A detailed scheme of carboxylate formation and retention by plant tissues as a result of ion uptake and utilization is given. By means of discontinuities in the supply with nutrient ions, carboxylate retention by the tissues of perennial ryegrass was followed as a function of growth. It was found that translocation of potassium nitrate to the shoot and subsequent nitrate metabolism was the only process capable of supplying the shoot with sufficient carboxylates and of removing the excess from the foliage to the root system with maintenance of the normal carboxylate content. Absorbed bicarbonate was a good source of carboxylates in the roots, but the rate of translocation to the plant tops was too slow relative to growth. Therefore, the carboxylate concentration in the foliage fell progressively to one half the normal value. Constancy of carboxylate concentration in the dry matter was related to the early establishment of the proportion of carboxylates to dry material in the new growth, making it independent of subsequent changes in water content of the tissues. Changes in carboxylate concentrations due to changes in the supply were continuous with time. Nitrate caused a depression in the roots during nitrate accumulation, but the nitrate metabolism in the follage made sufficient carboxylates available for replenishment and maintenance of their normal level in the whole plant.
- fodder grasses