Variation in nickel content in the nickel-hyperaccumulating shrub Psychotria douarrei (Rubiaceae) from New Caledonia

Plants that hyperaccumulate Ni contain > 1000 ppm (dry wt.) in their tissue s. Variation of Ni content within hyperaccumulating plant species is poorly explored. Using the Ni-hyperaccumulating shrub Psychotria douarrei, we doc umented variation of leaf Ni levels within individual shrubs, and variation with respect to plant size and leaf age. Plant. size did not correlate sig nificantly with leaf Ni content, and leaf Ni content did not correlate sign ificantly with soil Ni content. Older leaves contained twice as much Ni as younger leaves. Older leaves also contained greater concentrations of Ca, F e, and Cr bur less K, P, and Cu. Five elements (Zn, Pb, Co, Mn, Mg) showed no significant variation due to leaf age. We also examined the effect of le af age on epiphyll cover, finding increased epiphyll cover on the upper sur face of older leaves. The dominant leafy liverwort epiphyll had a relativel y high Ni content (400 ppm), suggesting that epiphylls of Ni hyperaccumulat ors obtain some Ni from host leaves. Individual shrubs differed in mean lea f Ni content almost two-fold (14,900-27,700 ppm). Variation among branches within individuals also ranged widely; however, this intraplant variability was not strongly correlated with the mean leaf Ni content of an individual shrub. We concluded that Ni contents in leaves of I? douarrei vary conside rably due to leaf age, among individual shrubs, and among branches within a shrub.