Now, researchers at ICRISAT have found that certain sorghums withstand heat better than others. No one has paid attention to this quality before, and almost all of today's sorghums produce seedlings susceptible to burning hot soils.
By sowing seed in hot fields and seeing which survived, lines with heat-tolerant seedlings have been identified. But such tests are expensive, time-consuming, and subject to hosts of uncertainties. Now, researchers at the Welsh Plant Breeding Station13 are devising mass-screening techniques that can be performed in a laboratory and with much more precision.
One Welsh technique, already adopted by ICRISAT, monitors the amount of protein synthesized by the germinating seeds. In hot surroundings, the most heat-tolerant types produce the most protein. However, this test is expensive and cumbersome to run on thousands of samples, so now the Welsh researchers are developing a second-generation test based on "heat-shock proteins" (HSPs).
All living things make HSPs when exposed to temperatures above their normal range. They do it quickly—often within 15 minutes. Once made, the proteins—which are similar in plants, animals, and bacteria—seem to confer an ability to prosper in the heat. Their exact function is still uncertain, but they may protect the organism's proteins, messenger RNA, or membranes from damage. One HSP—often called HSP70 because it has a relative molecular mass of 70,000—may ensure that heat-damaged proteins regain their proper shape so that they can continue working as enzymes, muscles, and antibodies.
The researchers now have found that briefly exposing a sorghum seedling to temperatures between 40°C and 45°C induces it to produce a characteristic set of HSPs. From then on, the plant can tolerate temperatures of 50°C or even more without suffering damage.
Although all sorghum seedlings make HSPs, those that tolerate heat best make HSPs much sooner after germinating. Speed is the secret of their success.
This response is being studied in the hope of finding an easily recognizable feature that can identify heat tolerance without torturing the seeds. If successful, this will open the way to mass screening so that farmers in the hottest areas will no longer face the heartbreak of seeing their fields wilting in the blazing sun before the plants have even grown more than knee-high.
Another approach is to find the regions of the chromosomes which are important for survival of heat stress. DNA probes are being used as markers by the researchers in Wales to follow regions of the chromosomes linked to the thermotolerance trait from parents to subsequent generations.