Abstract
The cause of reduced leaf-level transpiration under elevated CO2 remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22-40 months under elevated (eCO2; c. 860 ppm) or ambient (aCO2; c. 410 ppm) CO2. We assessed if eCO2-triggered reductions in canopy conductance (gc) alter the response to soil or atmospheric drought and are reversible or lasting due to anatomical adjustments by exposing eCO2 seedlings to decreasing [CO2]. To quantify underlying mechanisms, we analyzed leaf abscisic acid (ABA) level, stomatal and leaf morphology, xylem structure, hydraulic efficiency, and hydraulic safety. Effects of eCO2 manifested in a strong reduction in leaf-level gc (-55%) not caused by ABA and not reversible under low CO2 (c. 200 ppm). Stomatal development and size were unchanged, while stomatal density increased (+18%). An increased vein-to-epidermis distance (+65%) suggested a larger leaf resistance to water flow. This was supported by anatomical adjustments of branch xylem having smaller conduits (-8%) and lower conduit lumen fraction (-11%), which resulted in a lower specific conductivity (-19%) and leaf-specific conductivity (-34%). These adaptations to CO2 did not change stomatal sensitivity to soil or atmospheric drought, consistent with similar xylem safety thresholds. In summary, we found reductions of gc under elevated CO2 to be reflected in anatomical adjustments and decreases in hydraulic conductivity. As these water savings were largely annulled by increases in leaf biomass, we do not expect alleviation of drought stress in a high CO2 atmosphere.
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Claims 14
Claim · model-written summary
Elevated CO2 reduced leaf-level canopy conductance by 55%.
- Taxon
- Aleppo pine (Pinus halepensis)
Pinus halepensise1 - Treatment
- eCO2e0
- Outcome
- leaf-level gce0
- Direction
- reductione0
- Value
- -55%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Effects of eCO2 manifested in a strong reduction in leaf-level gc (-55%)”
e1“Aleppo pine (Pinus halepensis)”
Claim · model-written summary
The conductance reduction was not caused by ABA.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Compound
- ABAe0
- Outcome
- not caused by ABAe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“not caused by ABA”
e1“Aleppo pine (Pinus halepensis)”
Claim · model-written summary
Reduced conductance was not reversible under low CO2.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- low CO2 (c. 200 ppm)e0
- Outcome
- not reversiblee0
- Direction
- not reversiblee0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“not reversible under low CO2 (c. 200 ppm)”
e1“Aleppo pine (Pinus halepensis)”
Claim · model-written summary
Stomatal development and size were unchanged under elevated CO2.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- Stomatal development and sizee0
- Direction
- unchangede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Stomatal development and size were unchanged, while stomatal density increased (+18%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Stomatal density increased by 18%.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- stomatal densitye0
- Direction
- increasede0
- Value
- +18%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“stomatal density increased (+18%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Vein-to-epidermis distance increased by 65%.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- vein-to-epidermis distancee0
- Direction
- increasede0
- Value
- +65%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“An increased vein-to-epidermis distance (+65%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Branch xylem conduits were 8% smaller.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- conduitse0
- Direction
- smallere0
- Value
- -8%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“branch xylem having smaller conduits (-8%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Conduit lumen fraction was 11% lower.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- conduit lumen fractione0
- Direction
- lowere0
- Value
- -11%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“lower conduit lumen fraction (-11%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Specific conductivity was 19% lower.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- specific conductivitye0
- Direction
- lowere0
- Value
- -19%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“lower specific conductivity (-19%)”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Leaf-specific conductivity was 34% lower.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e3
- Outcome
- leaf-specific conductivitye0
- Direction
- lowere2
- Value
- -34%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“leaf-specific conductivity (-34%)”
e1“Aleppo pine (Pinus halepensis)”
e2“lower specific conductivity (-19%) and leaf-specific conductivity (-34%)”
e3“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Elevated CO2 adaptations did not alter stomatal sensitivity to soil or atmospheric drought.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- CO2e0
- Outcome
- stomatal sensitivity to soil or atmospheric droughte0
- Direction
- did not changee0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“These adaptations to CO2 did not change stomatal sensitivity to soil or atmospheric drought”
e1“Aleppo pine (Pinus halepensis)”
Claim · model-written summary
Xylem safety thresholds were similar under elevated CO2.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- eCO2e2
- Outcome
- xylem safety thresholdse0
- Direction
- similare0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“similar xylem safety thresholds”
e1“Aleppo pine (Pinus halepensis)”
e2“eCO2-triggered reductions in canopy conductance (gc)”
Claim · model-written summary
Leaf biomass increases largely annulled water savings.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Outcome
- water savingse0
- Qualifier
- increases in leaf biomasse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“water savings were largely annulled by increases in leaf biomass”
e1“Aleppo pine (Pinus halepensis)”
Claim · model-written summary
Authors do not expect elevated CO2 to alleviate drought stress.
- Taxon
- Aleppo pine (Pinus halepensis)Pinus halepensise1
- Treatment
- high CO2 atmospheree0
- Outcome
- alleviation of drought stresse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“we do not expect alleviation of drought stress in a high CO2 atmosphere”
e1“Aleppo pine (Pinus halepensis)”
The same questions in other papers
co2 · growth · Pinus halepensis (Aleppo pine)Positive and negative results
1 paper in the same direction as this paper · 1 in the opposite direction
- positive
- no effect
Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.