Experiments were carried out to study differences in physiological and biochemical properties between slow-growing and fast-growing cotton seedling roots.
The root length, fresh weight and dry weight increased with germination time. The difference between the relative fresh weights of long and short roots was greater than the difference between the corresponding dry weights. In other words, long roots contained more water than the short roots, at least during early germination. Short roots were fatter than long roots and they had more root tip fresh weight. Root elongation occurs in the tip region, while a second area of elongation was observed in the hypocotyls after 48 hour germination. Lateral root initiation occurred away from the root tip over a fairly extended range up to the root-hypocotyl junction.
In the tip region, cell elongation took place mostly between 2.5 mm and 6.0 mm from the tip. Long roots had longer cells as compared with short roots, but the age of the root had no significant effect on cell length. During early germination, long roots had wider cells and greater cell volume as compared with short roots, but this effect disappeared by 84 hours. The number of cell files decreased with increasing root length at all germination times. Younger seedling roots contained more cell files as compared with older ones. These changing parameters of cell length, cell width and file number determined the size and the shape of the roots.
During early germination, (Me-³ H) thymidine incorporation was greater in long roots than in short roots, but it was almost the same in the roots of all lengths at later germination times. Incorporation values were higher in younger roots than in older roots. It was highest in the tip region of the root. Total DNA levels were greater in short roots than in long roots and more in younger than in older roots. It was concluded from these data that cell division does not play an important part in root growth in cotton seedlings.
Soluble peroxidase activity decreased as germination time increased. The insoluble ( cell wall) peroxidase activity increased as germination time increased. On a per root basis, soluble peroxidase activity did not vary much with decreasing root length, but cell wall peroxidase activity increased. On a per gram fresh weight basis, both soluble and cell wall activities decreased progressively with decreasing root lengths. The distribution of both soluble and cell wall peroxidase activities in both long and short roots showed high activities in the first segment and a second area of high activity in segments further away from the tip.
Root elongation was slightly stimulated by 10^-5 to 10^-7 M GA₃ • ABA also slightly stimulated root growth up to 24 hours, but by 48 hours the effect was reversed. High IAA concentrations (10^-3 and 10^-4 M) inhibited growth. Low IAA concentrations (10^-6 to 10^-10 M) promoted growth. The high concentration caused a large increase in the tip fresh weight.
With the high IAA concentrations, soluble peroxidase activity on a per root basis decreased up to 24 hours, but then increased again up to 48 hours. On a per gram fresh weight basis, the activity decreased with time all the way to 48 hours. Insoluble peroxidase activity, on a per root as well as on a per gram fresh weight basis, decreased with time all the way to 48 hours. At low concentrations, IAA had no effect on soluble peroxidase activity, but the insoluble (cell wall) peroxidase activity was increased. It was concluded from the peroxidase studies results that enzyme had no significant role in root elongation.