Datensatz

Zusammenfassung

Five- to six-year-old 'Redblush' grapefruit (Citrus paradisi Macf.) trees on 'Volkamer' lemon [VL = C. volkameriana (Ten. & Pasq.)] or sour orange (SO = C. aurantium L.) rootstock, were grown individually in 7.9-m(3) lysimeters for 2.5 years using low to high rates of fertilizer N. Net CO₂ assimilation (A(CO₂)) of leaves and leaf dry mass per area (DM/a) increased with leaf N concentration, whereas leaf tissue C isotope discrimination (Delta) decreased. Leaf tissue Delta was negatively related to A(CO₂) and DM/a. Transient effects of rootstock on leaf N were reflected by similar effects on Delta. There was no effect of leaf N on water-use efficiency (WUE) of leaves (WUE(L) = A(CO₂)/transpiration); WUE(L) was not correlated with Delta. Although photosynthetic N use efficiency (A(CO₂)/N) consistently decreased with increased leaf N,Delta was not consistently related to A(CO₂)/N. Annual canopy growth, tree evapotranspiration (ET), and fruit yield increased with whole tree N uptake. Leaf tissue Delta was negatively related to all of these tree measurements at the end of the second year. By that time, whole-tree WUE (WUE(T), annual canopy growth per ET) also was negatively related to Delta. Larger trees on VL had higher ET than trees on SO, but there were no rootstock effects on WUE(T) or on Delta. Leaf tissue Delta was consistently higher than Delta values of trunk and woody root tissues. Citrus leaf tissue Delta can be a useful indicator of leaf N, characteristics of leaf gas exchange, tree growth, yield, and WUE(T) in response to N availability.