The effects of mechanically-induced stress and water stress on freezing resistance in lettuce and cauliflower seedlings

SummaryMechanically-induced stress, applied by brushing young lettuce and cauliflower plants for 90 s each day, reduced the freezing resistance of cauliflower but had no effect on that of lettuce. Brushing reduced fresh weight in both species and smaller plants were less freezing-resistant than larger ones. The levels of abscisic acid (ABA) in the shoots of brushed cauliflowers were slightly less than in unbrushed plants, whereas in lettuce the levels were similar in the two treatments. ABA sprayed onto lettuce plants had no effect on freezing resistance. With both lettuce and cauliflower, freezing resistance, and the osmolarity of sap extracted from the shoots, increased following water-stress and declined progressively following rehydration.