Stem elongation and gibberellins in alpine and prairie ecotypes of Stellaria longipes

The potential for gibberellins (GAs) to control stem elongation and its plasticity (range of phenotypic expression) was investigated in Stellaria longipes grown in long warm days. Gibberellin metabolism and sensitivity was compared between a slow-growing alpine dwarf with low stem elongation plasticity and a rapidly elongating, highly plastic prairie ecotype. Both ecotypes elongated in response to exogenous GA1, GA4 or GA9, but surprisingly, the alpine dwarf was relatively unresponsive to GA3. Endogenous GA1, GA3, GA4, GA5, GA8, GA9 and GA20 were identified and quantified in stem tissue harvested at commencement, middle and end of the period of most rapid elongation. The concentration of GAs which might be expected to promote shoot elongation was higher during rapid elongation than toward its end for both ecotypes. While there was a trend for certain GAs (GA3, GA4, GA9, GA20) to be higher in stems of the alpine ecotype during rapid elongation, that result does not explain the slower growth of the alpine ecotype and the faster growth of the prairie ecotype under a range of conditions. To determine if the two ecotypes metabolized GA20 differently, plants were fed [2H]- or [3H]-GA20. The metabolic products identified included [2H2]-GA1, -GA8, -GA29, -GA60, -3-epi-GA1, GA118 (-1-epi-GA60) and -GA77. The concentration of [2H2]-GA1 also did not differ between the two ecotypes and metabolism of [2H2]- or [3H]-GA20 was also similar. In the same experiments the presence of epi-GA1, GA29, GA60, GA118 and GA77 was indicated, suggesting that these GAs may also occur naturally in S. longipes, in addition to those described above. Collectively, these results suggest that while stem elongation within ecotypes is likely regulated by GAs, differences in GA content, sensitivity to GAs (GA3 excepted), or GA metabolism are unlikely to be the controlling factor in determining the differences seen in growth rate between the two ecotypes under the controlled environment conditions of this study. Nevertheless, further study is warranted especially under conditions where environmental factors may favour a GA:ethylene interaction.