Several succulent species in various families, including Agavaceae, Asphodelaceae, and Cactaceae, have contractile roots capable of pulling the plant further down in the soil, thereby positioning the shoot and/or roots in a more favorable location. Because many members of these families occur in hot, dry habitats, the relationship between the amount of root contraction, mean annual rainfall, and high and low annual temperatures was investigated using measurements of annual shoot descent as a proxy for root contraction, herbarium records for species ranges, and published climate data. The relative importance of rainfall versus temperature extremes in determining root contraction, and thus the importance of root contraction in evading unfavorable conditions, was initially assessed by regression analysis and analysis of covariance. In addition, root contraction and tissue temperature tolerances were measured for plants grown in environmental chambers set at 24/18 versus 10/5 degrees C daytime/nighttime temperatures. Root contraction and thermal tolerances were also measured for smaller succulents and seedlings in soil microcosms on the roof of the biology building at Occidental College using thermal sensors embedded in plants and soil.

Results/Conclusions

Greater root contraction was significantly associated with lower rainfall (P < 0.001). The regression coefficient averaged only about 0.60 for 24 species in the Agavaceae, however, indicating the likely importance of other variables. The regression coefficient for the relationship between root contraction and mean annual low temperature was higher, averaging 0.72 (P < 0.01). Species with noncontractile roots showed the least amount of low-temperature tolerance, their lethal low temperature averaging 5.2 degrees higher than for species with contractile roots. In addition, species from colder habitats showed greater root contraction and a greater difference in the amount of contraction between plants from the two growth chamber temperature regimes than did species native to warmer regions. For the small cacti and agaves in soil microcosms, diurnal patterns of temperatures in the shoot above the soil, at the soil line, and in the shoot below the soil line suggest that root contraction may be more important for low than for high temperature avoidance. For the agaves, cacti, and yuccas examined, both high and low temperature tolerances were positively related to the amount of root contraction, indicating the importance of extreme temperature avoidance for succulents, particularly when small.