Physiological aspects of seed quality related to field emergence in Phaseolus vulgaris L. and Glycine max (L.) Merrill
Thirty seed lots of 8 cultivars of soyabean and 30 seed lots of 17 cultivars of Phaseolus bean from the U.S.A., Brazil and the Netherlands of high and commercially acceptable laboratory germination were sown in two field emergence experiments in 1982 in Aberdeenshire. A wide variation in field emergence was observed which was indicative of differences in the vigour of the seeds. In soyabean, low field emergence was a feature of seed lots with high levels of damage to their seed coats. Such lots imbibed at faster rates, leaked higher amounts of electrolytes into the seed soak water, and had reduced levels of tetrazolium staining of their cotyledons after 24 imbibition in water in the laboratory. In the field these lots exhibited poor emergence and produced smaller seedlings. These findings were explained in terms of imbibition damage, a phenomenon described for peas in which an initial rapid uptake of water damages the outer cells of the cotyledons. In the present work on soyabeans a crucial observation in support of this view was the improved staining of cotyledons with tetrazolium chloride when the rate of water uptake was slowed by imbibing seeds in polyethylene glycol. While previous work on soyabeans had only laid emphasis on the production of low vigour seeds through the effect of deterioration, in the field, prior to harvest or during unsatisfactory storage conditions, this work focussed attention on the importance of the condition of the seed coat and on imbibition damage which was found to be more damaging to deteriorated seeds. In Phaseolus bean, as in soyabeans low field emergence was associated with rapid imbibition. Lots which imbibed at faster rates emerged less in the field, had poor tetrazolium staining of the cotyledons and higher leakage of electrolytes. However, in Phaseolus bean the influence of split seed coats in determining the rate of water uptake by the seeds was not the most crucial factor. Studies with the aim of determining the cause of differences in the rate of water uptake between two cultivars which had contrasting rates of imbibition, Provider (slow uptake) and Tenderette (fast uptake), revealed that both cultivars showed similar permeability of their isolated seed coats to water, as measured with a diffusion porometer. Similar rates of water uptake by both cultivars when imbibed through one end of the seed with intact seed coat also indicated similar permeability of their seed coats. The seed coats of the white seeded cultivar Tenderette tended to absorb water vapour at faster rates but this was not great enough to explain the considerable faster rate of water uptake by white seeds during soaking in: water in comparison with the brown seeded cultivar Provider. Furthermore, no differences were found in the permeability of the hilum/micropyle of both cultivars to water, or in the rate of water uptake of their isolated cotyledons. However, when both cultivars imbibed through a large split area of the seed coat in one end of the seed, the rapid movement of water upwards was observed between the loosely adhering seed coat and the cotyledons of the white seeded cultivar Tenderette, which has a fast irate of imbibition. This was in contrast to the brown seeded cultivar Provider, in which the tightly adherence of the feeed coat to the cotyledons did not allow suchi a movement of water, and therefore imbibition occurs slowly. Thus in both species (soyabean and Phaseolus bean) cultivars or lots which imbibed at faster rates gave rise not only to lower field emergence but also the emerged seedlings were smaller in size. The low vigour of seedlings originating from fast imbibing seed resulted from slower rate of transfer of food from the cotyledons to the shoot, as indicated by a significantly lower weight of the shoot, which was negatively and significantly correlated with the rate of water uptake. Suggestions based on the results obtained in this work have been put forward which could be incorporated into breeding programmes, seed processing methods and seed testing methods, with the overall objective of improving seed quality of both soyabean and Phaseolus bean.