Temperature and the Vegetative Growth of Three Saudi Arabian Herbs

In Saudi Arabia, Artemisia abyssinica is a perennial, sand-stabilizingspecies which is common in the great inland deserts; Rumex nervosusis a medicinal herb of lowland distribution; Verbesina encelioidesis an arable weed of similar distribution to R. nervosus, butis also locally common in certain fertile inland and montanehabitats. Typical environments of these species are sharplycontrasted in climate. To see if temperature played any partin limiting these species to their respactive habitats, seedlingsof all three (including two potentiallyseparate lowland andmontane populations of V. encelioides) were grown under controlled-environmentconditions over a range of different temperatures. Artemisia abyssinica proved to be innately slow-growing andto exhibit a high temperature optimum, though with a shallowresponse-gradient in respect of growth over-all and of partitioningbetween root and shoot. Rumex nervosus and both populationsof V. encelioides were quite unlike this, being faster-growing,plastic, and with clear, low temperature-optima Despite certain morphological distinctions, the two populationsof 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 constantor fluctuating temperatures. The lowland population, like R.nervosus, was not. Differential conbinations of germinativeand vegetative behaviour were thus postulated as facilitatingthe wide natural distribution of V. encelioides. General conclusions were also drawn concerning the locationon a mesic-xeric continuum of the three species, and also oftwo others studied in a set of experiments published previously. Artemisia abyssinica Sch. Bip., Rumex nervosus Vahl., Verbesina encelioides (Cav.) Benth. et Hook., F. ex A. Gray, growth analysis, temperature, rsponse surfaces, plant distribution