Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community |
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Authors: | Clark H; Newton P; Barker D |
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Institution: | AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand; Corresponding author e-mail: clarkh@agresearch.cri.nz |
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Abstract: | Periods of limited soil water availability are a feature of many temperate
pasture systems and these have the potential to modify pasture plant and
community responses to elevated atmospheric CO2. Using large pasture
turves, previously exposed to elevated CO2 concentrations of 350 or 700
mol mol-1 for 324 d
under well-watered conditions the morphological and physiological responses
of pasture species growing at these CO2 concentrations were compared when
subjected to a soil moisture deficit-and to recovery from the deficit-with
those that continued to be well watered.Net leaf photosynthesis of
Trifolium repens (C3 legume), Plantago
lanceolata (C3) and Paspalum dilatatum (C4)
was increased by exposure to elevated CO2, but there was no consistent
effect of CO2 on stomatal conductance. At low soil moistures, net
photosynthesis declined and stomatal conductance increased in these three
species. There was a strong CO2 x water interaction in respect of net
photosynthesis; in Trifolium repens, for example,
elevated CO2 increased net photosynthesis by approximately 50% under
well-watered conditions and this increased to over 300% when soil moisture
levels reached their minimum values. Similar values were recorded for both
Paspalum dilatatum and Plantago
lanceolata. Potential water use efficiency (net
photosynthesis/stomatal conductance) was increased by both exposure to
elevated CO2 and drought.Leaf water status was measured in three species:
Trifolium repens, Paspalum dilatatum and Holcus
lanatus (C3). Total leaf water potential (t) and osmotic
potential () were decreased by drought, but CO2
concentration had no consistent effect. t and
were highest in the C4 species Paspalum dilatatum and
lowest in the legume Trifolium repens.In the wet
turves, rates of leaf extension of the C3 grasses Holcus
lanatus and Lolium perenne at elevated CO2
were frequently higher than those at ambient CO2, but there was no effect
of CO2 concentration on the rate recorded in the C4 grass
Paspalum dilatatum or the rate of leaf appearance in
the legume Trifolium repens. Drought reduced leaf
extension rate irrespective of CO2 in all species, but in Holcus
lanatus the reduction was less severe at elevated CO2.
Immediately after the dry turves were rewatered the leaf extension rate on
tillers of Holcus lanatus and Lolium
perenne were higher than on tillers in the wet turves, but only
at ambient CO2. Consequently, despite the greater leaf extension rate
during the soil moisture deficit at elevated CO2, because of the
overcompensation after rewatering at ambient CO2, total leaf extension over
both the drying and rewetting period did not differ between CO2
concentrations for these C3 grass species. Further investigation of this
difference in response between CO2 treatments is warranted given the
frequent drying and wetting cycles experienced by many temperate
grasslands. |
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