Growth and Phosphate Absorption in Lettuce and Cabbage Plants in Dilute Solution Culture

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Abstract
Growth and phosphate absorption of lettuce and cabbage plants were compared at seven solution phosphate concentrations in the range 0.06-8.0 µM. Phosphate levels were maintained constant throughout the 35-day growth period by the use of large volume (450 litres per 24 plants) continuous- flow solution culture units. Both lettuce and cabbage achieved maximum relative growth rates of approximately 14 g dry matter per 100 g dry matter per day. For cabbage this rate of growth was achieved at a solution phosphate concentration of 0.5 µM. However, to attain the same rate of growth, lettuce required a solution phosphate level at least fourfold greater (between 2.0 and 4.0 µM) . These phosphate concentrations are many times lower than those previously reported for maximum yield of these species. The ability of cabbage to achieve maximum dry matter yield at lower solution phosphate concentrations than that required by lettuce appears to be due to a combination of its greater rate of phosphate absorption per unit of root at low phosphate concentrations, its higher proportion of total plant phosphorus and dry matter contained in the shoot, and its higher phosphate utilization quotient at maximum yield.