Effects of Environment on Vessel Characters in Cactus Wood

Abstract

Vessel size classes within several cactus species were studied to determine if theoretically advantageous patterns of vessel widths are produced in plants cultivated in dry conditions. Clones of five cacti (Cleistocactus, Harrisia, Pereskia, Rhipsalis, Selenicereus) were given either mesic or xeric watering regimens for more than 2 yr, and then were studied with light microscopy. Plants cultivated in drought conditions all produced a modified wood that had characters suited for surviving water deficits. All plants cultivated in xeric conditions produced wood with narrower vessels in both roots and shoots. Relative abundances of narrow and wide vessels varied with cultivation conditions: xeric‐cultivated plants had greater percentages of narrow vessels (rather than being normally distributed, vessel widths were positively skewed), whereas mesic‐cultivated plants had comparatively lower percentages of narrow vessels (negatively skewed, or at least less positively skewed). If wide vessels cavitate before narrow ones during water stress, then xeric‐cultivated plants produced wood that had so many narrow vessels that even if a plant were to lose half its conducting capacity from cavitation of some of its widest vessels, it would still have a large fraction of its vessels water filled and functional. In contrast, mesic‐cultivated plants would have smaller fractions of their vessels functional at the point when half their wood’s conducting capacity had been lost. Three of the species studied are capable of producing dimorphic wood: they produce one type of wood while young and a second type when older. Xeric cultivation inhibited or retarded the transition to the second wood type.