No abstract

Seeds of Sisymbrium officinale (L.) Scop, that are buried under natural conditions in soil pass annually through a seasonal pattern of changes in dormancy. Dormancy is broken in autumn-winter and re-induced in summer. To elucidate dormancy regulation in this species under natural conditions, a detailed analysis of the changes in sensitivity to some relevant germination factors was carried out Germination data fitted as logistic dose response curves showed that sensitivity to light and nitrate, both indispensable stimuli for germination of this species, varied with the seasons. Patterns of shifts in requirement for light and nitrate were remarkably similar. Sensitivity increased when both primary and secondary dormancy were alleviated, and it was reversed during induction of secondary dormancy. During alleviation of primary dormancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very-Iow-flueuce-range and in the low-fluence-range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurred in two distinct ranges as well. From interpretation of curve parameters, it is postulated that dormancy is regulated by changes in the number of phytochrome and nitrate receptors, in shifts in the hinding characteristics of the receptors and/or in shifts in the response chain initiated by the ligand-receptor interaction. Somewhere in this response chain, biosynthesis of gibbereiiins (GAs) is stimulated. By use of the GA biosynthesis inhibitor tetcyclasis, * it was indirectly proven that the capacity to synthesize GAs indeed varied with the seasons. Sensitivity to GAs gradually increased from burial onwards and was not particularly related to changes in dormancy. Thus, except for the first few months of burial, GA sensitivity may not be regarded as a limiting factor in controlling dormancy in this species.

The study described in this report was conducted with the aim of developing an unified database of ecological data and residue data to be used for the risk assessment of plant protection products for birds and mammals. The main sources of data were the information submitted in the context of approval of active substances and authorization of products and and additional information retrieved through a systematic literature review. The data were screened and organised in three Excel databases, one for birds, one for mammals and one for residue studies. The ecological information for birds and mammal risk assessment consisted of data that is used for the determination of focal species, estimation of the proportion of an animal's daily diet obtained in a treated habitat (PT) and assessment of the composition of the diet obtained from a treated area (PD). The information gathered on residues focussed on (initial) residue levels after treatment and on residue decline (the reported half‐life or DT50 and the DT90).

Derkx, M. P. M. and Karssen, C. M. 1993. Effects of light and temperature on seed dormancy and gibberellin-stimulated germination in Arabidopsis thaliana: studies with gibbereilin-deficient and -insensitive mutants. -Physiol. Plant. 89: 360-368.Effects of light and temperature on gibberellin (GA)-induced seed germination were studied in Arabidopsis thaliana (L.) Heynh. with the use of GA-deficient {gal) mutants, mutants with a strongly reduced sensitivity to GA {gal) and with the recombinant gai/gal. Seeds of the gal mutant did not germinate in the absence of exogenous GAs, neither in darkness, nor in light, indicating that GAs are absolutely required for germination of this species. Wild-type and gai seeds did not always require applied GAs in light. The conclusion that light stimulates GA biosynthesis was strengthened by the antagonistic action of tetcyclacis, an inhibitor of GA biosynthesis. In wild-type, gal and gai/gal seeds light lowered the GA requirement, which can be interpreted as an increase in sensitivity to GAs. In gai and gai/gal seeds light became effective only after dormancy was broken by either a chilling treatment of one week or a dry after-ripening period at 2°C during some months. The present genetic and physiological evidence strongly suggests that temperature regulates the responsiveness to light in A. thaliana seeds. The responsiveness increases during dormancy breaking, whereas the opposite occurs during induction of dormancy (8 days at 15°C pre-incubation). Since light stimulates the synthesis of GAs as well as the responsiveness to GAs, temperature-induced changes in dormancy may indirectly change the capacities to synthesize GAs and to respond to GAs. GA sensitivity is also directly controlled by temperature. It is concluded that both GA biosynthesis and sensitivity to GAs are not the primary controlling factors in dormancy, but are essential for germination.

The activities of several gibberellins in stimulating germination of wild-type and GA-deficient gal seeds of Arabidopsis thaliana were compared. Of the six compounds tested GA4 and GAY-isolactone had the highest activity and GA7 and GA9 the lowest; activities of GA1 and GA3 were intermediate. Combined application of pure GAS presented no indications that more than one GA receptor is involved. Four GAS were identified in extracts from wild-type and GA-insensitive gai seeds by combined gas chromatography mass spectrometry: GA1 , GA3, GA4 and GA9. Effects of light and chilling on levels of GAl, G& and GA9 were studied using deuterated standards. Light increased both GA levels and germination in unchilled wild-type and gai seeds. As a result of irradiation GA levels in gai seeds were 7-10 times as high as in wild-type seeds. In the dark germination was O%, in the light 14% of gai seeds and 95% of wild-type seeds germinated. A chilling pre-treatment of 7 days at 2 'C was required to enhance further the germination of gai seeds in the light. Light did not increase GA levels of chilled seeds of either genotype; levels of GA4 and GA9 of chilled gai seeds, in the light were respectively 7 and 12 times lower than in non-chilled seeds, whereas the latter seeds germinated better. Slightly elevated levels of GA4 were detected in darkness after chilling, but germination capacity was still 0%. These results strengthened the conclusion that GAS are required for germination of A. thaliana seeds, whereby GA4 has intrinsic biological activity. However, it is unlikely that light and chilling stimulate germination primarily by increasing levels of GA. Instead GA sensitivity is a possible alternative.