Net carbon dioxide assimilation, carbon isotope discrimination, growth, and water-use efficiency of Citrus trees in response to nitrogen status

Abstract

Five- to six-year-old 'Redblush' grapefruit (Citrus paradisi Macf.) trees on 'Volkamer' lemon [VL = C. volkameriana (Ten. and Pasq.)] or sour orange (SO = C. aurantium L.) rootstock, were grown individually in 7.9-m3 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 (Δ) decreased. Leaf tissue Δ was negatively related to A(CO2) and DM/a. Transient effects of rootstock on leaf N were reflected by similar effects on Δ. 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 Δ. Although photosynthetic N use efficiency (A(CO2)/N) consistently decreased with increased leaf N, Δ 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 Δ 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 Δ. Larger trees on VL had higher ET than trees on SO, but there were no rootstock effects on WUE(T) or on Δ. Leaf tissue Δ was consistently higher than A values of trunk and woody root tissues. Citrus leaf tissue Δ can be a useful indicator of leaf N, characteristics of leaf gas exchange, tree growth, yield, and WUE(T) in response to N availability.