrigation

Water stress alters biomass allocation strategies of crop plants, resulting in changes in root growth patterns in time and space. Understanding these changes will assist in screening cultivars for drought tolerance traits in melons. This study was conducted to investigate the effect of deficit irrigation (50% vs. 100% crop evapotranspiration, ETc) on root growth of three melon (Cucumis melo L.) cultivars (Mission and Da Vinci, reticulatus type and Super Nectar, inodorus type). Root length intensity (RL; mm·cm^-2) was measured using the minirhizotron method six times at biweekly intervals during 2012 growing season, and at final harvest with the soil core method. Minirhizotron data showed that RL increased significantly up to 70 days after planting (DAP), with no significant differences among 70, 84, 98, and 112 DAP. Both minirhizotron and soil core methods showed similar root growth trends at final harvest, concluding that deficit irrigation significantly enhanced root growth in 'Mission', but had no effect in ‘Super Nectar’. However, the two methods differed in ‘Da Vinci’, which showed 17% higher root growth under deficit irrigation with minirhizotron; whereas, soil core method showed no significant effect of deficit irrigation on root growth. As in previous studies, the minirhizotron method underestimated root growth in the upper soil layer (10–30 cm) as compared to the soil core method. Conversely, the minirhizotron estimates were higher than the soil core method at deeper soil layers (50–70 cm). Further, spearman’s correlation coefficient, showed poor association (r = 0.1043) between minirhizotron and soil core root growth estimates. These results indicate disagreements between the two methods for root growth measurements. This disparity may be due to the poor soil-tube interface contact in the upper soil layers in the minirhizotron method. However, better estimates under deeper layers indicate that minirhizotron is a more suitable method for root growth measurement under deficit irrigation.