Morphological and physiological properties of poinsettia (Euphorbia pulcherrima) leaves and bracts in relation to sulfur-dioxide sensitivity
Krizek, D.T.; Wergin, W.P.; Semeniuk, P.
Environmental & Experimental Botany 25(2): 165-174
Four cultivars of poinsettia belonging to 2 distinct groups and found previously to differ in sensitivity to SO2, were investigated to determine the possible morphological and physiological bases for differences in SO2 insensitivity. Leaves and bracts (lower and upper surfaces) were examined by scanning electron microscopy to determine the occurrence and frequency of stomata and trichomes. Comparative measurements of stomatal conductance and transpiration rates were made under controlled environments with a diffusive resistance porometer. Both insensitive cultivars (i.e., 'Ruff and Reddy' and 'Paul Mikkelsen') had leaves characterized by lower stomatal density, higher trichome density, lower stomatal conductance and lower transpiration rates than those of the 2 sensitive cultivars examined (i.e. 'Rudolph' and 'Mikkel Rochford Pink'). Stomatal behavior and certain leaf surface properties, e.g. stomatal and trichome density, which influence the rate of SO2 penetration into the plant, may be useful criteria in selecting and breeding cultivars of poinsettia that are insensitive to SO2. Since bracts contained relatively few stomata and variable numbers of trichomes, the surface properties of these organs do not appear to provide a useful criterion for assessing SO2 sensitivity.