Agavespecies are high‐yielding crassulacean acid metabolism (CAM) plants, some of which are grown commercially and recognized as potential bioenergy species for dry regions of the world. This study is the first field trial ofAgavespecies for bioenergy in the United States, and was established to compare the production ofAgave americanawith the production ofAgave tequilanaandAgave fourcroydes, which are produced commercially in Mexico for tequila and fiber. The field trial included four experimental irrigation levels to test the response of biomass production to water inputs. After 3 years, annual production of healthyA. americanaplants reached 9.3 Mg dry mass ha⁻¹ yr⁻¹(including pup mass) with 530 mm of annual water inputs, including both rainfall and irrigation. Yields in the most arid conditions tested (300 mm yr⁻¹water input) were 2.0–4.0 Mg dry mass ha⁻¹ yr⁻¹.Agave tequilanaandAgave fourcroydeswere severely damaged by cold in the first winter, and produced maximum yields of only 0.04 Mg ha⁻¹ yr⁻¹and 0.26 Mg ha⁻¹ yr⁻¹, respectively. The agave snout weevil (Scyphophorus acupunctatus) emerged as an important challenge forA. americanacropping, killing a greater number of plants in the higher irrigation treatments. Physiological differences inA. americanaplants across irrigation treatments were most evident in the warmest season, with gas exchange beginning up to 3 h earlier and water use efficiency declining in treatments with the greatest water input (780 mm yr⁻¹water input). Yields were lower than previous projections forAgavespecies, but results from this study suggest thatA. americanahas potential as a bioenergy crop and would have substantially reduced irrigation requirements relative to conventional crops in the southwesternUSA. Challenges for pest management and harvesting must still be addressed before an efficient production system that usesAgavecan be realized.

Climate‐resilient and highly water‐use efficient crops are needed to meet the future food, feed, fiber, and fuel demands of a growing human population. Cactus pear (Opuntia spp.) are highly productive yet have crop water demands that are approximately 20% that of traditional crops due to their ability to perform crassulacean acid metabolism (CAM). In the first long‐term field trial of Opuntia for bioenergy in the United States, the aboveground biomass (cladode and fruit) productivity and response to different water inputs of O. cochenillifera, O. ficus‐indica, and O. streptacantha were evaluated at a planting density of 1418 plants ha⁻¹ under three irrigation levels over a 5‐year period to investigate their response to different water inputs. Mean cladode fresh weight, cladode dry weight, cladode count, fruit fresh weight, and fruit dry weight increased linearly by 1.26‐, 1.88‐, 1.53‐, 1.89‐, and 2.13‐fold, respectively, with increasing irrigation from 300 to 716 mm year⁻¹. Significant differences in irrigation response appeared in years 4 and 5 as the cactus trees grew larger. However, no significant differences were observed among the three Opuntia spp. for all parameters measured. Biomass productivity increased over 5 years by 140.8‐ and 132.5‐fold for cladode fresh and dry weight, respectively. Mean annual total aboveground biomass at 300, 407, and 716 mm year⁻¹ water input resulted in 8.25, 11.16, and 15.52 Mg dry mass ha⁻¹ year⁻¹, respectively, at the sparse planting density used here to separate irrigation blocks, which could have been easily quadrupled. O. ficus‐indica performed better than O. cochenillifera and O. streptacantha in terms of cladode biomass and fruit count and quality. Biomass productivity was consistent with previous studies for Opuntia spp. demonstrating that cactus pear displays very high biomass and food/forage potential using substantially lower rates of supplemental irrigation than conventional crops grown in semi‐arid and arid regions.

The relative contribution of C₃ photosynthesis and crassulacean acid metabolism (CAM) during the earliest stages of development were investigated to assess how much each might contribute to cactus pear (Opuntia ficus-indica) productivity. The developmental progression of C₃ photosynthesis and CAM was assessed in seedlings and daughter cladodes of mature plants by titratable acidity, δ ¹³C carbon isotopic values, and diel gas exchange measurements. Nocturnal acidification was observed in seedling cladodes and cotyledons at the earliest stages of development and which became highly significant by 75 days of development. Seedlings cotyledons showed mean δ ¹³C values of -21.4‰ to -17.1‰ from 30 to 100 days of age, respectively. Seedlings cladodes showed mean δ ¹³C values of -19.4‰ to -14.5‰ from 30 to 100 days of age, respectively. These values were typical of CAM plants. Net CO₂ assimilation was negative, then occurred in both the day and the night, with nighttime fixation becoming predominant once the primary cladode reached five cm in size. Emergent daughter cladodes growing on mature plants showed nocturnal titratable acidity at the earliest stages of development, which became significant when daughter cladodes were >2.5-5 cm in height. Emergent daughter cladodes showed mean δ ¹³C values of -14.5‰ to -15.6‰ typical of CAM plants. CO₂ assimilation studies revealed that net CO₂ uptake was negative in daughter cladodes <12 cm in length, but then exhibited net positive CO₂ assimilation in both the day and the night, with net nocturnal CO₂ assimilation predominating once the daughter cladode grew larger. Developing O. ficus-indica primary and daughter cladodes begin as respiring sink tissues that transition directly to performing CAM once net positive CO₂ fixation is observed. Overall, these results demonstrate that CAM is the primary form of photosynthetic carbon assimilation for O. ficus-indica even at the earliest stages of seedling or daughter cladode development.

Cactus pear (Opuntia-ficus indica (L.) Mill.) is an important agricultural crassulacean acid metabolism (CAM) species used as a source of food, forage, fodder, and secondary products and as a biofuel feedstock. However, the preferred source of nitrogen for this species, whether it be nitrate (NO₃⁻), ammonium (NH₄⁺), or a combination of both, is not well understood. To investigate the nitrate and ammonium preference of cactus pear, we grew cladodes in sand culture with deionized water as a control or with a cross-factorial set of nutrient solutions of 0.0, 2.5, 5.0, and 10.0 mmol of nitrate and/or ammonium for one month. We then assessed a set of physiological parameters including cladode growth, relative water content, chlorophyll, tissue acidity, soluble sugars, starch, nitrate, ammonium, glyoxylic acid, nitrate reductase activity, and nitrogen and carbon content. We found significant differences in all measured parameters except for cladode length, relative water content, and carbon content. Cladodes provided with only deionized water produced no new cladodes and showed decreased soluble sugar content, increased starch content, and increased tissue acidity. We also determined the relative steady-state transcript abundance of genes that encode enzymes involved in N metabolism and CAM. Compared with control cladodes, nutrient-supplied cladodes generally showed increased or variable steady-state mRNA expression of selected CAM-related genes and nitrogen-metabolism-related genes. Our results suggest that O. ficus-indica prefers fertilizers containing either equal concentrations nitrate and ammonium or more nitrate than ammonium.