NET CARBON-DIOXIDE ASSIMILATION, CARBON-ISOTOPE DISCRIMINATION, GROWTH, AND WATER-USE EFFICIENCY OF CITRUS TREES IN RESPONSE TO NITROGEN STATUS

Five- to six-year-old 'Redblush' grapefruit (Citrus paradisi Macf.) tr ees 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 isot ope 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 lea f N on water-use efficiency (WUE) of leaves (WUE(L) = A(CO2)/transpira tion); 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 t ree 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 rel ated 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 Del ta was consistently higher than Delta values of trunk and woody root t issues. Citrus leaf tissue Delta can be a useful indicator of leaf N, characteristics of leaf gas exchange, tree growth, yield, and WUE(T) i n response to N availability.