ENVIRONMENTAL AND HORMONAL-REGULATION OF SEED DORMANCY AND GERMINATION IN CAPE FYNBOS LEUCOSPERMUM RBR (PROTEACEAE) SPECIES

The endogenous levels of abscisic acid (ABA), cytokinins (CKs) and gib berellins (GA(1)/GA(3) combined) in Leucospermum glabrum embryos were monitored in axes and cotyledons separately during normal germination. Plant growth substance changes were correlated with known morphologic al, structural and ultrastructural events in the embryo of Proteaceae. The effect of exogenous application of 6-benzyladenine (BA) and GA(47) under three known dormancy-enforcing environmental conditions were studied in L. glabrum and L. cordifolium. The endogenous levels of the hormone classes GAs and CKs changed phasically during normal germinat ion under a single alternating temperature regime. GA(1)/GA(3) levels increased in cotyledons within 3 d of hydration while at the same time initial CK levels decreased. Following this transient peak GAs fell t o a low level throughout the germinative period. Subsequently the CKs, Z and ZR, and to a lesser extent their dihydro-derivatives, appeared in both the axes and the cotyledons as fluctuating, transient peaks. E arly increases in GAs are thought to control the induction of the germ ination process. The CK pattern suggests that CKs control at least thr ee major processes of germination sensu stricto following induction: 1 ) early mobilization of protein and lipid reserves in the axis and lat er in cotyledons, 2) cotyledon expansion which causes the endotesta to split permitting radicle protrusion and 3) later, radicle growth. Our results indicate that dormancy in intact Leucospermum seeds is enforc ed by embryo anoxia, regulated by the impermeable exotesta. In additio n synthesis of or tissue sensitizing to both hormone classes GAs and C Ks depends on moderately low temperature as the primary environmental requirement. For GA synthesis a secondary, daily pulse of high tempera ture is required. Inhibitory hormones, specifically ABA, appear not to play a role.