Abstract

Hypocotyl elongation in germinating lettuce (Lactuca sativa L., cv. Diana) M2 seedlings from an ethyl methanesulfonate treated population was used to select lines that were less sensitive to ethylene (C2H4) than the progenitor line. Individuals were identified that had hypocotyls at least twice as long in the presence of hormonal C2H4 levels (> 1 ?l l-1) as wild-type plants. This phenotype was confirmed in M3 seedlings for three independent mutants. The phenotype was normal in terms of growth for the mutant lines, so there were no obvious confounding growth defects or effect of other induced mutations. Changes in response to C2H4 and wounding (chlorophyll loss, induced respiration, induced phenylalanine ammonia-lyase (PAL) activity) in mature heads of lettuce were variable among the three mutant lines. Selection for C2H4 insensitivity at the seedling stage did not produce mature plants that were consistently less sensitive to C2H4-induced increases in PAL activity or CO2 production. However, all the selected mutated plants did have lower rates of C2H4-induced chlorophyll loss. All of the mutants differed from the wild-type in at least 3 of the 7 phenotypes measured, while one line differed in all the measured phenotypes. The apparent random distribution of induced changes observed in the mutant lines suggests that screening additional mutants may identify lines that combine a number of useful horticultural traits. © 2002 Elsevier Science B.V. All rights reserved.