uptake of phosphate of plants from flowing nutrient solution. III. Effect of changed phosphate concentrations on the growth and distribution of phosphate within plants of Lolium perenne L

Perennial ryegrass (Lolium perenne L.) was grown from seed for 29 d in flowing solution culture containing 0.1, 0.4 or 6.4 mmol m—3 P before the concentrations were changed (0.1 and 0.4 raised to 6.4; 6.4 lowered to 0.4; controls unchanged) for an experimental period of two weeks to test the hypothesis that after the seedling stage, the maximum rate of plant growth could be sustained by a lower concentration of phosphate at the root/solution interface than was necessary for the maximum rate of seedling growth. During the 29 d seedling period growth was greatest on 6.4 mmol m-3 P achieving 179 mg per plant dry weight compared with 122 and 26 mg on 0.4 and 0.1 mmol m—3 P respectively. During the experimental period growth on the treatment 6.4 lowered to 0.4 mmol m—3 P continued at the same rate as the 6.4 control achieving 981 and 983 mg per plant dry weight respectively. Similarly growth of the treatment 0.4 raised to 6.4 mmol m—3 P was unaffected by the change in concentration and was comparable with the 0.4 control. Both results support the hypothesis for seedlings exceeding about 100 mg per plant dry weight. In contrast the small plants of the treatment 0.1 raised to 6.4 mmol m—3 P behaved similarly to seedlings and responded rapidly to the increased concentration of phosphate in solution, achieving high rates of phosphate uptake and increasing the growth of shoot more than the growth of root so that the ratio of root : shoot declined from 0.65 to 0.34, a value similar to that for the seedlings grown on 6.4 mmol m—3 P.