Carbon isotope discrimination and oxygen isotope composition in clones of the F₁ hybrid between slash pine and Caribbean pine in relation to tree growth, water-use efficiency and foliar nutrient concentration

The objectives of this study were: (1) to examine how foliar carbon isotope discrimination (Δ) and oxygen isotope composition (δ¹⁸O) are related to tree growth, ash mineral nutrient concentration and foliar nutrient concentration in 7-year-old clones of the F₁ hybrid between slash pine (Pinus elliottii Engelm.) and Caribbean pine (P. caribaea var. hondurensis Barr. et Golf.) in subtropical Australia; and (2) to evaluate the potential of using foliar Δ, ash mineral nutrient concentration and δ¹⁸O measurements for selecting F₁ hybrid pine clones with high water-use efficiency (WUE) and growth potential. There were significant differences in tree growth, foliar Δ, δ¹⁸O and ash mineral nutrient concentration among the eight clones tested. Significant negative linear relationships existed between tree growth and Δ, extrapolating to zero growth at Δ = 24-30‰. There were strong genetic correlations (r= -0.83 to -0.96) between Δ and tree growth, particularly tree height. Significant non-genetic correlations (r = -0.62 to -0.80) existed between Δ and foliar K concentration. Foliar δ¹⁸O, ash mineral nutrient concentration and foliar nutrient concentration were unrelated to tree growth. In the F₁ hybrid pine clones, variation in tree WUE, as reflected by Δ, was largely attributed to a genetic effect on leaf photosynthetic capacity rather than on stomatal conductance, as reflected by liar δ¹⁸O.