Jaume Flexas talks with
ScienceWatch.com and answers a few questions about
this month's Fast Breaking Paper in the field of Plant
& Animal Science. The author has also sent along
images of his work.
Article Title: Mesophyll conductance to CO2:
current knowledge and future prospects
Authors:
Flexas, J;Ribas-Carbo, M;Diaz-Espej, A;Galmes,
J;Medrano, H
Journal: PLANT CELL ENVIRON
Volume: 31, Issue: 5, Page: 602-621, Year: MAY 2008
* Univ Illes Balears, Grp Recerca Biol Plantes Condic
Mediterranies, Dept Biol, Carretera Valldemossa Km 7-5,
Palma de Mallorca 07122, Balears, Spain.
* Univ Illes Balears, Grp Recerca Biol Plantes Condic
Mediterranies, Dept Biol, Palma de Mallorca 07122, Balears,
Spain.
* CSIC, Inst Recursos Nat & Agrobiol, E-41080
Seville, Spain.
Why do you think your paper is highly
cited?
Recently, mesophyll conductance has become one of the most important
aspects of photosynthetic regulation. During photosynthesis, CO2
has to move from the atmosphere surrounding the leaf to the sub-stomatal
internal cavities through the stomata, and from there to the site of
carboxylation inside the chloroplast stroma through the
mesophyll—passing leaf internal gas, cell wall, liquid, and lipid
phases (Figure 1). The resistances inside the leaf mesophyll, or its
inverse mesophyll conductance to CO2
(gm)—i.e., the facility for CO2
diffusion inside leaves—have often been overlooked in photosynthesis
studies.
However, this field of research has exponentially increased in activity
over the past 23 years, as judged by the evolution of the number of
publications on this subject (Figure 2). Very renowned research groups in
photosynthesis—including those of Graham Farquhar, John Evans, and
Suzanne von Caemmerer (ANU, Australia), Ichiro Terashima (University of
Tokyo, Japan), Steve Long (Urbana, IL, USA), Tom Sharkey (Michigan State
University, USA), Ülo Niinemets (Estonian University of Life Sciences,
Estonia), Francesco Loreto (CNR Rome, Italy) and Bernard Genty (CEA
Cadarache, France), among others, are involved in this research and leading
the field.
Micrograph
of the abaxial surface
of an olive
leaf...
Evolution
of publications on
mesophyll conductance
to...
As an increasing number of researchers have become involved in the same
area, our paper, along with another by Charles Warren of the University of
Sydney's School of Biological Sciences, entitled: "Stand aside stomata,
another actor deserves centre stage: the forgotten role of the internal
conductance to CO2 transfer," (Journal of Experimental
Botany 59:[7]: 1475-87, 2008) were the first and only complete reviews
about gm published in nearly a decade. This may be one
of the main reasons why it is highly cited.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
It synthesizes all the aspects on current knowledge about mesophyll
conductance, including an historical review of the term and concept, a
brief description of available methodology for its estimation, an
evaluation of its variability among plant groups, a synthesis of
gm responses to environmental variables, both in the
short term and in acclimation, a summary of current knowledge on its
physiological and molecular basis, its implications for plant ecology,
ecophysiology, and modelling, and a proposal of future prospects for
research.
Would you summarize the significance of your paper in
layman's terms?
Classically, photosynthesis in higher plants has been considered limited by
two factors: stomatal conductance, which regulates the CO2
supply, and leaf biochemistry—understood as the basic photochemistry,
carboxylation, and Calvin cycle reactions—which regulate the
CO2 demand.
In contrast, gm has been implicitly assumed to be
infinite and therefore non-limiting for photosynthesis. The evidences
accumulated along the past 20 years clearly show that
gm is sufficiently small so as to significantly limit
photosynthesis, especially in species with thick leaves, such as most
evergreens.
Moreover, it can be regulated, it has a structural as well as metabolic
basis, and it responds rapidly to changes in environmental conditions. In
other words, it is now clear that there is a third player in the
photosynthesis game, of quantitative importance similar to that of stomata
and Rubisco in terms of photosynthesis regulation and/or restriction. This
leads to the need of reconsidering the current understanding of the
regulation of photosynthesis in vivo, as well as reformulating
current models of prediction of photosynthesis.
How did you become involved in this research, and were
there any problems along the way?
As I had already mentioned in a previous Fast Moving Front
commentary, I began to be involved in research on
gm during my Ph.D. thesis, particularly in relation
to the fact that drought-induced decreases of gm
could explain drought-induced depressions of photosynthesis, and help
solve a long-standing controversy regarding stomatal versus metabolic
limitations to photosynthesis under drought conditions.
Later on, I became interested in many aspects of the regulation of
gm, such as its response to environmental variables
other than drought, along with CO2 concentration or vapor
pressure deficit, its variations with leaf ageing, and the involvement of
some aquaporins in its regulation.
The main problems when doing research on gm concern the
accuracy and validity of the methods for its estimation. All methods rely
on quite a few assumptions and are very sensitive to variations in key
input parameters.
For this reason, it is necessary to perform accurate tests on the validity
of several assumptions and parameters, and use at least two independent
methods for the estimation of gm, which makes these
studies very dependent on experimental rigor and also time-consuming.
Concerning the review itself, I became involved on it when I was invited to
write a review on gm by Plant, Cell and
Environment editors Keith Mott and Tom Sharkey, in recognition for the
contribution of our research group to the present knowledge on
gm. Considering the excellence of the above mentioned
researchers and others in this field, I was really honored at being
selected to write this review, and I'm grateful to the editors for this
opportunity.
Where do you see your research leading in the
future?
There are needs to explore interspecific differences in
gm and its responses to the environment, as well as to
study the structural, physiological, and molecular mechanisms underlying
the regulation of gm. I believe that this is a field
which shall develop rapidly in the near future, and that is where my
research will mostly focus.
Do you foresee any social or political implications for
your research?
Stomatal diffusion conductance and the enzymatic capacity of foliage
photosynthetic apparatus have been recognized as important regulatory and
limiting factors for photosynthesis and these are the two sole
physiological components of most commonly used photosynthesis
models—which are the basis for predictions of crop yields, plant
responses to climate change, etc.
The accuracy of these prediction models is to some extent limited by the
fact that they ignore variations of gm. Therefore,
incorporating algorithms for the estimation of gm and
its variability will improve our prediction capacity.
On the other hand, improved knowledge on the mechanisms regulating
gm and its genetic basis may, in the near future, open
new opportunities for the biotechnological improvement of plant
photosynthesis, water use efficiency, and yield.
Jaume Flexas, PH.D.
Grup de Recerca en Biología de les Plantes en Condicions
Mediterrànies
Universitat de les Illes Balears
Departament de Biología
Illes Balears, Spain