TEMPERATURE INFLUENCES ON ROOT-GROWTH FOR ENCELIA-FARINOSA (ASTERACEAE), PLEURAPHIS-RIGIDA (POACEAE), AND AGAVE-DESERTI (AGAVACEAE) UNDER CURRENT AND DOUBLED CO2 CONCENTRATIONS
Pm. Drennan et Ps. Nobel, TEMPERATURE INFLUENCES ON ROOT-GROWTH FOR ENCELIA-FARINOSA (ASTERACEAE), PLEURAPHIS-RIGIDA (POACEAE), AND AGAVE-DESERTI (AGAVACEAE) UNDER CURRENT AND DOUBLED CO2 CONCENTRATIONS, American journal of botany, 83(2), 1996, pp. 133-139
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.