Cherry seedlings (Prunus avium) were grown from seed for
two growing seasons in three ambient [CO2] (∼350
μmol mol−1) and three elevated [CO2]
(ambient+∼350 μmol mol−1) open-top chambers, and
in three outside
blocks. A drying cycle was imposed in both the growing seasons to half
the seedlings: days 69–115 in the first
growing season, and in the second growing season days 212–251 on
the same seedlings which had already
experienced drought. Stomatal conductance was significantly reduced in
elevated [CO2]-grown, unstressed
seedlings in both the first and second growing seasons, but was not caused
by a decrease in stomatal density.
Droughted seedlings showed little or no reduction in stomatal conductance
in response to elevated [CO2].
However, stomatal conductance was highly correlated with soil water status.
Photosynthetic rate increased
significantly in response to elevated [CO2] in both
water regimes, leading to improvement in instantaneous
transpiration efficiency over the whole duration of the experiment, but
there was no relationship between
instantaneous transpiration efficiency and long-term water use efficiency.
The Amax was strongly reduced in the
second growing season, but unaffected by [CO2] treatment.
Although photosynthetic rate was not down-regulated,
Rubisco activity was decreased by elevated [CO2],
possibly because of the increased leaf carbon: nitrogen ratio
which had occurred by the ends of the two growing seasons. Elevated [CO2]
did not improve plant water relations
(for example, bulk leaf – water potential, osmotic potentials at
full and zero turgor, relative water content at zero
turgor, bulk modulus of elasticity of the cell) and thus did not increase
water-stress tolerance of cherry seedlings.