Soybean Varieties Face Low Germination Glitch

Indian soybean varieties face a severe problem of seed germination. Seeds of soybean lose their germination capacity if harvesting is delayed after maturity. The problem is aggravated, if high temperature and humidity prevail during maturity and harvest.

Therefore, for realisation of optimum yield potential through desired plant stand in the field, it is necessary to check the germination potential for the seeds before planting.

At physiological maturity (maximum accumulation of seed dry weight), the seed reaches its maximum germination potential. Unfortunately, this capacity is short-lived in soybean as compared to other grain crops. Hence, its seeds are grouped in the least storable category.

These seeds not only lose their germination capacity during storage, but are also prone to field weathering.

Field weathering is the process by which the seed quality deteriorates from physiological maturity to harvesting. High temperature and moisture in the form of high humidity or precipitation or both, which are typical of major soybean-growing areas in India, are mainly responsible for field weathering in soybean seeds. The degree of deterioration in such weather-ridden seeds gets enhanced during storage. However, the soybean genotypes are known to differ in their response to field weathering. Therefore, by proper selection of genotypes and following the recommended practices, the loss in seed-germination potential could be minimized.

Soybean ranks third in oilseed production in India with an area of more than 4.5 million hectares, which continues to expand by half a million per annum. But, the average productivity has remained very low at one tonne per hectare as against the world average of more than 1.8 million tonnes per hectare.

But, poor crop stands due to low quality of seeds is primarily responsible for low productivity in India.

To assess the extent of field weathering and the subsequent seed germination problem, a detailed study was undertaken in the predominant soybean growing region of Madhya Pradesh.

It was found that due to late-seasonal rains in the Malwa Plateau during the last week of September and the first week of October in 1992, the mature pods in the standing crop of predominantly early-maturing soybean varieties of the region viz, Punjab-1 and JS 71-05, suffered from alternate wetting and drying periods before they could be harvested.

Samples of such seeds prior to sowing in June 1993 were collected from 34 farmers' fields from the villages located around Indore.

All samples were subjected to germinability tests under laboratory conditions.

The results showed that JS 71-05 was highly prone to field weathering, whereas Punjab-1 possessed resistance to the process.

It was interesting that the samples which showed field weathering, were associated with larger seed size compared with the small-seed samples, that showed resistance to it.

According to SOPA, to obtain the desired plant population in the field, a minimum of 80 per cent germination in seed at the time of sowing is recommended.

As soybean seeds deteriorate during storage - and the degree of deterioration depends on the quality of seeds stored - their performance potential and vigour decline. Hence, germinability tested soon after harvest is not an indicator of their storage potential.

Therefore, to find out the degree of seed deterioration during storage, the recommended accelerated ageing treatments were performed on the seeds collected from three weathering treatments and their germinability was tested in laboratory.

The results showed that:

* Highly significant genotypic differences in seed germination (%)

* Even seeds collected at harvested maturity, resulted in loss of longevity. The degree of deterioration increased as harvesting was delayed and coupled with high moisture conditions created by spraying water on the standing crop.

It was also concluded that:

* Both the early and late maturing genotypes are prone to seed deterioration, whereas bold-seeded genotypes are relatively more susceptible as against small-seeded ones.

* If seeds that have lost germinability are used for sowing, they result in low productivity.