Research PaperA shoot based Na+ tolerance mechanism observed in pumpkin-An important consideration for screening salt tolerant rootstocks

- There is an absence of inverse relationship between shoot Na+ accumulation and salinity tolerance in pumpkin germplasms from various genetic backgrounds.
- A novel salt tolerant mechanism has been found in pumpkins which accumulated Na+ in leaf veins rather than restricted in roots.
- The salt tolerance of rootstock will be lost when salt sensitive cucumbers are grafted on them if the rootstocks adopted a “shoot-based” tolerance mechanism.

Ion accumulation and growth of 28 commercial pumpkin rootstocks and 16 inbred lines were investigated under saline conditions. Significant variation and correlations were observed in seven morphological traits and three ionic traits. 44 genotypes were clustered into three groups, namely, tolerant (low Na+ concentration in the shoot, low salt injury index, and high relative shoot dry weight), sensitive (high Na+ concentration in the shoot, high salt injury index, and low relative shoot dry weight) and intermediate (high Na+ concentration in shoot, low salt injury index, and high relative shoot dry weight) by three key traits. Na+ concentration in four parts of seedlings (root, stem, leaf veins, and mesophyll) were determined. Results indicated that different pumpkins groups show various Na+ distribution patterns (tolerant group restricted Na+ in roots, intermediate group restricts Na+ in their stems and leaf veins and sensitive group transported more Na+ to mesophyll tissues). Six genotypes were selected from three groups as rootstocks and grafted with a salt sensitive cucumber scion for evaluation the salt tolerance in grafting combinations. Results showed that Na+ concentration in the shoot of cucumbers grafted onto tolerant pumpkins was significantly lower than onto the sensitive and intermediate pumpkins. However, no significant differences were observed between the cucumbers grafted onto sensitive and intermediate group of pumpkins. These findings indicate that beside Na+ restriction in root, an alternative mechanism in shoot may be adopted by some pumpkin genotypes; if those genotypes are used as rootstocks they cannot perform as well as in self-root condition for their shoots have be replaced with cucumber scions. Our finding provides an insight into the screening of rootstocks tolerant to salinity.