EFFECT OF SINK SIZE ON GROWTH-RESPONSE TO ELEVATED ATMOSPHERIC CO2 WITHIN THE GENUS BRASSICA

Many plants grown at elevated CO2 concentrations exhibit enhanced phot osynthetic rates. However, this increase in photosynthesis is often re duced after prolonged exposure to elevated CO2. This reduction may be related to the capacity of plants to utilize the extra photosynthate p roduced at elevated CO2. This study examined the effect of source to s ink ratio on the capacity of plants to respond to elevated CO2. Seven species or cultivars within the genus Brassica were germinated and gro wn at either 350 or 1000 ppm CO2. Broccoli (Brassica oleracea L.) and cauliflower (B. oleracea L.) have large carbon sinks in the reproducti ve structures; Chinese broccoli (Brassica campestris L.) and marrow st em kale (B. oleracea) have carbon sinks in the stem; turnip (B. campes tris) stores carbon in the root; rape (Brassica napus L.) and white mu stard (Brassica alba (L.) Rabenh.) have no obvious carbon storage stru ctures and were assumed to have a lower sink strength relative to the above cultivars. Plants were harvested at three stages of development and total plant weight, leaf area ratio, and allocation to leaf, root, and stem determined. As young seedlings, all cultivars responded posi tively to elevated CO2. The long-term growth response of different cul tivars to CO2 was independent of sink location, but was dependent on s ink size. Cultivars with no obvious carbon storage structures showed n o significant growth enhancement by elevated CO2 by the end of the exp eriment. However, neither leaf area ratio nor biomass allocation patte rn were reliable predictors of response to CO2 suggesting that assessi ng differences in source to sink ratio is not necessarily straightforw ard.