Gibberellin Relationships in the ‘Alaska’ Pea (Pisum Sativum)

'Alaska’ peas ( Pisum sativum L.) grown under a 16‐hr photoperiod at 20 ± 1 C and an 8‐hr dark period at 16 ± 1 C in their ontogeny exhibit two periods of sensitivity to applied gibberellin (GA), namely, prior to and subsequent to but not during the linear phase of stem elongation. This paper describes experiments conducted primarily with seedlings. Growth‐saturating doses of GA, applied to dry seeds before planting (10 −3 m) and to the shoot tips of 3‐day‐old seedlings (10 μg), evoked growth rates equal to the growth rate of etiolated seedlings. Sensitivity of seedlings to applied GA decreased with age through the first 2 to 3 weeks of development; by the time seedlings were about 14 days of age and had four elongating internodes they no longer responded to applied GA. As endogenous growth rate diminished late in ontogeny, the plants again became sensitive to applied GA. Growth response was used as a criterion for determining apparent translocation of applied GA. ‘Alaska’ pea seedlings appeared to transport GA, both acropetally from the cotyledons and basipetally from the shoot tip, to all internodes with remaining extension potential. Excision of both cotyledons at any time during the first 9 days of development caused a significant reduction of growth rate, and applied GA did not restore normal growth rate. No evidence was found that the cotyledons supply endogenous GA to the shoot axis in normal seedling development. It is suggested that the normal growth rate of light‐grown ‘Alaska’ peas is correlated with the rate of synthesis of GA and that GA is rate‐limiting for stem elongation during early seedling development and during the period of decreasing growth rate and onset of apex senescence.