Allelopathic activities of three carotenoids, neoxanthin, crocin and β-carotene, assayed using protoplast co-culture method with digital image analysis

Allelopathic activities of three carotenoids of a natural pigment group, neoxanthin, crocin and β-carotene, were assayed by the protoplast co-culture method with digital image analysis (DIA-PP method). Effects on three different growth stages of lettuce protoplasts, i.e., cell wall formation, cell division, and yellow pigment accumulation, were investigated using 96-well culture plates. Cell division was inhibited 65-95% by all three carotenoids at 33-100 µM. Inhibition of cell division stage was stronger than at the cell wall formation stage in neoxanthin, and the water-soluble carotenoid, crocin, whose yellow pigment was incorporated into the vacuole of lettuce protoplasts. Neoxanthin at 33 µM and crocin at higher than 100 µM inhibited more than 100% of the yellow pigment accumulation. By contrast, at low concentrations (0.01-1 µM) β-carotene stimulated growth at the cell division stage. At high concentrations of β-carotene (100-500 µM), inhibition was prominent at all three stages, and also in neighboring wells of zero control, which suggested emission of a volatile compound by β-carotene. They were compared with the report of the volatile compound, tulipalin A. Differences in patterns of inhibition of carotenoids on lettuce protoplast growth were compared with reports of another natural pigment, anthocyanin, and anthocyanin-containing red callus cultured in the light, and with that of neoxanthin-containing yellow callus cultured in the dark.