Ontogenetic Changes in Respiration and Photosynthesis in a Uniculm Barley1

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Abstract
Respiratory (R) and photosynthetic (P) rates, together with root and shoot dry weights (W) and leaf areas were measured at intervals throughout the ontogeny of a uniculm barley (Hordeum vulgate L.) grown at three temperatures (15, 20, 28 C). Increasing shoot photosynthesis per unit area of leaf lamina as the plants aged was probably related to the increasing contribution by non‐laminar tissue. Absolute values appeared to be higher than those reported in other cultivars. The rates of both shoot and root respiration on a dry weight basis decreased throughout the life cycle with a more rapid decline in the early stages and with a more rapid decline in the shoots than in the roots. The R/P ratio decreased from an initially high value to a minimum and increased again as the plants matured. The period during which minimum ratios were obtained depended on the temperature. The relationship between R/W and P/W appeared curvilinear with a greater slope in the early growth stages. The results emphasize that care should be taken in the choice of coefficient's for application in a two parameter respiration model.