Effects of Abscisic acid on the growth pattern of the shoot apical meristem and on flowering in Chenopodium rubrum L.

Abscisic acid (ABA) at 1 × 10-4 M or 3 × 10-4M was applied to the apical buds of Chenopodium rubrum plants exposed to different photoperiodic treatments and showing different patterns of floral differentiation. Stimulation of growth in width of the apical meristem of the shoot and/or inhibition of growth in length was obtained under all photoperiodic treatments. This change of growth pattern was followed by different effects on flowering. In non-induced plants grown under continuous light ABA stimulated periclinal divisions in the peripheral zone and the initiation of leaves as well as the growth in width of bud primordia. In plants induced by two short days reduced growth of the meristem coincided with ABA application. Longitudinal growth of the meristem was inhibited in this case and only a temporary stimulation of inflorescence formation took place. In plants induced at a very early stage, ABA exerted a strong inhibitory effect on flowering. A permanent and reproducible stimulatory effect on flowering was obtained in plants induced by three sub-critical photoperiodic cycles if ABA was applied to apices released from apical dominance. In this case formation of lateral organs and internodes was promoted by ABA and was followed by stimulated inflorescence formation. Gibberellic acid (GA2) at 1× 10-4 M or 3 × 10-4 M brought about a similar effect on flowering as ABA, although the primary growth effect was different, i.e. GA2 stimulated longitudinal growth. The effects of ABA and GA2 on floral differentiation have been compared with earlier results obtained from auxin and kinetin applications. These growth hormones are believed to regulate flowering by changing cellular growth within the shoot apex. Depending on the actual state of the meristem identical growth responses may result in different patterns of organogenesis and even in opposite effects on flowering