Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress - II. Recent photosynthate distribution and growth

Northern red oak in the western Lake States area of the USA exists on the m ost xeric edge of its distribution range. Future climate-change scenarios f or this area predict decreased water availability along with increased atmo spheric CO2. We examined recent photosynthate distribution and growth in se edlings as a function of CO2 mole fraction (400, 530 and 700 mu mol mol(-1) CO2), water regime (well watered and water-stressed), and ontogenic stage. Water stress effects on growth were largely offset by elevated CO2. Water stress increased root mass ratio without concurrently increasing allo cation of recent photosynthate to the roots. However, apparent sink strengt h of water-stressed seedlings at the completion of the third growth stage t ended to be greater than that of well watered seedlings, as shown by contin ued high export, which may contribute carbon reserves to support preferenti al root growth under water-stressed conditions. Elevated CO2 decreased apparent shoot sink strength associated with the rap id expansion of the third flush. Carbon resources for the observed enhanced growth under elevated CO2 could be provided by enhanced photosynthetic rat e over an increased leaf area (Anderson & Tomlinson, 1998, this volume). Increased sink strength of LG seedlings under water-stressed conditions, to gether with decreased apparent shoot sink strength associated with growth i n elevated CO2 provide mechanisms for offsetting water stress effects by gr owth in elevated CO2. Careful control of ontogeny was necessary to discern these changes and prov ides further evidence of the need for such careful control in mechanistic s tudies.