Comparative study on carbon-nitrogen metabolism and endogenous hormone contents in normal and overgrown sweetpotato

Sweetpotato is easily overgrown because of improved production conditions and soil fertility. In this study, normal and overgrown sweetpotato plants were obtained from the long vine cultivar J18 (J18) and the short vine cultivar X32 (X32) in a field experiment with different nitrogen (N) treatments (0, 120, and 240 kg N ha− 1). The carbon (C)-N metabolism and endogenous hormone contents were compared between normal and overgrown sweetpotato plants. Results revealed that the total C content and C/N ratios in leaves of the overgrown plants were significantly lower than those of the normal plants at 75 days after planting (DAP) and 105 DAP. Also, the sucrose phosphate synthase (SPS) activities significantly decreased whereas the nitrate reductase (NR) and glutamine synthetase (GS) activities significantly increased. In the overgrown plants, the sucrose content in leaves significantly decreased at 75 DAP and 105 DAP, whereas the free amino acid and soluble protein contents significantly increased at 50 DAP, 75 DAP and 105 DAP. The indole-3-acetic acid (IAA) and zeatin riboside (ZR) contents in leaves of the overgrown plants were significantly higher than those of the normal plants at 50 DAP, 75 DAP and 105 DAP. The changing of IAA and ZR contents in the stem tips of different growth types showed similar treads to those in leaves. The abscisic acid (ABA) content in the stem tips of the overgrown plants at 75 DAP and 105 DAP was significantly lower than that of the normal plants. The ZR and ABA contents in the storage roots of the overgrown plants at 50 DAP, 75 DAP and 105 DAP were significantly lower than those of the normal plants. Compared to the normal plants, the overgrown plants exhibited more vigorous N metabolism in leaves. Meanwhile, the levels of endogenous hormones that promote cell elongation and division in the leaves and stem tips increased whereas those restraining stem tip elongation and promoting storage root enlargement decreased, which could promote leaf hypertrophy and stem tip extension at above-ground parts and restrain storage root enlargement at under-ground parts. Thus, source-sink growth became incoordinate and sweetpotato underwent overgrowth.