The growth and productivity of Cotton (Gossypium hirsutum L.) is dependent on successful seedling establishment to help insure vigorous growth during the remainder of the season. The development of the root system obviously plays an important role in the establishment process. Seedling establishment may be hindered however, through a reduction in root growth due to low soil temperatures at the time of planting. If the optimum temperature for root growth in cotton were lower, then more consistent seedling establishment might be possible. Therefore, studies were conducted to determine the influence of various temperature regimes on root growth of cotton seedlings as compared to another species (sunflower) which has been observed in field studies to grow a root system at a significantly faster rate than cotton under the same conditions. The thermal dependence of a respiratory enzyme necessary for growth of roots, malate dehydrogenase, was also investigated since differences in the activity of this enzyme between the two species may provide some insight as to a mechanism for temperature response.

Seedlings of both cotton (cv 'Paymaster 145') and sunflower (cv 'PD11') were grown in the dark at constant temperatures (10, 15, 20, 25, 30, 35, and 40C for the sunflower and 20, 25, 30, 35, and 40C for the cotton) for a period of 10 days using growth pouch techniques. The lengths of both the primary roots (taproots) and lateral roots, as well as the total number of lateral roots produced were measured for each seedling at the end of the ten day period. Twenty seedlings were evaluated for each species for each temperature treatment. Malate dehydrogenase was also extracted from the roots of each species grown at each temperature and the activity expressed as the change in apparent Km of the enzyme as a function of temperature.

The optimum temperature for the growth of the taproot of both the cotton and sunflower seedlings was 30C. Growth was reduced at both the low (20C) and the high (40C) temperatures for both species, but the reduction at the low temperatures was significantly greater for the cotton. There was growth of the sunflower taproots at 10C whereas the cotton did not germinate below 20C during the ten day test period. The optimum temperature for lateral root initiation was 35C for cotton and 28C for sunflower with a significantly higher number of lateral roots being produced in the sunflower (90/plant vs 25/plant for cotton). No lateral roots were initiated in the cotton grown at 20C for the ten day period. Lateral roots were initiated however, in the sunflower at 15C. The total lateral root length per plant was also significantly higher in the sunflower. The results of the enzyme analysis indicated that the thermal dependence of the Km for malate dehydrogenase differed between the cotton and the sunflower with the thermal optimum being lower for the sunflower. A more complete understanding of the genetic control of the thermal dependence of the enzyme may allow us to alter the growth habits of the cotton root system to optimize growth and seedling establishment at low soil temperatures.