Cell elongation and cell division in elongating lettuce hypocotyl sections.

The roles of cell division and cell elongation in the growth of sections excised from hypocotyls of lettuce (Lactuca sativa L. cv. Arctic) were investigated. Elongation of sections incubated in the light is inhibited compared to dark-grown sections and this inhibition is reversed by gibberellic acid (GA3). The elongation of both dark-grown and GA3-treated, light-grown sections can be enhanced by 10mM KCl. Under all conditions of incubation, elongation growth is greatest in the uppermost quarter of the hypocotyl section while the basal quarter does not elongate. In darkness the two apical segments of sections marked into four equal parts grow at the same rate, while in light, growth of the apical segment exceeds that of the second segment. Cell division in cortical or epidermal cells, as measured by mitotic index or cell number, is not affected by illumination conditions nor by GA3 or KCl treatments. Although γ-irradiation and FUDR pretreatment eliminate or cause a marked reduction in cell division in the excised hypocotyl, sections from seeds irradiated with γ-rays or incubated in 5-fluorodeoxyuridine elongate in response to GA3 and KCl treatment as do sections from non-pretreated controls. Therefore, since neither GA3 nor darkness affect celldivision activity and since treatments which eliminate or significantly reduce cell division do not affect growth, we conclude that the effect of GA3 and darkness in this material is to increase cell elongation.