Shoot Apex Sugars in Relation to Long-Day Induction of Flowering in Lolium temulentum L

Abstract

Inflorescence initiation in Lolium temulentum is induced by a single long day with a photoperiod extension of 16 h under low photon flux density (12 μmol PAR m^-2 s^-1) from incandescent lamps. Under these conditions the content of sucrose, the predominant free sugar in the shoot apex, fluctuates diurnally in the same way as in short day apices. There was no evidence of a greater apical sucrose content at any time during the long day or in the following period of high irradiance when floral evocation occurs. Thereafter, however, the diurnal fluctuation in apical sucrose content became more pronounced. Increasing the sugar supply to the apex by raising the photon flux density during the daily light period did not lead to flowering of non-induced plants; nor did the high contents of apical sugars reached in apices cultured in vitro on 5% sucrose medium. By contrast, when apices were excised after receipt of the floral stimulus from long day leaves, increase in the sugar content enhanced inflorescence development in vitro, this response being most pronounced after the inflorescences were initiated. Thus, floral evocation in L. temulentum does not require an increase in the content of sucrose at the apex although inflorescence development is highly responsive to it.

When photoperiodic extensions with incandescent or fluorescent lamps were compared for their effects on apical sugars and flowering response, there was no interaction between light quality and photon flux density. Thus the shoot apex response to the low irradiance, photoperiodic time-measurement processes of leaves is distinct from the apical response to sugar supply. In Lolium temulentum floral evocation is controlled by the photoperiodic processes, the response to which is amplified by high sugar supplies but not replaced as it is in Sinapis alba.