But the acacia in 1,000 ppm of CO2 grew an average of 23 feet, compared with 15 feet for plants in 700 ppm and only about five feet for plants in 350 ppm. Rising CO2 levels during the past century could help explain how rangeland brush has proliferated in western states as plants' increased water use efficiency enables them to grow in areas where water is scarce. But researchers found this shouldn't happen if growers can harvest the melons within a day or two after a heavy rain. Or, if growers can wait a week, the fruit's sugar stocks will rebuild. Growers have thought that heavy rain dilutes the sweet taste when the fruit takes in excess water via the roots.
Scientists now blame a sugar shortage rather than a water surplus.
The results of laboratory studies by ARS and North Carolina State University were confirmed in field tests by ARS and the University of Arizona. After heavy rain, the roots stop working. They can't take up needed oxygen from rain-saturated soil. This shutdown signals leaves to halt photosynthesis, the plant's mechanism for using solar energy to produce sugars. The reduced sugar content shows up about four days after the heavy rain.