Plant Productivity and Water Use Efficiency of Sweetpotato (Ipomoea batatas) as Affected by Nitrogen Supply

A pot experiment was conducted in a tropical mid-elevation environment (861 m altitude) to evaluate sweetpotato (Ipomoea batatas (L.) Lam.) clones of different origin and breeding intensity for traits related to growth phenom-ena, nitrogen-use efficiency, and transpirational water-use efficiency (WUE) as affected by different levels of N fertilization. Genotypes with small cano-pies were associated with a consistently positive response in their final storage root dry matter (DM) yields to increasing N supply, and with efficient allocation of DM and N to storage roots. Genotypes with high canopy net assimilation rates (NARs) had a high proportion of leaves exposed to the sun and high chlorophyll content in leaves. Nitrogen stress led to increased transpiration per unit leaf area and decreasing WUE. Decreasing WUE under N stress was due to lower total plant DM production rather than to increased total water transpiration per plant. As do most other field crops, sweetpotato responds well to improved management practices, among which N fertilization plays an important role in producing satisfactory yields. Previous investigations did not indicate an optimum range of N fertilization because the response of sweetpotato to N fertilization depends highly on genotypic and environmental variation (Villagarcia, 1996). Jones and Bouwkamp (1992) reported that an appli-cation of 60 kg N/ha increased yields of three USA varieties, but decreased yields of three African varieties. Hill et al. (1990) showed that total biomass, storage root yields, and foliage weight were not significantly affected by the addition of fertilizer N. But there is general agreemen