Temperature and vegetative growth of three Saudi Arabian herbs

In Saudi Arabia, Artemisia abyssinica is a perennial, sand-stabilizing species which is common in the great inland deserts; Rumex nervosus is a medicinal herb of lowland distribution; Verbesina encelioides is an arable weed of similar distribution to R. nervosus, but is also locally common in certain fertile inland and montane habitats. Typical environments of these species are sharply contrasted in climate. To see if temperature played any part in limiting these species to their respactive habitats, seedlings of all three (including two potentiallyseparate lowland and montane populations of V. encelioides) were grown under controlled-environment conditions over a range of different temperatures. Artemisia abyssinica proved to be innately slow-growing and to exhibit a high temperature optimum, though with a shallow response-gradient in respect of growth over-all and of partitioning between root and shoot. Rumex nervosus and both populations of V. encelioides were quite unlike this, being faster-growing, plastic, and with clear, low temperature-optima Despite certain morphological distinctions, the two populations of V. encelioides differed little in their whole-plant physiology. However, previous work had established that the montane population, like A. abyssinica, was protected from germination at high constant or fluctuating temperatures. The lowland population, like R. nervosus, was not. Differential conbinations of germinative and vegetative behaviour were thus postulated as facilitating the wide natural distribution of V. encelioides. General conclusions were also drawn concerning the location on a mesic-xeric continuum of the three species, and also of two others studied in a set of experiments published previously.