Seasonal, light, and temperature influences on organ initiation for unrooted cladodes of the prickly pear cactus Opuntia ficus-indica

Detached, unrooted cladodes (stem segments) of the widely cultivated prickly pear cactus Opuntia ficus-indica (L.) Miller (Cactaceae), which remain alive for at least 12 months, provide a model system for investigating stem responses to environmental factors, Initiation of organs varied seasonally; on average 2.14 new organs were initiated per cladode over a 16-week period in a glasshouse for cladodes detached in winter, 0.76 when detached in late spring, and only 0.07 when detached in late summer, Shading by 45% halved new organ initiation and shading by 95% decreased it by 96%. The seasonal and light responses for new organ initiation are consistent with field observations on O. ficus-indica. For detached cladodes maintained in environmental chambers for 14 weeks, the new organs were 10 times more likely to be fruit than daughter cladodes at day/night temperatures of 15/5 degrees C, equally likely to be either organ at 25/15 degrees C, and 10 times more likely to be daughter cladodes than fruit at 35/25 degrees C., Decreasing the shading or the temperature favored stomatal opening, as shown by increases in the dry mass/fresh mass ratio of the detached cladodes, Such increased stomatal opening was accompanied by increased photosynthetic activity, as shown by greater starch content and higher concentrations of sucrose, glucose, and fructose, Why low day/night temperatures favored reproductive structures and high temperatures favored vegetative ones is not clear, but future research using unrooted cladodes may help elucidate the mechanisms involved.