Photosynthetic and respiratory acclimation and growth response of antarctic vascular plants to contrasting temperature regimes

Air temperatures have risen over the past 50 yr along the Antarctic Peninsu la, and it is unclear what impact this is having on Antarctic plants. We ex amined the growth response of the Antarctic vascular plants Colobanthus qui tensis (Caryophyllaceae) and Deschampsia antarctica (Poaceae) to temperatur e and also assessed their ability for thermal acclimation, in terms of whol e-canopy net photosynthesis (P-n) and dark respiration (R-d), by growing pl ants for 90 d under three contrasting temperature regimes: 7 degrees C day/ 7 degrees C night, 17 degrees C day/7 degrees C night, and 20 degrees C day /7 degrees C night (18 h/6 h). These daytime temperatures represent subopti mal (7 degrees C), near-optimal (12 degrees C), and supraoptimal (70 degree s C) temperatures for P-n based on field measurements at the collection sit e near Palmer Station along the west coast of the Antarctic Peninsula. Plan ts of both species grown at a daytime temperature of 20 degrees C had great er RGR (relative growth rate) and produced 2.2-3.3 times as much total biom ass as plants grown at daytime temperatures of 12 degrees or 7 degrees C. P lants frown at 20 degrees C also produced 2.0-4.1 times as many leaves, 3.4 -5.5 times as much total leaf area, and had 1.5-1.6 times the LAR (leaf are a ratio; leaf area:total biomass) and 1.1-1.4 times the LMR (leaf mass rati o; leaf mass:total biomass) of plants grown at 12 degrees or 7 degrees C. G reater RGR and biomass production at 20 degrees C appeared primarily due to greater biomass allocation to leaf production in these plants. Rates of P- n (leaf-area basis), when measured at their respective daytime growth tempe ratures, were highest in plants Brown at 12 degrees C, and rates of plants grown at 20 degrees C were only 58 (C. quitensis) or 64% (D. antarctica) of the rates in plants grown at 12 degrees C. Thus, lower P-n per leaf area i n plants grown at 20 degrees C was more than offset by much greater leaf-ar ea production. Rates of whole-canopy P-n (per plant), when measured at thei r respective daytime growth temperatures, were highest in plants grown at 2 0 degrees C, and appeared well correlated with differences in RGR and total biomass among treatments. Colonbanthus quitensis exhibited only a slight a bility for relative acclimation of P-n (leaf-area basis) as the optimal tem perature for P-n increased from 8.4 degrees to 10.3 degrees to 11.5 degrees C as daytime growth temperatures increased from 7 degrees to 12 degrees to 20 degrees C. There was no evidence for relative acclimation of P-n in D. antarctica, as plants grown at all three temperature regimes had a similar optimal temperature (10 degrees C) for P-n. There was no evidence for absol ute acclimation of P-n in either species, as rates of P-n in plants grown a t a daytime temperature of 12 degrees C were higher than those of plants gr own at daytime telnperaturcs of 7 degrees or 20 degrees C, when measured at their respective growth temperatures. The poor ability for photosynthetic acclimation in these species may be associated with the relatively stable m aritime temperature regime during the growing season along the Peninsula. I n contrast to P-n, both species exhibited full acclimation of R-d, and rate s of R-d on a leaf-area basis were similar among treatments when measured a t their respective daytime growth temperature. Our results suggest that in the absence of interspecific competition, conti nued warming along the Peninsula will lead to improved vegetative growth of these species due to (1) greater biomass allocation to leaf-area productio n (as opposed to improved rates of P-n per leaf area) and (2) their ability to acclimate R-d, such that respiratory losses per leaf area do not increa se under higher temperature regimes.