employs genetically engineered bacteria. The bacteria, which live on plant leaves, have protein coats that stimulate ice crystals to form around them when air temperatures drop below freezing point. The ice crystals rupture plant cells, releasing nutrients, which the bacteria use for their growth. In the absence of these bacteria, frost does not form so readily on the leaf surface and the plants can survive without damage at lower temperatures.

Since it would be practically impossible to remove bacteria from the leaves, scientists have instead made competing bacteria, in which the genes controlling production of the ice-forming protein coat have been deleted. Plants are sprayed with these engineered ''non-icing" bacteria to prevent the buildup of the normal ice-forming microbes. In California, where fruit crops are especially vulnerable to frost, the technique has protected sprayed crops from frost damage at temperatures as low as -10°C (15°F). As an interesting sideline, the ice-forming bacterial protein is marketed by producers of artificial snow for ski slopes.

Apart from the weather, another major factor affecting crop productivity is soil quality. Modern intensive farming methods usually require large inputs of fertilizer to maintain the level of soil nutrients demanded by high yield crops. But it may be easier and cheaper in the long run to change plants to suit the soil than to change soil to suit the plants. For example, researchers are studying ways to engineer salt tolerance into crops. This could make it possible to expand farmland into marginal areas with poor soils, or even to irrigate fields with seawater.

Another focus of much study is the enormous untapped genetic resources of beneficial soil microbes. One important group of bacteria, Rhizobium, lives in