Grafting onto different rootstocks as a means to improve watermelon tolerance to low potassium stress

To test the possibility that using appropriate rootstock could improve the tolerance of watermelon to low potassium (K) stress, a greenhouse experiment was conducted to determine plant growth and some physiological parameters of watermelon plants [Citrullus lanatus (Thunb.) Matsum. and Nakai, cv. Zaochunhongyu], either ungrafted or grafted onto the rootstock 'Hongdun' (C lanatus sp.), Jingxinzhen No.4' (Cucurbita moschata Duch.) and 'Nabizhen' (Lagenaria siceraria Standl.). Plants were subjected to two levels of K (6.0 and 0.6 mM, supplied as K2SO4) for 20 days in hydroponic systems. Compared with plants treated with 6.0 mM K, those supplied with 0.6 mM K produced less biomass as indicated by the significantly decreased shoot and root dry weight in both ungrafted and grafted plants. However, a less decrease in shoot dry weight was observed in plants grafted onto 'Hongdun' (5%) and lingxinzhen No.4' (3%) than in the ungrafted plants (23%). The xylem sap volume, total amount of K in the xylem sap, shoot K accumulation, K uptake efficiency, and leaf transpiration rate were significantly higher in the plants grafted onto 'Hongdun' and lingxinzhen No.4' than in the ungrafted plants under 0.6 mM K conditions. However, the K utilization efficiency of shoot was quite similar among all the plants. In addition, the reduction in the shoot dry weight induced by low potassium stress (0.6 mM K) was similar in the plants grafted onto 'Nabizhen' (18%) and ungrafted plants (23%). Taken together, the above results supported the hypothesis that watermelon grafted onto adequate rootstocks ['Hongdun' (C lanatus sp.) and 'Jinxinzhen No.4' (C moschata Duch.)] can enhance plant tolerance to low potassium stress. The enhanced tolerance is mainly attributed to higher K uptake efficiency, rather than K utilization efficiency. (C) 2012 Elsevier B.V. All rights reserved.