Te. Huxman et al., Gas exchange and chlorophyll fluorescence responses of three south-westernYucca species to elevated CO2 and high temperature, PL CELL ENV, 21(12), 1998, pp. 1275-1283
The ability of seedlings to tolerate temperature extremes is important in d
etermining the distribution of perennial plants in the arid south-western U
SA, and the manner in which elevated CO2 impacts the ability of plants to t
olerate high temperatures is relatively unknown, Whereas the effects of chr
onic high temperature (30-38 degrees C) and elevated CO2 are comparatively
well understood, little research has assessed plant performance in elevated
CO2 during extreme (> 45 degrees C) temperature events, We exposed three s
pecies of Yucca to 360 and 700 mu mol CO, mol(-1) for 8 months, then 9 d of
high temperature (up to 53 degrees C) to evaluate the impacts of elevated
CO2 on the potential for photosynthetic function during external high tempe
rature. Seedlings of a coastal C-3 species (Yucca whipplei), a desert C-3 s
pecies (Yucca brevifolia), and a desert CAM species (Yucca schidigera), wer
e used to test for differences among functional groups. In general, Yuccas
exposed to elevated CO2 showed decreases in carboxylation efficiency as com
pared with plants grown at ambient before the initiation of high temperatur
e, The coastal species (Y. whipplei) showed significant reductions (33%) in
CO2 saturated maximum assimilation rate (A(max)), but the desert species (
Y. brevifolia and Y. schidigera) showed no such reductions in A(max). Stoma
tal conductance was lower in elevated CO2 as compared with ambient througho
ut the temperature event; however, there were species-specific differences
over time. Elevated CO2 enhanced photosynthesis in Y. whipplei at high temp
eratures for a period of 4 d, but not for Y. brevifolia or Y. schidigera, E
levated CO2 offset photoinhibition (measured as F-v/F-m) in Y. whipplei as
compared with ambient CO2, depending on exposure time to high temperature.
Stable F-v/F-m in Y. whipplei occurred in parallel with increases in the qu
antum yield of photosystem II (Phi PSII) at high temperatures in elevated C
O2. The value of Phi PSII remained constant or decreased with increasing te
mperature in all other treatment and species combinations, This suggests th
at the reductions in F-v/F-m resulted from thermal energy dissipation in th
e pigment bed for Y. brevifolia and Y. schidigera, The greater efficiency o
f photosystem II in Y. whipplei helped to maintain photosynthetic function
at high temperatures in elevated CO2. These patterns are in contrast to the
hypothesis that high temperatures in elevated CO2 would increase the poten
tial for photoinhibition, Our results suggest that elevated CO2 may offset
high-temperature stress in coastal Yucca, but not in those species native t
o drier systems, Therefore, in the case of Y. whipplei, elevated CO2 may al
low plants to survive extreme temperature events, potentially relaxing the
effects of high temperature on the establishment in novel habitats.