This dissertation consists of an introduction and three independent research chapters. Chapter one (introduction) presents a review of CAM (Crassulacean Acid Metabolism) biology, chlorophyll fluorescence, and leaf optical properties. Chapter two addresses the physiology, anatomy, and biochemistry of two tropical Peperomia species in the field in the El Cielo Biosphere Reserve in Tamaulipas, Mexico. Chapters three and four address similar studies in two morphs of Agave striata (red vs. green leaves) under natural condition in the high plateau desert region of Miquihuana, Tamaulipas, Mexico. The summary for each of the research chapters is discussed.
In chapter two, I tested, under field conditions, the hypotheses that relative thickness of the spongy mesophyll layer is an indication of CAM and that water distribution within the leaf tissues of the two Peperomia species studied changes differently in response to drought. The results showed that when well-watered, P. obtusifolia has typical C3-activity and P. macrostachya has typical CAM-activity, with spongy mesophyll layer thickness of 26% and 76% respectively. In addition, the results also showed that the distribution of water in the leaf tissue in P. obtusifolia changes in response to drought, from the hydrenchyma to the chlorenchyma, but this is not the case in P. macrostachya.
In chapter three, I tested the hypothesis that leaf epidermal pigments screen light of particular wavelengths from reaching the photosynthetic machinery, reducing dependence on the xanthophyll cycle as an energy dissipation process in two morphs of Agave striata (red vs. green leaves). The results showed that Fo (dark level fluorescence), Fv/Fm (maximum PSII efficiency), and ΦPSII (quantum yield of PSII) were higher under sun in leaves of the red morph compared to the green morph. Leaf surface temperature, xanthophyll conversion, and qN (non-photochemical quenching) were significantly higher during the day for leaves of the green morph compared to the red morph. From these results, it is clear that the sub-epidermal anthocyanins serve as a photoprotective barrier in the leaves of the red morph.
Finally, in chapter four, the first goal of this project was to test the hypothesis that leaf wax development is a delayed response to sunlight exposure following cutin development. The second goal of this project tested the hypothesis that during leaf ontogeny in the two morphs of A. striata, energy dissipation shifts from non-photochemical to photochemical quenching. The results showed that on both the adaxial and abaxial leaf surfaces of young and mature leaves of both morphs, leaf wax thickness increased from the basal end of the leaf whereas cutin thickness decreased from the basal end of the leaf. Thus, as leaf age in both morphs of A. striata, cutin deposition exceeds wax deposition at the early stages of development. Furthermore, chlorophyll fluorescence data supported the photoprotective function of anthocyanin, indicating that the deposition of anthocyanins may help to reduce dependence of the leaf on energy dissipation via the xanthophyll cycle and non-photochemical quenching during leaf ontogeny.