The effects of root intervention on stand establishment rate of grafted watermelon (Citrullus aedulis L) seedlings under saline conditions

A commercial watermelon (Citrullus aedulis L) variety was grafted onto a commercial variety rootstock (Cucurbita maxima x C. moschata). Two different grafting methods; splice grafting (SG) and root pruned splice grafting (RPSG) were simultaneously applied. End of nursery period, a sufficient number of 14 day old seedlings of each grafting method were transplanted in large pots filled with a mixture of peat compost and vermiculite (3:1). The plants were split in three equal groups and in the following 14 days each group was periodically irrigated with equal amounts of respectively tap water or saline water (50 mM NaCl and 100 mM NaCL). The relative growth rate (RGR) and its components; net assimilation rate (NAR) and leaf area ratio (LAR), root relative growth rate (RRGR), stem elongation rate (SER) and leaf expansion relative rate (LER) were computed for each experimental plot. Additionally to that, equal weights of fresh roots from seedlings of both grafting methods were washed out carefully with tap water and immersed into equal quantities of methanol. Several diluted solutions (10% vol/vol, 15% vol/vol and 20% vol/vol) were prepared from each root extracts, parallel with control solutions (0.01, 0.05 and 0.1 ppm) of artificial cytokinins, and used as rooting medium for 20 day old cuttings of tepary bean (Phaseoulus acutifolius). The average number of root nodes and lateral roots as well as the average length of lateral roots of tepary bean cuttings was used to indicate the difference between RPSG and SG seedlings regarding the nature and quantity of growth hormones extracted from the fresh rootstock roots. A significantly higher relative growth rate was found for root pruned splice grafted seedlings after transplanting, mostly due to higher net assimilation rate, as well as a higher root relative growth rate indicating a faster stand establishment rate for RPSG seedlings. Generally speaking, the relative growth rate of transplanted seedlings was drastically reduced due to the increase of nutrient solution salinity. However, significantly higher values were recorded for root pruned splice grafted seedlings compared to splice grafted ones. The same was true regarding root relative growth rate, stem elongation rate and leaf expansion rate. A higher concentration of cytokinins, was indicated by a higher rooting index of Phaseoulus acutifolius, from the extracts of RPSG seedlings and to that is attributed their significantly higher stand establishment rate.