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Accueil du site → Doctorat → Australie → 2009 → Effects of sclerophylly of photosynthesis and gas diffusion

University of Western Australia (2009)

Effects of sclerophylly of photosynthesis and gas diffusion

Hassiotou, Foteini

Titre : Effects of sclerophylly of photosynthesis and gas diffusion

Auteur : Hassiotou, Foteini

Université de soutenance : University of Western Australia

Grade : Doctor of Philosophy (PhD) 2009

[Truncated text] Sclerophylly comprises a suite of structural traits that result in tough long-lived leaves, but which also have the potential to influence leaf photosynthetic performance. Sclerophyllous traits such as leaf dry mass per area (LMA), the abundance of sclerified tissues and cell wall thickness, have been shown to influence the conductance to CO2 diffusion in the mesophyll (gm), and through it, the rates of CO2 assimilation per unit leaf area (Aarea). However, key aspects of the photosynthetic process at the high end of the LMA spectrum and the conditions in which photosynthesis takes place at the tissue and cellular level are not well understood. The present study focused on the impact of leaf structure on CO2 diffusion and photosynthesis in the genus Banksia, which displays a great diversity of leaf morphologies, with the aim to determine whether high-LMA leaves differ from lower-LMA leaves in the organisation of the mesophyll or if the mesophyll itself is also different in its physiology. A prominent leaf feature of many Banksia species is the presence of epidermal invaginations called crypts on the abaxial surface, which host the stomata. Stomatal crypts have been assumed to have a transpiration-reducing function. However, the occurrence of species with crypts in both wet and arid environments suggests that the primary role of these structures may not be moderation of water loss. The diffusion resistance of stomatal crypts was estimated in ten Banksia species using simple equations formulated for perforated or porous layers, and was also modelled in detail using finite-element modelling. Crypts reduced leaf transpiration by less than 15% compared with non-encrypted, superficially positioned stomata. Moreover, the trichomes that are often present within the crypts, and have also been assumed to reduce transpiration, had virtually no influence on transpiration. An alternative hypothesis was formulated that crypts facilitate CO2 diffusion to adaxial palisade cells in thick leaves, which was supported by evidence showing that stomatal encryption becomes more pronounced as leaf thickness and other indicators of sclerophylly increase. Furthermore, the possibility that crypts increase photosynthetic water-use efficiency was examined using an electrical resistance analogue model. This showed that crypts improve water-use efficiency only when the diffusivities for water vapor and CO2 in the crypts differ from those at the stomatal level. It was also demonstrated that the greater the part of the resistance that is due to stomata and crypts relative to mesophyll, the greater the benefit for diffusion of CO2 relative to water vapour. Interrelationships between leaf structural traits and photosynthetic characteristics were investigated in 49 Banksia species and subsets of this group, and the contributions of the two components of LMA, leaf thickness and density, to the variability in LMA observed were determined. Leaf thickness and density contributed similarly to variation in LMA, but to different extents in different species, indicating that there are various ways to be sclerophyllous in this genus. The increasing amount of leaf structural tissues with increasing LMA resulted in lower mass-based chlorophyll, nitrogen and thus, photosynthesis (Amass) at high LMA...

Mots Clés : Banksia—Physiology—Western Australia — Leaves—Physiology — Photosynthesis — Gases from plants


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