A two-year field study was designed to investigate the effect of leaf-age on photosynthesis, epicuticular wax, and foliar-appled (15)N absorption in a cotton (Gossypium hirsutum L.) leaf. A 20-d old leaf was the most physiologically active with photosynthesis and (15)N absorption of 16.8 mol CO2 m-2 s-1 and 79.6%, respectively. As the leaf aged, it progressively lost its activity, as exhibited by the 30, 40, and 60-d old leaves. In contrast to photosynthesis and (15)N absorption, leaf aging increased epicuticular wax content in combination with alterations in the wax composition. Scanning electron microscopy further supported this observation by exhibiting waxier leaves with increased age. Dotriacontane (3.9%) was the major long-chain wax constituent in 20-d old leaves, whereas n-Tetratriacontane (31-34%) was the predominant long-chain alkane in the older leaves. Increased wax content coupled with increased levels of long-chain alkanes were associated with reduced (15)N absorption in the older leaves. While chloroplast ultrastructural alterations such as disintegration of grana in combination with increased size and number of plastoglobuli were associated with lower photosynthetic rates of the older leaves. The reduced photosynthesis and (15)N absorption during the peak boll filling period could be a limitation to optimum boll growth.