The Change of Dormancy and Mustard Oil (Allyl-isothiocyanate) Content in Seeds of Brassica juncea Czern. et Coss. and B. carinata Braun

Abstract

In Brassica juncea (designated as B. cernua in the preceding papers of this series) and B. carinata, when freshly harvested seeds are stored in the desiccator, the breaking of their dormancy is partially prevented. The similar preventing effect is also recognized when seeds are preserved in harvested fruits without separation. On the other hand, it has been reported that mustard oils, mostly allyl-isothiocyanate and β-phenethyl-isothiocyanate, contained in fruits and seeds of some cruciferous vegetables are germination-inhibiting substances. The present investigation was undertaken to examine whether or not allyl-isothiocyanate is a substance which prevents the breaking of seed dormancy in Brassica juncea and B. carinata.
After the harvest of mature fruits, they were divided into two groups. In one group seeds were isolated separately from fruits, and in the other group seeds were left intact in fruits. Then, seeds in each group were either placed on the desk in the room or stored in the desiccator (containing silica gel). After that, the germination percentage, allyl-isothiocyanate content, and water content of seeds were examined at one or two week intervals during 13 weeks. Allyl-isothiocyanate was analyzed by liquid-chromatography.
Results obtained are as follows. The breaking of dormancy was prevented only when the seeds preserved in fruits were stored in the desiccator. There were little differences in seed germination among three other treatments, in which dormancy was broken almost completely in 5 to 6 weeks after harvest. The prevention of breaking of dormancy in the seeds with fruits in the desiccator was due to the germination-inhibiting effect of desiccation, as demonstrated by the rapid decrease of water content of seeds; in other treatments there was no such decrease of water content in early stages after harvest. In the separated seeds in the desiccator, water content did not reach the percentage low enough to inhibit germination until 5 to 6 weeks. This was perhaps caused by the close packing of seeds in a bag, which might prevent the drying of seeds. The germination of seeds was not affected by fruits in B. juncea, but in B. carinata the breaking of dormancy was prevented in some degree when seeds were preserved in fruits.
The content of allyl-isothiocyanate showed similar variations through the process of seed storage after harvest in all treatments. Differences in allyl-isothiocyanate were hardly noticed between seeds with and without fruits and between seeds in the room and in the desiccator. Therefore, it is considered that allyl-isothiocyanate had no relation to a germination-inhibiting substance(s) which may prevent the breaking of dormancy. In each treatment, however, the curve of allyl-isothiocyanate presented similarity to the curve of germination percentage though the latter showed the lag of time. This similarity may suggest that a certain physiological change (such as so-called biorhythm) occurred in seeds, and that mustard oils and germination-inhibiting substances were influenced by this physiological change in their respective ways.