The Nature of Summer Dormancy in Perennial Grasses

Abstract

1. The summer dormancy behavior of twenty species of perennial grass was investigated in the field at Davis, California, where the climate is a winter-rain-fall, arid-summer type. 2. Of the twenty species, seven became summer dormant in the field even though supplied with water throughout the dry season. 3. Those species which continued growth as long as water was available were found to resume growth in summer if watered after a period of dormancy. This was not true of the species going dormant while watered. 4. Stipa cernua, Oryzopsis miliacea, and Phalaris tuberosa var. stenoptera were observed to make vigorous vegetative growth in September after being summer dormant and before effective autumn rains. Portions of the root system of P. tuberosa were severed while the plant was dormant. The typical September growth occurred following such treatment only when the roots at depths below 4 feet were left intact. 5. Poa scabrella was selected for critical study of the factors, other than moisture, involved in the summer dormancy behavior of a species becoming dormant though watered throughout the dry season. Field observations indicated that high temperatures and long day lengths were related to the initiation of dormancy in this plant and that moderating temperatures and availability of water were associated with the breaking of dormancy. Greenhouse studies confirmed these deductions. 6. Dormancy in P. scabrella was induced in the greenhouse in midwinter while soil moisture was available by subjecting plants at least 15 weeks of age to an 18-hour photoperiod and to high-temperature stresses in a controlled environment chamber. The conditions of the most effective high-temperature treatments were 130⚬ F. and 50% relative humidity, the plants being illuminated by fluorescent light while in the chamber. Three such exposures each of 4-hour duration were given on alternate days. 7. Dormancy was broken by relative coolness and water. Plants of P. scabrella dormant 20 months in the greenhouse resumed growth 5 days after being placed at temperatures not exceeding 75⚬ F. daily and receiving water.