Altitudinal Temperature Gradients Research Articles

Germination of Verbesina enceliodies and Rumex nervosus from South Hijaz

Verbesina encelioides, an annual herb, is a common weed of cultivated fields and waste land. Rumex nervosus is a perennial herb which inhabits lower mountain slopes. Their geographical distribution in Saudi Arabia is limited to the mountainous region of South Hijaz. Comparisons were made between the germination responses to temperature of freshly harvested seeds of four populations of V. encelioides and two of R. nervosus collected from different localities. The seeds were germinated over a temperature range of 5–40 °C at intervals of 5 °C, and also over a range of alternating temperatures (18/8, 21/10, 25/15, 31/16 and 36/21 °C). The geobotanical distribution of the two species is reflected in their responses: R. nervosus shows wide ecological amplitude with regard to its germination temperature responses. The possibility that V. encelioides may have developed an ecotype in response to altitudinal temperature gradients is inferred. This ecotype widens the ecological amplitude of the species with regard to its germination temperature responses and may extend the distribution of the species into warmer regions.

C3 photosynthesis and high temperature acclimation of CAM in opuntia basilaris engelm. and bigel.

Carbon fixation by CAM at high night temperatures was examined in the stem succulent, Opuntia basilaris. Nighttime accumulation of titratable acids was uniformly high among plants growing naturally along an altitudinal temperature gradient in Death Valley, California during the hot summer period. Plants grown at high temperature regimes (40°/30°C) had rates of CAM and C3 fixation similar to rates observed in plants maintained at a cool temperature (20°/10°C). C3 fixation comprised 30% of the total carbon fixed by the potted, well watered plants. However, when pads were excised, C3 fixation was suppressed while CAM continued unabated.