Cotton leaf age, epicuticular wax, and nitrogen-15 absorption

Foliar N fertilization is sometimes used as a supplement to soil application in cotton (Gossypium hirsutum L.) production; however, information is lacking about the absorption of foliar-applied N in relation to leaf age and epicuticular wax. A 2-yr field study was conducted to examine the effect of leaf age and epicuticular wax on foliar-applied N-15-urea absorption and subsequent distribution within the subtended bell. Cotton (cv. Stoneville 506) leaves of different ages (20, 30, 40, and 60 d), at main-stem node 10, fruiting position 1, were selected and treated with N-15-urea solution using a paint brush. The N-15 analysis of tissues 1 wk after treatment indicated that maximum absorption of 80% was achieved by 20-d-old leaves compared with 27 to 38% for 60-d-old leaves. As the leaves aged, N-15 absorption decreased in concert with increased total wax concentration. A negative relationship between N-15 absorption and total wax concentration of leaves of different ages (r(2) = 0.97) suggested waxes were an important barrier to N-15 absorption in older leaves. The subsequent translocation and distribution of N-15 within the boil indicated that most of the absorbed N-15 was retained in the 20-d-old leaf. As the leaves aged, more N-15 was translocated to the subtended boil. Within the bell, most of the N-15 was compartmentalized to lint and seed, and less was partitioned to the capsule wall, which implicated lint and seed as the major sinks. This study demonstrated that increased wax concentration during leaf aging was associated with reduced N-15 absorption.