Influence of Plant Maturity and Plant Part on Nitrate Content of the Sweetclover Weevil-Resistant Species Melilotus infesta12

Abstract

All parts of 6-week-old M. infesta plants were analyzed to determine nitrate distribution within a plant of a given age. The nitrate content was low in the newly developed pinched leaves but became progressively higher with each successive stage of leaf development. The nitrate content of the oldest leaves was 2, 3, 9, and 80 times higher than that of the first fully expanded, unfolded, loosely pinched, and tightly pinched leaves, respectively. The nitrate levels in petioles and stems were about equal to those of the older fully expanded leaves and nearly twice the amount in the roots. Susceptibility of the leaves of young M. infesta plants to feeding of the sweetclover weevil, Sitona cylindricollis Fåhraeus, was associated with their respective nitrate contents. Pinched leaves were fed on extensively, unfolded leaves were fed upon less extensively than pinched leaves, and fully expanded leaves were not fed upon. The nitrate content of leaves of a particular stage of development was affected by the age of the plant. With the exception of the pinched leaves, which at all plant ages had a low nitrate content, the leaves of each developmental stage had a high nitrate content in young plants but a low level in the mature plants. The low-nitrate leaves of the mature M. infesta plants were resistant to sweetclover weevil feeding, which fact indicated that some factor other than nitrate was responsible for the feeding resistance in leaves of these mature plants.