Abstract
Red light (R) has a dual effect on the seed germination of the two related species Arabidopsis thaliana and Sisymbrium officinale. The two species provide different means to separate the light-effects. In S. officinale, stimulation of germination by R depends on the stimultaneous presence of nitrate (light-effect I). The effect of both factors is completely blocked by tetcyclacis, an inhibitor of gibberellin (GA)-biosynthesis. Addition of a mixture of gibberellins A4 and A7 (GA4+7) antagonizes the inhibition. In the absence of nitrate, R shifts germination to lower GA-requirement (light-effect II). In A. thaliana a similar second light-effect is seen on the GA-requirement of GA-deficient ga-1 mutant seeds. R stimulates germination of wild type seeds in water (light-effect I). For both species, light-effect I shows a fluence threshold value of approximately 10−5 moles per square meter, which is independent of the nitrate concentration. Increasing nitrate concentrations narrow the fluence-range required for maximal germination whereby the product of nitrate concentration and fluence value determines the germination level, indicating a multiplicative interaction between R and nitrate. Fluence-response curves for light-effect II are similar for both species. Germination occurs in the range of 10−6 to 10−2 moles per square meter fluence. The maximal level of germination is determined by the level of dark-germination and light-effect II. Increasing GA4+7 concentrations induce a shift to lower fluence values. It is shown that in the second effect the co-action of R and exogenous GA4+7 is 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.
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