Stem-swelling and photosynthate partitioning in stem mustard are regulated by photoperiod and plant hormones

The experiment was conducted to study the relationship between stem-swelling and photoperiod and growth hormones by comparing stem swelling with non-stem-swelling stem mustard (Brassica juncea var. tsatsai) plants about their growth characteristics and levels of endogenous gibberellin and cytokinin under different photoperiods. The results here showed that plant biomass was higher in 12-h photoperiod compared to that in long day (LD) and short day (SD), whereas stem growth was much stronger in LD compared to 12-h photoperiod and SD. Exogenous application of 1.0 mM gibberellic acid (GA₃) accelerated stem elongation in SD, but 8.9 μM benzyladenine (BA) failed. The shape of the swollen stem was also found to be associated with day length: a LD promoted stem elongation, while a 12-h photoperiod made the stem oval swollen. Also, stem was shown to have no sign of swelling in plants in SD with a relatively poor growth. The further studies showed that the largest proportion of ¹⁴C photosynthate was allocated to the swelling stems in stem-swelling plants, but to expanded leaves in non-stem-swelling plants, and endogenous gibberellin A₁ (GA₁) and zeatin + zeatin riboside (ZRs) were higher in LD compared to 12-h photoperiod and SD. These results from this experiment indicate that stem growth and swelling is a physiological process of hormonal control, and the photoperiod possibly exerts its influence by altering the balance between the levels of endogenous gibberellins and cytokinins.