Mechanism responsible for restricted boron concentration in plant shoots grafted on pumpkin rootstocks

The use of wastewater as an alternative water source for irrigation needs specific studies since this water contains organic matter, salt, heavy metals and high boron concentrations, which may be toxic to many crops. Grafting of vegetable plants became a common practice with the main goals to control soil-borne diseases and nematodes and improve tolerance to environmental stresses such as flooding, salinity and high boron concentrations. The aim of the present study was to explore how boron damage to melon plants under wastewater irrigation is reduced by grafting onto pumpkin rootstocks. Six melon/pumpkin combinations were examined. Boron decreased shoot and root dry weights of all plant types, but by much less in those with pumpkin root systems than in those on melon roots, indicating that pumpkins are more boron-tolerant than melons and can impart this tolerance onto plants grafted on them. Boron concentrations in roots were much lower than in shoots, and similar in all plant types, implying that boron was not retained within the pumpkin root system. Shoot dry weights were much higher in plants grafted on pumpkin than in those grafted on melon, therefore transpiration rates and consequently boron transport to the shoots were expected to be higher in the former; in fact, they were significantly lower, implying that boron exclusion by the pumpkin root system at least partly accounted for the lower boron damage to plants grafted onto pumpkin. The concentrations of boron in xylem sap exudates were significantly lower for plants with pumpkin root systems than for those on melon roots, and exudation rates were markedly higher for the former, regardless of boron concentrations. However, the total amounts of boron exuded from plants grafted on pumpkin were still lower than those from plants grafted on melon, indicating that pumpkin roots blocked boron uptake from the growth media, to some extent. The study demonstrates that pumpkin root systems are capable of partially excluding boron when its concentrations in the growing media is high, thus avoiding damage to boron-sensitive plants, such as melons grafted onto pumpkins. The grafting technique can thus be adopted for agricultural setups where effluents with high boron concentration are used for irrigation.