Cool weather curbs plants' ability to respond to light, researcher finds

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?"