The Effect of Defoliation on the Carbon Balance in Dactylis glomerata

Measurements were made of the carbon dioxide exchange of roots and shoots, changes in soluble carbohydrates, rates of root extension, and rate of phosphorus uptake of young plants of Dactylis glomerata (Cocksfoot) during eight days following defoliation. The results indicated that the soluble carbohydrates formed part of a labile pool which was used in respiration and for providing substrates for new growth. Where defoliation was not severe, the changes in reserve carbohydrates could account for net respiratory losses and amounts of new growth made. Where defoliation was severe, even high concentrations of reserve carbohydrates were inadequate and other substances, presumably proteins, must have been remobilized for use in respiration and new growth. The content of soluble carbohydrate in the root was completely inadequate to meet the needs of root respiration; transfer from the tops and/or remobilization of other substances in the roots must have occurred. Following a severe defoliation, root extension stopped and rates of respiration and phosphorus uptake fell markedly. Phosphorus uptake remained at a low level for the eight days considered. After a light defoliation the roots recovered relatively rapidly. It is suggested that, following a severe defoliation, regrowth during the first week is limited in turn first by the soluble carbohydrate content in the bases of expanding leaves, then by the rate of photosynthesis, and then in the later Stages by the rate of nutrient uptake sustained by the roots.