Crisp et ai. ' suggested that the development of lettuce tipburn and other calcium-related physiological disorders may, in part, be due to supra-optimal levels of the auxin indole-3-acetic acid (IAA). These high levels may stimulate cell expansion to such an extent that calcium cannot be supplied fast enough to maintain the structure of the rapidly extending cell walls with the result that the cell collapses and ultimately the plant tissue becomes necrotic. Concentrations of auxin can become excessive because some polyphenols, particularly chlorogenic acid ( CGA) can occur in plants at concentrations sufficient to inhibit IAA oxidase.2
To find whether the large cultivar variation in the expression of tipburn in lettuce could be related to CGA concentrations in the tissues, experiments were carried out in which the CGA concentrations of seven cultivars (Cobham Green, Avoncrisp, Fairton, Portage, Salinas, Vanguard and Minetto) were measured at six stages of growth. At maturity the cultivars were assessed for tipburn incidence.
At all stages of growth there were significant differences between the CGA concentrations in the cultivars. CGA concentration decreased with increasing plant size and at all growth stages tested it was positively correlated (r=0.46-0.62, d.f. =26), with the incidence of tipburn at maturity. However, the cultivars Salinas and Vanguard suffered less tipburn (0 and 6.7 incidence, respectively) than expected from the concentrations of chlorogenic acid in them.
These results, therefore, lend some support for the view that the concentration of chlorogenic acid in young lettuce plants could be a useful indicator of the susceptibility of cultivars to tipburn at maturity.