Abstract Detail

Ecological Section

North, Gretchen [1], Garrett, Tadao [2].

Root contraction helps “living rock” cacti avoid lethal temperatures when growing in rocky soil.

Small desert succulents such as Ariocarpus fissuratus, or “living rock” cactus, endure some of the hottest habitats occupied by terrestrial plants. The shoot surface of A. fissuratus is usually flush with the soil surface in the field, suggesting two hypotheses: 1) shoots are pulled down in the soil column by root contraction, and 2) shoot temperatures are lower for plants that are lower in the soil. Root contraction was investigated by inserting wires at the stem base in greenhouse-grown plants and measuring changes in wire height under well-watered and drying conditions, correcting for contraction in the soil and shoot. Root contraction pulled shoots down by ca. 7 mm over six months under well-watered conditions and 13 mm under drying conditions, decreases that were significant with respect to time but not level of soil moisture. Analysis of root xylem conduits (wide-band tracheids) further demonstrated root contraction. Specifically, the distance between lignified bands in uncontracted root regions was 22.1 micrometers versus 19.1 micrometers in contracted root regions, which together with the sinuous appearance of the tracheids was taken as evidence of longitudinal contraction. Plant temperature were investigated by embedding small temperature sensor/recorders in stem tissue of A. fissuratus with shoots at 30 mm above the soil surface versus shoots level with the soil surface (buried stems) in two soil conditions: bare soil or in soil covered by small rocks; plants were grown in large containers outdoors on the Occidental College campus in Los Angeles. During one week of extended high temperatures, buried stems in rocky soil reached a maximum of 56 degrees C, whereas buried stems in bare soil exceeded 60 degrees C. High-temperature tolerance of comparable plants as concurrently measured by stain uptake was 57 degrees C. Indeed, all plants with buried stems in rocky soil survived, whereas those in bare soil did not. Thus, root contraction does help living rock cacti avoid lethal high temperatures, but only in rocky soil.

Broader Impacts:
Intellectual: Despite the physiological adaptation of contractile roots, plants of Ariocarpus fissuratus appear to be growing at temperatures within a few degrees of their high temperature tolerance. Further warming of their habitat due to anthropogenic climate modification could threaten the survival of this species in much of its current range. Personal: Almost all of the data in this presentation was collected by an Occidental College undergraduate. He and I were funded by NSF grant #0517740.

1 - Occidental College, Biology, 1600 Campus Rd, Los Angeles, California, 90041, US
2 - Occidental College, Biology, 1600 Campus Rd, Los Angeles, CA, 90041, USA

Keywords:
root contraction
desert biota
xylem.

Presentation Type: Oral Paper:Papers for Sections
Session: 33
Location: 555B/Convention Center
Date: Tuesday, August 3rd, 2010
Time: 10:15 AM
Number: 33001
Abstract ID:548