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Park S. Nobel and Miguel Castañeda

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 °C, equally likely to be either organ at 25/15 °C, and 10 times more likely to be daughter cladodes than fruit at 35/25 °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.

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Avinoam Nerd and Park S. Nobel

Water relations and fruit development were studied for up to 100 days after anthesis for potted plants of Opuntia ficus-indica (L.) Mill. (a prickly pear) that were either well-watered or water-stressed, each plant consisting of a medium-sized cladode bearing two or three fruit. Even though cladodes of water-stressed plants lost up to 50% of their thickness, their fruit continued to gain water and to develop; at ripening such fruit had only 16% less water than fruit of watered plants. Maturation indicated by the decrease in fractional peel content and increases in pulp weight and in pulp soluble sugar content was hastened by water stress, leading to ripening ≈88 days after anthesis for water-stressed plants, which was 10 days earlier than for watered plants. Fruit had a lower stomatal frequency than the cladodes but both exhibited Crassulacean acid metabolism behavior. Transpiration occurred mainly at night, and the daily amount of water transpired per unit fruit surface area decreased with time, especially for fruit of water-stressed plants. This decrease was related to fruit expansion (leading to decreased stomatal frequency) for watered plants and to both fruit expansion and water stress for water-stressed plants. At 75 days after anthesis, daily diameter changes of fruit were correlated with transpiration, contraction occurring at night and expansion during the daytime, and changes were greater for watered plants for which daily transpiration was higher.

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Eulogio Pimienta-Barrios and Park S. Nobel

Flower and fruit production by the columnar cactus, Stenocereus queretaroensis (Weber) Buxbaum, occurred during the dry season in the late winter and spring, and the relatively small annual stem extension occurred primarily during the fall. Thus, reproductive growth does not directly compete with vegetative growth for resources such as reducing sugars, which increased during the wet summer season, a period when total sugars were decreasing. Stem extension, reproductive demography, fruit quality, seed size, and seed quality were not influenced by irrigation. Final fruit size and seed germination, however, were enhanced by applying water. The times from flower bud differentiation to flower opening and from anthesis to fruit ripening were relatively short and unaffected by irrigation.

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Victor Garcia de Cortázar and Park S. Nobel

Productivity of irrigated prickly pear cactus [Opuntia ficus-indica (L.) Miller] was studied over 3 years in central Chile using two planting densities. A low-density planting (0.25 plants/m2), traditionally favored for fruit production, had maximal fruit productivity in the 2nd year (6 Mg dry weight/ha per year). A high-density planting (24 plants/m2), which assured almost full interception of incident solar radiation, led to an extremely high shoot dry-weight productivity (50 Mg·ha-1·year-1) in the 2nd year and maximal fruit productivity (6 Mg·ha-1·year-1) in the 3rd year. Cladode dry weight tended to increase with cladode surface area. However, fruit production did not occur until the dry weight per cladode exceeded the minimum dry weight for a particular cladode surface area by at least 33 g. The year-to-year variation in fruit production apparently reflected variations in such excess dry weight and, hence, in the storage reserves of individual cladodes.