Developmental changes in the gibberellin-induced growth response in stem segments of light-grown pea genotypes

The effects of GA on stem elongation were studied using segments from one tall and three dwarf light-grown pea genotypes varying in endogenous hormone content. Stem segments were cut at two distinct ages: when the fourth internode was at about 6-13 % of full expansion (early-expansion) or at 18-25 % of full expansion (mid-expansion). Light microscopy and how cytometry were used to demonstrate that GA does not induce cell division in excised pea stem segments. The growth studied here was strictly elongation. Measurement of final segment length after 48 hours and high resolution measurement of growth kinetics over 20 hours using an angular position transducer were done on segments treated with hormone solutions. Our data indicate that the action of GA on stem elongation can be classified into two distinct modes. The first, apparent in early-expansion stem segments, shows distinct growth kinetics and is independent of the endogenous IAA concentration of the segments. Quantitation of IAA by GC/MS in early-expansion segments of wild type pea incubated with gibberellin shows that an increase in IAA concentration is part of the GA response in such segments. The second mode of GA action is evinced in mid-expansion segments. Whereas there is no short term (< 20 h) response to GA alone (as determined by growth kinetics), there is a long term (48 h) response whose magnitude decreases across the genotypes with decreasing endogenous hormone content. Growth responses indicate that in mid-expansion segments exogenous GA acts by enhancing IAA action but appears to be unable to augment endogenous IAA content. Contradictory reports of the response of excised stem segments to GA can be reconciled when tissue genotype and developmental stage are considered.