Wide-Band Tracheids from a Southern African Succulent and Their Responses to Varying Light Intensities: A Pre-Adaptation for Future Water Stress?

Examination of leaf and stem tissues in a broad range of genera resulted in the discovery and analysis of a novel tracheid type termed Wide-Band Tracheids (WBTs; the term is derived from the comparatively wide secondary wall) in derived genera of Aizoaceae, Cactaceae and Portulacaeae (Caryophyllales). In Aizoaceae, WBTs are only found in genera of Ruschioideae; in Cactaceae, WBTs are found in xylem of leaves and stems in genera of Opuntioideae and Cactaceae. However, in the genus Anacampseros (Portulacaceae), WBTs are found in leaf xylem, but not as part of the xylem of the stems and instead, WBTs are found in piths and rays. It was hypothesized that the wide secondary wall prevents primary wall contact during extreme water stress and thus WBTs were thought to differentiate in response to water-stress. In order to determine what factors cause WBT initiation and differentiation, seedlings of Anacamperos rufescens (Portulacaceae) were exposed to varying light intensities that mimicked spring and summer light levels found in southern Africa. In this experiment, results show that WBTs are generally formed in advance of probable water-stress event times. Furthermore, the number of WBTs are directly correlated to the intensity of light received as a seedling; however, the mean WBT size remained relatively unchanged, presumably due to a rigid genetic control. Results suggest that, in Anacampseros rufescens, the later a seedling germinates, the greater the number of WBTs differentiate, which pre-adapts the plant for future water-stress events.