Carbon balance of Yucca elata Engelm. during a hot and cool period in situ

Respiratory activity of intact, attached roots was measured under field and controlled conditions. Root respiration of Yucca elata Engelm. was highly temperature dependent: Q10values decreased from 2.1 (12-22° C) to 1.7 (26-36° C) as temperatures increased. Respiration ceased after 5 h at 42° C. In the field, in August, when net leaf photosynthesis was severely depressed, the diel fluctuation in the respiration rate of suberized and partially suberized roots was predominantly a function of temperature. A photoperiod-associated rise in respiration rates apart from temperature response occurred in February for nonsuberized, partially suberized, and suberized roots when active net photosynthesis occurred throughout the photoperiod. In whole-plant root systems, respiratory CO₂ was 3.2 and 4.3 mg CO₂·g$\text{DW}^{-1}\cdot \text{d}^{-1}$in August and February, respectively, when adjusted for the proportion of suberized and nonsuberized lateral roots. On a whole-plant basis, 0.89 mg C·g$\text{DW}^{-1}\cdot \text{d}^{-1}$was gained during February and 0.46 mg C·g$\text{DW}^{-1}\cdot \text{d}^{-1}$was lost in August. The belowground:aboveground ratio of whole-plants in situ was 0.42 on a shallow soil where vertical root growth was limited to a soil depth of 68 cm and ranged from 1.29 to 5.94 (x̄=3.31) in deep sands. No leaf dark fixation of CO₂ was observed in field plants during August and February, nor in well-watered plants or plants subjected to drought in laboratory studies. Although small diel fluctuations in leaf acidity occurred in both field and greenhouse-grown plants, results of this study suggest that Y. elata is a C₃ plant.