Journal of Experimental Botany, Vol 50, 1381-1391, Copyright © 1999 by Oxford University Press
T Lawson and J Weyers
This paper describes spatio-temporal variation in gas exchange over the
lower surface of primary leaves of glasshouse-grown Phaseolus
vulgaris L. plants. Simultaneous measurements of assimilation
and water vapour conductance were made with a small area cuvette attached
to an infra-red gas analyser. The plants were kept in glass chambers so
that the external gaseous environment could be controlled. Observations are
reported from four half hour periods during a day in which the ambient
PPFD, while variable, was close to saturating for photosynthesis.
'Snapshot' measurements of gas exchange were made at 20 positions on the
leaf surface using a stratified random out-to-in strategy, which avoided
disturbance of yet-to-be-measured sites. Data were mapped using the
'Unimap' cartographic program. For any given measurement period, gas
exchange varied greatly over the leaf surface: typically, net assimilation
(A) varied by over 4-fold and leaf conductance
(gi) by over 3-fold. Estimated intracellular pressures
of CO2 and leaf temperatures showed less relative variation both in space
and time. Comparing measurement periods, the spatial patterns of variation
in A and gi were dissimilar.
Moreover, at different sites on the leaf, the trends in a given variable
could be in opposite directions, while external conditions were relatively
constant. Although the correlation between A and
gi was significant overall, there was a large degree
of scatter in the data and zones of high gi often
corresponded to areas of low A. Depending on the basis
of calculation, A was as much as 63% lower than a
value predicted on the basis of steady-state measurements for the
appropriate mean irradiance. It was not possible to deduce from the
relationships between pairs of variables which factors were most important
in determining A and gi at any
given time or space, but gi did not appear to be the
only factor limiting A. It is hypothesized that the
observed variation in gas exchange, the lack of close correspondence
between gi and A and the
reduction in photosynthesis compared with the apparent potential value are
all phenomena that arise from differences in the induction times for these
variables following changes in conditions, interacting with other factors
associated with position on the leaf.Key words:
Assimilation, conductance, heterogeneity, infrared gas analysis,
photosynthesis, stomata
ARTICLES
Spatial and temporal variation in gas exchange over the lower surface of Phaseolus vulgaris L. primary leaves
Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, UK; Present address: Department of Biology, University of Essex, Colchester CO4 3SQ, UK; Corresponding author e-mail: j.d.b.yeyers@dundee.ac.uk
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