The hypothesis that shoot growth responses of C4 grasses
to elevated CO2 are dependent on shoot water relations
was tested using a C4 grass,
Panicum coloratum (NAD-ME subtype). Plants were grown
for 35 days at CO2 concentrations of 350 or 1000
µL CO2
L-1. Shoot water relations were altered by growing
plants in soil which was brought daily to 65, 80 or 100% field capacity
(FC) and by maintaining the vapour pressure deficit (VPD) at 0.9 or 2.1 kPa.
At 350 µL CO2
L-1, high VPD and lower soil water content depressed
shoot dry mass, which declined in parallel at each VPD with decreasing soil
water content. The growth depression at high VPD was associated with increased
shoot transpiration, whereas at low soil water, leaf water potential was
reduced. Elevated CO2 ameliorated the impact of both
stresses by decreasing transpiration rates and raising leaf water potential.
Consequently, high CO2 approximately doubled shoot mass
and leaf length at a VPD of 2.1 kPa and soil water contents of 65 and
80% FC but had no effect on unstressed plants. Water use efficiency was
enhanced by elevated CO2 under conditions of stress but
this was primarily due to increases in shoot mass. High
CO2 had a greater effect on leaf growth parameters than
on stem mass. Elevated CO2 increased specific leaf area
and leaf area ratio, the latter at high VPD only. We conclude that high
CO2 increases shoot growth of C4
grasses by ameliorating the effects of stress induced by either high VPD or
low soil moisture. Since these factors limit growth of field-grown
C4 grasses, it is likely that their biomass will be
enhanced by rising atmospheric CO2 concentrations.
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