The effect of mechanically induced stress on the growth of cauliflower, lettuce and celery seedlings

Brushing cauliflower, lettuce and celery seedlings with paper for 1.5 min each day for 11–13 d, 10–12 d or 21–28 d, respectively resulted in smaller, more compact, plants than the unbrushed controls. In all three species shoot fresh and dry weights and leaf area were reduced following brushing. In cauliflower and celery the largest growth reduction was in petiole length. In lettuce, which has no discernible petioles, the reduction in leaf length caused by brushing was proportionally greater than the reduction in leaf width. Brushing reduced hypocotyl length in cauliflowers and to a lesser extent in lettuce. Petiole and hypocotyl thickness was reduced in cauliflower, whereas hypocotyl thickness was increased in lettuce following brushing. Brushing increased leaf thickness in cauliflower, celery and to a lesser extent in lettuce and increased the percentage dry matter content of lettuce shoots. The weight of chlorophyll per fresh weight of leaf tissue increased following brushing in celery and lettuce and declined in cauliflowers. Root length and the number of branches per root system were reduced in all three species following brushing. Root dry weight was reduced and the root:shoot dry weight ratio was increased in lettuce, reduced in celery and unaffected in cauliflowers. There were different patterns of response to brushing, the reduction in leaf weight being greatest in the youngest leaf of cauliflower and least in the youngest leaf of lettuce and celery. Growth responses to brushing were seen several days after brushing had ceased, noticeably in leaves which were barely visible at the time of brushing. It is suggested that growth retardation of cauliflowers, lettuce and celery, induced by mechanical stress such as brushing may prove valuable as a means of ‘conditioning’ the seedlings to withstand both the physical and physiological stresses which occur at and during transplanting.