TEMPERATURE INFLUENCES ON ROOT-GROWTH FOR ENCELIA-FARINOSA (ASTERACEAE), PLEURAPHIS-RIGIDA (POACEAE), AND AGAVE-DESERTI (AGAVACEAE) UNDER CURRENT AND DOUBLED CO2 CONCENTRATIONS

To help evaluate root distribution patterns, elongation rates of indiv idual roots were measured as a function of soil temperature for Enceli a farinosa (a C-3 species), Pleuraphis rigida (C-4), and Agave deserti (CAM), sympatric codominants in the northwestern Sonoran Desert. Meas urements were made at current and doubled CO2 concentrations under win ter and summer conditions of air temperature (day/night temperatures o f 17 C/10 C and 33 C/22 C, respectively). The three species had differ ent optimal temperatures for root elongation (T-opt) under winter cond itions (25 C for E. farinosa, 35 C for P. rigida, and 30 C for A. dese rti); T-opt increased by 2-3 C under summer conditions for all three s pecies. The limiting temperatures for elongation also acclimated from winter to summer conditions. The rate of root elongation at T-opt was higher under summer than winter conditions for E. farinosa (3 vs. 6 mm d(-1)) and P. rigida (20 vs. 14 mm d(-1)), reflecting conditions for maximum photosynthesis; no difference occurred for A. deserti (9 vs. 1 0 mm d(-1)). Decreased elongation rates at extreme temperatures were a ssociated with less cell division and reduced cell extension. The doub led CO2 concentration increased average daily root elongation rates fo r A. deserti under both winter (7%) and summer (12%) conditions, refle cting increased cell extension, but had no effect for the other two sp ecies. Simulations of root elongation as a function of soil temperatur es showed that maximum elongation would occur at different depths (16- 20 cm for E. farinosa, 4-8 cm for P. rigida, and 0-4 cm for A. deserti ) and during different seasons (winter to spring for E. farinosa, spri ng to summer for P. rigida, and all year for A. deserti), contributing to their niche separation. Shading of the soil surface moderated dail y variations in soil temperature, reducing seasonal root elongation fo r winter and spring and increasing elongation for summer. Shading also altered root distribution patterns, e.g., optimal rooting depth for A . deserti and especially P. rigida increased for a hot summer day.