Plant morphology and gas exchange in Hesperaloe: Influence on its adaptation for cultivation.

Abstract

The working hypothesis for this dissertation was that the contrasting plant architecture, and the distinct native environments of Hesperaloe funifera and H. nocturna should be reflected in differences in their eco-physiological responses, and that these, in turn, would determine these species' adaptability limits and productivity under cultivation. The objective of this dissertation project was to evaluate plant morphological characteristics and their effect on physiological processes, and how environmental factors interact with these processes in H. funifera and H. nocturna. Common to both Hesperaloe funifera and H. nocturna is the possession of constitutive crassulacean acid metabolism (CAM). The major implication of CAM is a prominent water use efficiency (WUE). Although instantaneous net CO₂ uptake rates of both Hesperaloe funifera and H. nocturna are low when compared with C₃ plants, integrated 24-hr net CO₂ uptake rates are within the lower range of C₃ species. Because there is a relatively large range of conditions in which photosynthesis occurs (i.e., both Hesperaloe species have a long growing season), net CO₂ uptake in Hesperaloe integrated throughout the year may reach or surpass that of C₃ crops. Daily integrated CO₂ uptake during late fall, winter and early spring were similar in Hesperaloe funifera and H. nocturna. A major peak in CO₂ uptake was found during the fall for H. funifera but not for H. nocturna. Also, no reduction in integrated CO₂ uptake was found in H. nocturna during the summer months, while H. funifera showed a considerable reduction in CO₂ uptake during this season. The increase in photosynthetic activity in H. nocturna coincided with the emergence of the inflorescence. Contrastingly, flowering did not appear to increase net CO₂ uptake in H. funifera. In this species a flush of new leaf production (increased sink demand) coincides with the observed increase in photosynthesis during the fall. The late fall peak of CO₂ uptake found in H. funifera could also be caused by a photoperiodic stimulation of CO₂ uptake. Significant differences in WUE between the two Hesperaloe species were found. These differences were, at least, partially explained by contrasting plant architecture and differences in leaf morphology.