Root Pruning Effects on Seedlings' Growth and Plant Stand Establishment Rate of Watermelon Grafted Seedlings

Two different commercial rootstocks were grafted with each of two commercial watermelon (Citrullus aedulis L.) cultivars. For each combination, splice grafting (SG) and root pruning splice grafting (RPSG) were simultaneously applied. At the end of the nursery period, a sufficient number of 14-day-old seedlings of each grafting combination was transplanted in larger pots filled with vermiculite, divided in three equal groups. In the following 10 days all plants were periodically irrigated with equal amounts of the same nutrient solution, but differing from each other in the quantity of NaCl added to the nutrient solution. On the first and second week after grafting, and 10 days after transplanting, 10 plants of each experimental plot were randomly selected. Root, stem and leaf dry matter was weighted, and leaf area of plants was measured successively for each harvested plant. The relative growth rate (RGR) - and its components, net assimilation rate (NAR) and leaf area ratio (LAR) were computed for each experimental plot. Compared to root pruned splice grafted seedlings, a higher plant dry matter and larger leaf area per plant were recorded for splice grafted seedlings at the end of nursery stage. Despite that, a significantly higher relative growth rate was found for root pruned splice grafted seedlings, mostly due to higher net assimilation rate. Most plant assimilates were allocated to the growth of new root system. The same advantage, in terms of relative growth rate and assimilate allocation, was maintained by RPSG seedlings during the stand establishment period. As a result, a faster establishment rate was assumed. The stand establishment rate of transplanted seedlings was significantly reduced due to the increase of nutrient solution salinity, but again, significantly higher values were recorded in case of root pruned splice grafted seedlings.