INHIBITORY AND PROMOTIVE EFFECTS OF GIBBERELLIC-ACID ON FLORAL INITIATION AND DEVELOPMENT IN POA-PRATENSIS AND BROMUS-INERMIS

Flowering in Poa pratensis L. cv. Holt and Bromus inermis Leyss. cv. Manchar requires exposure to short days (SD) for primary induction to occur, followed by long days (LD) to allow the inflorescence to develop. Weekly sprays with gibberellic acid (GA3) during primary induction inhibited flower initiation in both P. pratensis and B. inermis. With 10-4 M GA3 flowering of P. pratensis was suppressed even after an induction period of 10 weeks. Since both GA3 and non-inductive LD conditions greatly stimulate leaf elongation, the degree of primary induction was closely negatively correlated with plant height (leaf sheath and blade length) at the end of the induction period. GA3 application or the interpolation of LD during SD induction were most inhibitory during the later middle part of the SD period, whereas they were stimulatory near the beginning or immediately before the SD period. We suggest that changes in the portfolio or levels of endogenous gibberellins mediate photoperiodic control of growth and floral initiation in these plants. However, GA3 sprays could not substitute for LD in causing heading and culm elongation in SD induced plants of the two species. The results are discussed in the light of results with other plants with dual floral induction requirements.