Relationships between relative growth rate (RGR), endogenous gibberellin (GA) concentration and the response to application of gibberellic acid (GA3) were studied for two inbred lines of Plantago major L., which differed in RGR. A4, the fast-growing inbred line, had a higher free GA concentration than the slow-growing W9, as analyzed by enzyme immunoassay. GA3 application increased total plant weight and RGR3 particularly for the slow-growing line. Chlorophyll a content and photosynthetic activity per unit leaf area were decreased, while transpiration rate was unaffected by GA3 application. The increase in RGR by GA3 application was associated with an increased leaf weight ratio; specific leaf area and percentage of dry matter in the leaves were only temporarily affected. Root respiration rate per unit dry weight was unaffected.
The correlation between low RGR, low GA concentration and high responsiveness to applied GA3 supports the contention that gibberellins are involved in the regulation of RGR. However, the transient influence of GA3 application on some growth components suggests the involvement of other regulatory factors in addition to GA.