Dual effect of light on the gibberellin- and nitrate-stimulated seed germination of Sisymbrium officinale and Arabidopsis thaliana.
In S. officinale, stimulation of germination by red light (R) depended on the simultaeous presence of nitrate (light-effect I). The effect of both factors was completley blocked by tetcyclacis, an inhibitor of gibberellin (GA)-biosynthesis. Addition of a mixture of gibberellins A4 and A7 (GA4+7) antagonized the inhibition. In the absence of nitrate, R shifted germination to lower GA-requirement (light-effect II). In A. thaliana a similar second light effect was seen on the GA-requirement of GA-deficient ga-1 mutant seeds. R stimulated germination of wild type seeds in water (light-effect I). For both species light-effect I showed a fluence threshold value of approx. 10-5 mol/m2, which was independent of the nitrate concn. Increasing nitrate concn narrowed the fluence-range required for max. germination whereby the product of nitrate concn and fluence value determined the germination level, indicating a multiplicative interaction between R and nitrate. Fluence-response curves for light-effect II were similar for both species. Germination occured in the range of 10-6 to 10-2 mole/m2 fluence. The max. level of germination was determined by the level of dark-germination and light-effect II. Increasing GA4+7 concn induced a shift to lower fluence values. It was shown that in the second effect the co-action of R and exogenous GA4+7 was clearly additive. It is concluded that light-effect I induces a chain of events leading to GA biosynthesis. Light-effect II seems to enhance the sensitivity of the seeds to GAs.