Evolutionary innovations in the stomatal control of vascular plants

Stomata are the numerous pores on the leaves of land plants. These pores occur between two adjustable guard cells. Guard cells are primarily responsible for regulating the aperture of the stomatal pore through changes in turgor pressure. Studies in angiosperms have demonstrated that the control of guard cell turgor pressure is metabolic, regulated by a diverse array of guard cell ion pumps. A range of environmental and endogenous signals activate these ion pumps including the hormone, abscisic acid (ABA), photosynthetic rate, red light and CO\\(_2\\) concentration. As such all of these signals are instrumental in regulating efficient water use. However, until the work undertaken in this thesis, it was largely unknown whether this metabolically regulated stomatal signalling observed in angiosperms evolved once with the first stomata over 400 million years ago, or whether the various components of this complex system were assembled over the course of land plant evolution. The aim of my thesis was therefore to investigate the evolution of stomatal control and its possible ecological implications in the four extant lineages of vascular plants (lycophytes, ferns, gymnosperms and angiosperms). Using a phylogenetic approach, I examined the responses of stomatal conductance, aperture and morphology from representative species to exogenous and endogenous ABA, water stress, changes in the rate of photosynthesis and CO\\(_2\\) concentration. The stomata of ferns and lycophytes were insensitive to both exogenous and endogenous ABA. High concentrations of exogenous ABA fed into the transpiration stream of a diversity of fern and lycophyte species did not cause stomata to close despite very high levels of ABA in the leaf. In addition the stomata of drought stressed seed plants close with high levels of endogenous ABA in the leaf; however fern and lycophyte stomata from droughted leaves that were instantaneously rehydrated, rapidly reopened to pre-droughted levels despite the high levels of endogenous ABA in the leaf. In spite of this lack of control by ABA, the stomata of ferns and lycophytes were sensitive to changes in leaf water status, with highly predictable stomatal behaviour, passively responding to leaf water balance in the light. These results suggest that active stomatal regulation by ABA in response to changes in leaf hydration evolved after the divergence of ferns from an ancestral passive-hydraulic stomatal control system. Unlike angiosperms, the stomata of conifers display very little short-term response to changes in atmospheric CO\\(_2\\) concentration. Also, the sensitivity of angiosperm stomata to CO\\(_2\\) concentration is regulated by foliar ABA level, but increased ABA levels following drought stress did not augment the stomatal sensitivity to CO\\(_2\\) concentration in conifers. The capacity of guard cells to actively optimise water use is another crucial component of the metabolic control mechanisms of seed plant stomata. I examined the possibility that this stomatal capacity was also derived in seed plants by comparing the stomatal response to light intensity in 13 species of ferns and lycophytes with a diverse sample of seed plants. While seed plant stomata were capable of maintaining a high ratio of photosynthesis to water use at different light intensities, fern and lycophyte stomata were unable to sustain similar ratios at low light intensities. The behaviour of stomata on excised epidermis indicates that the reason for this difference is the evolution of a feedback signal from photosynthetic tissue to the guard cells, unique to seed plants. I further investigated the adaptive mechanisms adopted by a diverse morphological and ecological sample of ferns and lycophytes with passively controlled stomata that enable survival in response to water stress. Ferns and lycophytes survive prolonged soil drought stress by making significant changes to physiology (by increasing tolerances of low relative water content) and morphology (by increasing the volume of available leaf water). These adaptations are integrally governed by a passive stomatal response to leaf water status and not metabolic stomatal control by ABA as seen in seed plants. A passive stomatal control in the light has influenced the adaptations adopted by ferns and lycophytes over 360 million years of competition and persistence with the dominant seed plant lineages. My results therefore show that the suite of metabolic stomatal control mechanisms found in angiosperms did not evolve synchronously in the earliest stomatal bearing land plants. The results from this thesis expand our understanding of stomatal function in modern land plants; offer physiological explanations for the rise and fall in dominance of the different lineages of vascular plants over geological time and may explain differences in the ecological strategies employed by the diversity of extant and extinct land plants.