Gibberellins and the photoperiodic control of leaf growth in Poa pratensis

The role of gibberellins (GAs) in photoperiodic control of leaf elongation in Poa pratensis was studied by both application of exogenous GAs and analysis of endogenous GAs. Leaf elongation was strongly increased under long day (LD, 24 h) conditions at both 9 and 21°C, leaf length at 9°C LD being similar to that in plants grown in short days (SD, 8 h) at 21°C. However, even at 21°C leaf elongation was enhanced by LD. Exogenous GA₁ could completely compensate for LD at both 9 and 21°C. Gibberellins A₂₀, A₁₉ and A₄₄ could also partly replace LD, but they were significantly less active than GA₁, GA₅₃ was inactive when applied to plants grown at 9°C in SD and exhibited only marginal activity at 9°C LD and 21°C SD. The total level of GAs of the early 13-hydroxylation pathway (A₅₃, A₄₄, A₁₉, A₂₀ and A₁) increased rapidly when plants were transferred from SD to LD at 9°C. After transfer from 9 to 21°C, there was an increase in GA levels at both LD and SD, followed by a decrease under LD conditions. In all cases, GA₁₉ was the predominant GA, accounting for 60 to 80% of the analysed GAs. Levels of the bioactive GA₁ were low and increased transiently by LD four days after transfer from SD to LD. At both temperatures, the ratio GA₁₉ to GA₂₀ and GA₂₀ to GA₁ at 9°C was enhanced by LD compared with SD. Taken together, these results support the hypothesis that photoperiodic regulation of leaf elongation in Poa pratensis is GA-mediated, and they indicate a photoperiodic control of oxidation of GA₅₃ to GA₄₄ and GA₁₉ to GA₂₀, and perhaps also of 3β-hydroxylation of GA₂₀ to GA₁.