By Jim Barlow A bright sunny day may seem to be the right medicine for tomato plants cooled by overnight temperatures. The cold, however, lingers in the plant and puts the ice on a fast recovery, says UI agronomist William Ogren. Scientists and growers have known for a long time that frost can have a devastating effect on gardens and field crops. More recently, it was found that temperatures merely in the low 40s can have an adverse effect on the photosynthesis of tomatoes and other plants that originated in tropical climates, such as soybeans, tobacco and cucumbers. Although many of the affected plants may recover, the lost time of photosynthesis can reduce both crop growth and yield, Ogren said. In a report to the American Society of Plant Physiologists and the Canadian Society of Plant Physiologists meeting last month in Minneapolis, Ogren said that cool temperatures "turn down" a tomato plant's ability to respond to the presence of light; thus the photosynthesis needed for growth is greatly reduced. In June 1992, crop production in the upper Midwest was heavily damaged by unusually cool weather. Many soybean crops had to be replanted. Crops also are susceptible to periods of cool temperatures in the fall, when a cold snap can completely stop a crop's growth. Plants use sunlight as a source of energy in photosynthesis. The light is absorbed and transformed into a form of energy by chlorophyll, a group of green pigments in the membranes of the chloroplast, the part of the cell where photosynthesis occurs. In tomatoes, the process is initiated by ribulose bisphosphate carboxylase/oxygenase, an enzyme known as rubisco that previously was identified by Ogren as a major controlling factor in photosynthesis. Ogren is a pioneer in research involving the impact of photosynthesis on crop production. Rubisco, in essence, takes carbon dioxide out of the atmosphere and begins to make sugar for energy. During chilly weather, rubisco activase, a regulatory enzyme that interacts with the chloroplast membranes to turn on rubisco, is unable to tell the plant how much light is present, so photosynthesis is diminished. "Basically, the plant is unable to correctly sense the environment," said Ogren, a UI agronomy professor and U.S. Department of Agriculture researcher. "Because of this inability, the available sunshine is absorbed but not properly transformed by chlorophyll in the plant's membranes." The problem has been traced to "something in the plant's membrane," Ogren said. "We hope to find which component of the membrane is affected. The big question is: Can we do something to prevent that problem?"