Abstract: The stomotal conductance, transpiration and water use efficiency (WUE) were measured using a LI-6400 portable photosynthesis system for 5 tropical rain forest species and 5 desert species in Biosphere 2, USA. All the species have experienced in very high CO2 ( > 2 200 μmol• mol- 1 ) for more than 4.5 years. The results showed that the stomatal conductance and transpiration of rain forest species decreased from ( 127.4 ± 65.6) and (2.04 ± 0.61 ) mmol• m- 2•s- 1 to (61.3 + 30.5) and ( 1.54 ± 0.65 ) mmol• m-2• s -1 respectively, while WUE increased from (2.90 ± 0.55) to (8.45 ± 2.71) μmol CO2 •mmo1-1 H2O, with CO2 increasing from 350 – 400 to 700 – 820 μmol• mol-l. For the desert species, stomatal conductance and transpiration decreased from respectively (142.8±94.6) and (2.09±0.71) mmol•m-2•s-1 to (57.7±35.8) and (1.36±0.52) mmolm-2•s-l, but WUE increased from (4.69 ± 1.39) to (9.68 ± 1.61) μmol CO2•mmo1-1 H2O, with the CO2 increase from 320 - 400 to 820 – 850 μtmol• mol- 1. The stomatal conductance, transpiration and WUE were less influenced by light intensity under high CO2 than low CO2 concentrations. Most rain forest species reached their light saturation points at light intensity of 500 μmol• m-2•s-1, while desert species at 1 000 μmol•m-2•s-1. Among different species, the desert C3 tree, Nicotiana glauca Grah., had the highest decrease in stomatal conductance and transpiration and the highest increase in WUE, by 78%, 69% and 310% respectively. The enhancement of increasing CO2 to the stomatal, transpiration and WUE of species with different photosynthesis pathway and life forms in Biosphere 2 could be concluded as: C3 species > C4 species, and desert C3 species > rain forest C3 species.

Key words: Stomatal conductance, Transpiration, Water use efficiency, CO2 elevation, Biosphere 2