CHANGES IN THE ACTIVITY OF AN ENZYMATIC MARKER BOUND TO PLASMALEMMA DURING THE PHOTOPERIODIC FLOWERING INDUCTION IN SOYBEAN

Soybean plants (Glycine max (L.) MERR., CV. Williams), were grown unde r either non-inductive long day (LD = SD + night break) or inductive s hort day (SD = 9 h light + 15 h darkness) conditions to study the poss ible involvement of plasmalemma in the photoperiodic induction of flow ering. Pelletable peroxidase activity from three subcellular fractions isolated from both leaves and stem terminal buds was tested for its u sefulness as a marker for flowering. Significant differences in values for peroxidase activity (PA) were found among the fractions isolated from leaves. The mitochondria-enriched fraction (MEF) had a rhythmic b ehaviour of PA with maximal values during light and minimal values dur ing dark periods. In contrast, PA from the plasmalemma-enriched fracti on (PEF) continuously increased and that from the microsomal + soluble fraction (MSF) continuously decreased. The inductive SD treatment, wh ich had a clear effect only on PA from PEF, impeded the increase in ac tivity during the first 3 days of SD floral induction. Thereafter, the activity increased to values similar to those for leaves under LD con ditions. Electrophoretic analysis of the three subcellular fractions d id not reveal an effect of the SD treatment either on the pattern of p eroxidase isozymes from MEF, PEF or MSF. For terminal buds neither the time-courses of PA found in the subcellular fractions isolated from l eaves nor the SD effect on PA from PEF were observed, which indicates that the photoperiodic effect on plasmalemma status is restricted to t he leaves. In vivo application of Ca2+ inhibited PEF-PA. of LD-treated leaves to mimick the SD effect. A23187, a Ca2+ ionophore, also inhibi ted PEF-PA while Verapamil, a Ca2+-channel blocker, partially reverted the ionophore effect after one SD exposure. In vitro application of C a2+, as well as Ca2+-chelators (EDTA and EGTA), inhibited both PEF and MSF-PA. These results suggest that a Ca2+ uptake into the cell may be triggered as a response to SD application and its possible correlatio n with perception of the photoperiodic stimulus and the synthesis of t he flowering signal by the leaves is discussed.