Alicia Perera
Title: Interplay of photosynthetic capacity and tolerance to abiòtic stresses in Bryophytes: A possible role of cell wall
Thesis supervisor: Dr. Jaume Flexas
Summary: Photosynthesis is the process that sustains almost the entire primary productivity
of ecosystems. It is thought that, after land colonization, the photosynthetic capacity
increased gradually with the acquisition of complexity by plants, from low
photosynthetic rates of plants with bryophyte-like structure to the high rates of the more
complex photosynthetic tissues of angiosperms. Together with several structural
innovations, such as tissue differentiation, stomatal regulation and the presence of
cuticle and hydraulic system able to control water lost, the frequency of desiccation
tolerant species decreased and the cell wall, so multifunctional in bryophytes, started
relegating from some functions. Thin cell wall and structures gradually more enhancers
to CO2 diffusion are a constant in the evolution of land plants. Bryophytes, which
maintain a similar morphology to that of the first land plants according to the fossil
record, constitute the group of land plants with the lowest photosynthetic capacity,
lower CO2 conductance and higher cell wall thickness. Even so, they represent the
second group of land plants in terms of number of species, only surpassed by
angiosperms and, in cold ecosystems, they account for more than half the ecosystem net
primary productivity. Tissue differentiation allowed epidermis and cuticle to play the
role of photoprotection to ultraviolet radiation, so relevant for land colonization. The
accumulation of phenolic compounds that absorb ultraviolet light is relegated in
bryophytes to the cell walls and symplast of the photosynthetic tissues. Furthermore, in
bryophytes, unable to avoid water loss of their tissues in dry environments, the cell
walls tend to fold gradually with dehydration and, therefore, the physical properties of
cell walls may partly influence the capacity to survive to the high levels of desiccation
that bryophytes deal with. Question arises as to how the physico-chemical properties of
bryophyte cell walls condition their photosynthetic capacity, UV light tolerance and
desiccation tolerance.
The present thesis aims to respond to this question by means of several studies
which are packaged in a total of six articles ‒ three of them already published in
scientific journals and another three in different stages of preparation ‒ that build up the
three chapters of the thesis. In the first chapter, the photosynthetic capacity of Antarctic
bryophytes species and their mechanism for tolerating low temperatures and obtaining
positive carbon balances is explored. In the second chapter, the relationship between the
protection against ultraviolet light, the accumulation of phenolic compounds in cell
walls and the photosynthetic capacity of bryophytes is tested. Finally, in the third
chapter, the relationship between physical properties of cell wall of bryophytes,
desiccation tolerance and the photosynthetic capacity is explored.
The most relevant results obtained indicate that the abundance of bryophytes in
cold ecosystems could be mainly related to the capacity to deal with daily temperature
variation and sustain the downregulation of their metabolism and the inhibition of
carbon loss by respiration at low temperatures, with independence of their high
temperature optima for photosynthesis. For species growing in different conditions
exposed to different levels of ultraviolet light, a trade-off between the capacity to
accumulate phenolic compounds within cell walls and photosynthetic capacity was
observed, probably due to the negative effect for the CO2 diffusion provoked by these
molecules of the cell wall. On the contrary, a consistent relationship between
desiccation tolerance and photosynthetic capacity was not observed for the studied
bryophytes, since even being elasticity of tissues a physical property that enhance the
long-term survivance after strong loss of hydric content, this elasticity is not linked to
the photosynthetic capacity, contrary to what happens in vascular plants. Overall, these
results contribute to understanding the physiology of bryophytes and the limiting factors
of their photosynthetic capacity, constrained or not by their capacity to tolerate extreme
environments.