Photoperiod-induced changes in gibberellin metabolism in relation to apical growth and senescence in genetic lines of peas (Pisum sativum L.)

In an early-flowering line of pea (G2) apical senescence occurs only in long days (LD), while growth in short days (SD) is indeterminate. In SD, G2 plants are known to produce a graft-transmissible substance which delays apical senescence in related lines that are photoperiod-insensitive with regard to apical senescence. Gibberellic acid (GA₃) applied to the apical bud of G2 plants in LD delayed apical senescence indefinitely, while N⁶-benzyladenine and -naphthaleneacetic acid were ineffective. Of the gibberellins native to pea, GA₉ had no effect whereas GA₂₀ had a moderate senescence-delaying effect. [³H]GA₉ metabolism in intact leaves of G2 plants was inhibited by LD and was restored by placing the plants back in SD. Leaves of photoperiod-insensitive lines (I-types) metabolized GA₉ readily regardless of photoperiod, but the metabolites differed qualitatively from those in G2 leaves. A polar GA₉ metabolite, GA_E, was found only in G2 plants in SD. The level of GA-like substances in methanol extracts from G2 plants dropped about 10-fold after the plants were moved from SD to LD; it was restored by transferring the plants back to SD. A polar zone of these GA-like materials co-chromatographed with GA_E. It is suggested that a polar gibberellin is synthesized by G2 plants in SD; this gibberellin promotes shoot growth and meristematic activity in the shoot apex, preventing senescence.Abbreviations GA gibberellin - GA₃ gibberellic acid - SD short days - LD long days