Dried to perfection

The benefit from initial drying at high temperature is not unique to cultivated varieties of Oryza. Figure 9 (below) shows the results for two wild relatives. The two wild species remained viable for longest when they had initially been dried at 45°C (the blue curve) before a second stage of drying at temperatures of around 15°C. They remained viable for less time when they had only been dried at 15°C (the black curve) throughout the drying process. Even seeds that were initially dried at a sweltering 60°C (the pink curve) performed slightly better in germination tests than the seeds grown according to Genebank Standards (the black curve).

There are two graphs here, showing the declines in viability over time associated with different initial drying temperatures for two different species of rice. The x axis shows the storage period (days). The y axis shows percentage germination. The decline in viability approximates to an S-shaped curve for all samples, of both species. The decline in viability for samples initially dried at fifteen degrees is steepest, and results in the earliest loss of viability, at about twelve days. The decline in viability for seeds dried at forty-five degrees is the least steep, and viability is maintained for longest, dropping to zero at about twenty-seven days. Even drying at sixty degrees, a very high temperature, results in viabilty dropping off at about eighteen days: a better result than drying at fifteen degrees.
Figure 9: benefits of drying at a higher temperature

These findings suggest that for some species, longevity in storage can be improved by adapting the conditions under which the seeds are dried. These results were compelling enough to convince IRRI to change their procedures. Instead of drying rice according to the Genebank Standards, IRRI now carry out two-stage drying, where seeds are initially dried at 30°C for a couple of days, then finished under conditions the Genebank Standards recommend: 15% humidity and 15°C. This has allowed IRRI to make demonstrable improvements to seed viability.

There is evidence that other species may benefit from drying at different temperatures too. Scientists at the Institute for Tropical Agriculture (IITA) in Nigeria found similar improvements in soya beans and Bambara groundnuts. Although there is a lot we still don’t know, it is likely that if seeds have had to be harvested early, for instance due to poor weather, drying them under conditions that simulate natural conditions in the field during a good year can compensate for losses in viability as a result of being harvested prematurely.

  

Pause for reflection

You have discovered that genebanks can improve longevities of seeds in their care. Give yourself five minutes to think about whether, if genebanks were able to implement better and better conditions, the improvements in longevity could continue indefinitely? Are there limits to these improvements?

Humidity and temperature

Time between harvest and storage