Interspecific rootstocks improve the low-temperature resistance of bitter gourd through sucrose and nitrogen metabolism regulation

Low temperature (LT) disrupted the osmotic regulation system and sucrose and nitrogen metabolism of Bitter gourd (Momordica charantia L.). The osmotic regulation system and the sucrose and nitrogen metabolism in bitter gourd were investigated in ungrafted (M), self-grafted (MS), and grafted onto rootstocks of pumpkin (Cucurbita moschata Duch., MC) and loofah (Luffa cylindrica Roem., ML) exposed to LT (8 degrees C) for 24 h and then allowed to recover growth (RG; 24/18 degrees C) for 24 h. MC and ML maintained better osmotic adjustment ability. Gas exchange parameters of MC and ML are higher than M and MS under both LT and RG. However, chloroplast fluorescence parameters varied considerably for the different groups under LT and RG. The soluble sugar and sucrose contents and sucrose phosphatase and sucrose synthase synthesis activities of MC leaves under LT and RG were higher than those of M and MS, however, the changes of these indexes in ML are not as obvious as that of MC. The activities of the sucrose-decomposing enzymes neutral invertase and acid invertase in ML and MC were lower than those of M and MS. After the induction of LT and RG, the nitrogen metabolites and enzyme activities of M and MS leaves were significantly reduced, while the MC and ML seedlings were on the contrary and the ammonium content in the leaves was significantly reduced. Pumpkin as rootstock can improve the LT tolerance of bitter gourd and using this method may help expand the planting area and season of bitter gourd.