Net CO2 assimilation rates and chlorophyll
a fluorescence were measured on leaves of an
ABA-unresponsive poplar (Populus trichocarpa ×
koreana cv. Peace) during a period of drought.
Stomata of fully expanded leaves of ‘Peace’ partly closed
during drought, while those of Populus euramericana
cv. Robusta closed almost completely. The measured data
were used to calculate total light-driven electron flows and to
derive an estimate of a CO2 mole fraction in the
chloroplasts. Two major results were obtained. (i) In well watered
plants, photosynthesis operated at lower CO2 mole
fractions in the chloroplasts of cv. Peace than in those of
cv. Robusta, whereas CO2 mole fractions in the
substomatal spaces were of the same order of magnitude. We concluded
that a higher resistance to CO2 influx in the mesophyll
contributed to the lower net assimilation rates in
‘Peace’. (ii) Drought induced an important decrease in
CO2 availability in ‘Robusta’ but not in
‘Peace’. This suggested that reduced CO2 influx
was a major cause of the limitation of net CO2
assimilation during drought in ‘Robusta’, but not in
‘Peace’, where drought probably reversibly reduced the
apparent carboxylation efficiency of Rubisco. Measurements of
O2 evolution under saturating CO2 supported
this view, as photosynthesis decreased in droughted plants of
‘Peace’ but not of ‘Robusta’. Moreover,
estimates of the 13C/12C isotope ratio in
the leaves of both cultivars showed drought-related decreases in
discrimination, which, in the case of ‘Peace’, could only
be explained by changes in activity of the photosynthetic carbon
reduction cycle.