Poster on coconut wue for SEB Barcelona, 2005

1
Do homobaric and heterobaric leaves differ in
internal lateral CO2 diffusion?
Tracy Lawson1, Gabriel Cornic2 and James I.L.
Morison1 1Department of Biological Sciences,
University of Essex, Colchester, CO4 3SQ,
UK 2Laboratoire dEcophysiologie Végétale,
Université de Paris XI, Orsay, France
Introduction Methods CO2 can potentially
diffuse laterally in leaves, depending on the
permeability of the leaf tissue and any gradients
in photosynthetic activity. Heterobaric leaves
have physical barriers to CO2 diffusion with
bundle sheaths that extend to the upper and lower
epidermes, whereas homobaric species lack these
extensions. When stomatal apertures are not
spatially uniform, open stomata in one area could
supply an adjacent region with CO2 through
lateral diffusion. Chlorophyll fluorescence
imaging illustrates that when stomata are blocked
by applying grease different species show varying
amounts of lateral diffusion, depending on leaf
anatomy, photosynthetic CO2 consumption and
external CO2 concentration. Measurements and
modelling show that lateral CO2 diffusion can
contribute significantly to photosynthesis, and
that there is no simple dichotomy between
heterobaric and homobaric leaves.
2. Lateral diffusion and photosynthetic
capacity Antisense Rubisco (SSu) tobacco plants
with reduced carboxylation capacity were compared
with WT. Even with reduced Rubisco activity and
very high ambient CO2, the 4 mm patch was not
eliminated. However, the area affected by the
patch declined with increasing ambient CO2
showing that there was considerable lateral
diffusion over short distances.

• Chlorophyll a fluorescence imaging
• Effect of grease patches (approx 4 5.6 mm) on
  the quantum efficiency of PSII in intact,
  attached leaves in 0.5 O2. High CO2
  concentrations reduced or eliminated the grease
  effect where there was substantial lateral
  diffusion.

Species
1500 µmol mol-1
800 µmol mol-1
360 µmol mol-1
Arabidopsis thaliana
1500 µmol mol-1
800 µmol mol-1
360 µmol mol-1
Phaseolus vulgaris
Wild type
Helianthus annuus
(A)
Vicia faba
SSu
Nicotiana tabacum
Zea mays
distance, mm
Fq'/Fm'
Pisum sativum Argentum
WT
Glycine max
Petunia hybrid
SSu
(Morison Lawson 2007, Plant Cell Environ,
published on line June)
Fq' /Fm' 0.0
0.5
3. Modelling lateral diffusion into large
patches A 2-D diffusion model was used to
calculate CO2 assimilation rates in patched
leaves using an empirical A f(Ci) curve fit, for
a range of assumed CO2 diffusion coefficients
(DCO2), from 100 to 2 of the free air value.

• Conclusions
• Chlorophyll fluorescence imaging shows that
  blocking stomata restricts CO2 uptake and in
  present atmospheric CO2 concentrations lateral
  diffusion is not sufficient to supply
  photosynthesis into the middle of 3-6 mm diameter
  patches, in either homobaric or heterobaric leaf
  anatomies.
• However, at the periphery of patches there is
  substantial lateral CO2 flux, which can support
  significant amounts of photosynthesis, except in
  monocots. With large artificial patches, the
  lateral flux supports up to 20 additional CO2
  assimilation across a range of light
  intensities.
• From the model calculations the observed rate of
  lateral flux suggests that the effective bulk CO2
  diffusion coefficient in leaves is 15-20 of that
  in free air.
• The substantial lateral CO2 fluxes over small
  distances may be significant when stomatal
  patchiness occurs in natural situations such as
  with sun and shade flecks, raindrops, some types
  of herbivory or during pathogen infection.

The model confirms that if DCO2 is lt 25, most
of the patch is at or near the CO2 compensation
concentration. However, there is substantial CO2
assimilation in the periphery of the patches,
even at high light, which amounts to
approximately a 20 increase in A. ( Morison
et al. 2005, Plant Physiol. 139 254-266)
Acknowledgements We are grateful to the Univ.
Paris XI for providing a Visiting Professorship
to JILM during this work, and Prof Neil Baker
(Univ. of Essex) for helpful contributions. John
Andrews (ANU, Canberra) is thanked for providing
the antisense Rubisco plants.