Growth and predicted productivity of Opuntia ficus-indica for current and elevated carbon dioxide

Opuntia ficus-indica (L.) Mill., a prickly pear cactus cultivated worldwide for its fruits and stem segments, can have an annual dry weight productivity exceeding that of many crops. Using a recently introduced environmental productivity index (EPI), the influences of water status, temperature, and photosynthetically active radiation (PAR) on its productivity can be predicted. This investigation calculated the water index, the temperature index, and the PAR index, whose product equals EPI, for 169 sites distributed approximately uniformly across the contiguous USA for present climatic conditions as well as for those associated with an elevated CO2 concentration of 650 microliter L-1. The effect of elevated CO2 on growth of O. ficus-indica was directly measured, and low temperature limitations on productivity were considered. The dry weight gain of O. ficus-indica during 6 mo in an environmental growth chamber was 23% greater at 650 compared with 350 microliter L-1 CO2 and increased as the duration of the wet period increased, in agreement with predictions of the water index (the fraction of maximal net CO2 uptake during a 24-h period for the prevailing plant water status). For closely spaced plants that lead to a high productivity per unit ground area, EPI averaged about 0.10, except in desert regions where the water index lowered EPI, in the far North or South and at high elevations where the temperature index lowered EPI, and in the Northeast and Northwest where the PAR index lowered EPI. The predicted annual dry weight productivity for O. ficus-indica was 12.8 Mg ha-1 yr-1 under current conditions, and 16.3 Mg ha-1 yr-1 under those associated with 650 microliter L-1 CO2. Both productivities are relatively high compared with other agronomic plants. The percentage of sites where temperatures fall below -15 degrees C at least once during the 10 years simulated, which would be lethal to most prickly pear cacti, was reduced from 49 to 18% by the general warming expected to accompany an approximate doubling of the atmospheric CO2 concentration.