Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress - I. Carbon assimilation and biomass production

The interactive influences of elevated carbon dioxide, water stress, and on togeny on carbon assimilation and biomass production were investigated in n orthern red oak, a species having episodic shoot growth characteristics. Se edlings were grown from acorns through three shoot-growth hushes (8-11 wk) in controlled-environment chambers at 400, 530 or 700 mu mol mol(-1) CO2 an d under well watered or water-stressed soil-moisture regimes. Increasing CO 2 growth concentration from 400 to 700 mu mol mol(-1) resulted in a 34 % in crease in net assimilation rate (A), a 31 % decrease in stomatal conductanc e to water vapour (g(s)) and a 141 % increase in water use efficiency (WUE) in well watered seedlings. In contrast, water-stressed seedlings grown at 700 mu mol mol(-1) CO2 demonstrated a 69 % increase in A, a 23 % decrease i n g(s), and a 104 % increase in WUE. However, physiological responses to in creased CO2 and water stress were strongly modified by ontogeny. During act ive third-flush shoot growth, A in first-flush and second-flush foliage of water-stressed seedlings increased relative to the quiescent phase followin g cessation of second-flush growth by an average of 115 %; g(s) increased b y an average of 74 %. In contrast, neither A nor gs in comparable foliage o f well watered seedlings changed in response to active third-flush growth. Whereas seedling growth was continuous through three flushes in well watere d seedlings, growth of water-stressed seedlings was minimal following the l eaf-expansion stage of the third flush. Through three growth flushes total seedling biomass and biomass allocation to root, shoot and foliage componen ts were very similar in water-stressed seedlings grown at 700 mu mol mol(-1 ) CO2 and well watered seedlings grown at 400 mu mol mol(-1) CO2. Enhanceme nt effects of elevated CO2 on seedling carbon (C) assimilation and biomass production may offset the negative impact of moderate water stress and are likely to be determined by ontogeny and stress impacts on carbon sink deman d.