The Influence of the Plant Growth-regulator, 2-Methyl-4-Chlorophenoxyacetic Acid, on the Metabolism of Carbohydrate, Nitrogen and Minerals inSolanum lycopersicum (Tomato)

Abstract

SUMMARY The effects of the foliar application of phytocidal concentrations of 2-methyl 4-chlorophenoxyacetic acid (MCPA) on change in total dry weight, and in 'available carbohydrate' (starch, 'total' and 'reducing' sugars), total nitrogen, phosphorus, potassium, calcium, and magnesium of 'tops' and roots of tomato plants have been followed over a period of 14 days following spraying. There were two main treatments—-'nutrient' (nutrient supply to roots con tinued after spraying) and 'water' (distilled water only supplied to roots after spraying)—the sub-treatments consisting of 'MCPA' versus 'no-MCPA' for each of the main treatments. Twelve different times of sampling were used. In analysing the present data, the quantity 'residual dry weight' (total dry weight less 'available carbohydrate'), which was originally introduced by Mason and Maskell as a basis of reference for analyses of plant organs in short-period experiments not involving appreciable growth, has been used as an estimate of the permanent structure of plant growth. This new use of the 'residual dry weight' basis has brought out important features which were obscured when the data were left in their primary form (as percentages of total dry weight or amounts per plant). Growth, as measured by increase in 'residual dry weight', was greatly inhibited by 2-methyl-4-chlorophenoxyacetic acid shortly after spraying, in both the pre sence and the absence of nutrient. In the presence of 2-methyl-4-chlorophenoxyacetic acid, net assimilation rate (estimated as rate of increase in total dry weight per gram 'residual dry weight' of the 'tops') was greatly diminished while uptake of total nitrogen and of P205 (estimated as increase in total nitrogen or of P205 of the whole plant per day per i g. 'residual dry weight' of the roots) appeared to undergo a similar but much smaller diminution. It seemed probable, however, that in the presence of MCPA a larger proportion of the carbohydrate actually formed was utilized for synthesis of amino-acids and protein. In the plant as a whole there was no evidence of actual depletion of 'available carbohydrate' as a result of MCPA treatment, this fraction showing a steady increase in all treatments throughout the experiment. The rate of increase was, however, much reduced by MCPA treatment. The 'tops' presented much the same picture as the whole plant, but for the roots the situation was quite different. While the roots of the 'no-MCPA' plants and also of the 'MCPA-water' plants showed a steady increase in available carbohydrate, those of the 'MCPA-nutrient' plants rose only very slightly (from the initial value of 8 mg. per plant to about 10 mg.) during the first 2 days, and then in the next 2 days declined to a value