ELEVATED ATMOSPHERIC CO2 ALTERS STOMATAL RESPONSES TO VARIABLE SUNLIGHT IN A C-4 GRASS

Native tallgrass prairie in NE Kansas was exposed to elevated (twice a mbient) or ambient atmospheric CO2 levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C-4 grass, An dropogon gerardii, was subjected to fluctuations in sunlight similar t o that produced by clouds or within canopy shading (full sun > 1500 mu mol m(-2) s(-1) versus 350 mu mol m(-2) s(-1) shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO2 when com pared to plants at ambient CO2. This was due primarily to the 50% redu ction in stomatal conductance at elevated CO2, but was also a result o f more rapid stomatal responses. Time constants describing stomatal re sponses were significantly reduced (29-33%) at elevated CO2. As a resu lt, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure pote ntial during periods of sunlight variability provided additional evide nce that more rapid stomatal responses at elevated CO2 enhanced plant water status. CO2-induced alterations in the kinetics of stomatal resp onses to variable sunlight will likely enhance direct effects of eleva ted CO2 on plant water relations in all ecosystems.