Three new papers have come out recently by doctoral students on a diverse range of topics.
This article is fairly specialized and deserves a bit of context:
1. Drought and plant physiology
2. What is mesophyll conductance?
A CO2 molecule has a long way to travel from the atmosphere to leaf chloroplasts, where photosynthesis takes place. We often measure stomatal conductance, the rate at which water or CO2 moves between the stomatal aperture and the atmosphere, to understand rates of photosynthesis.
But, this only describes a part of the path a CO2 molecule travels! It must go from the gaseous phase, diffuse into an aqueous environment, and then travel through cell walls and membranes before it arrives in the chloroplast. Here is a nice summary of this process (link). Mesophyll conductance, simplified, is the ease to which a CO2 molecule travels from the stomatal cavity to the chloroplast.
3. In our study, we exposed seedlings of trees (Acer platanoides and Fraxinus excelsior) and herbaceous forest understory species (Impatiens noli tangere, Allium ursinum and Mercurialis annua) to moderate drought. We measured leaf water loss (transpiration rate), photosynthesis, stomatal and mesophyll conductance, water use efficiency and intrinsic water-use efficiency (WUEi = A/gs).
We hypothesized:
Interestingly, we found assimilation rate decreased before stomatal conductance was negatively affected. This has been found before and one thought is that ribulose bisphosphate regeneration and ATP production are reduced as a response to drought, therefore reducing assimilation. However, for only two species did we see WUE increase and gm increase. Furthermore, we did not see a clear response of gm to the effective leaf path length.
All in all, I think it is fair to say that the drought we imposed was not strong enough to push the plants out of their "comfort" zone, physiologically speaking. I address this in an upcoming conceptual paper, until this is out you can read about climate extremes research. Yet, this study does contribute to the our growing understanding of how flexible plants are in managing the crucial balance of carbon gain and water loss.
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Contributions of terrestrially derived dissolved organic carbon (DOC) is increasing with climate change and land-use management. This is a significant because: 1) terrestrial DOC sources represent a big increase in available energy to heterotrophic microbes, and 2) the bacterial community composition may change, both of which may have feedbacks on flows of energy and nutrients through aquatic systems.
To test the influence of DOC from terrestrial sources on the bacterial metabolism and community composition we used labeled (13C) leaf water extracts, and unlabeled lysates from phytoplankton in an incubation experiment. Katrin designed her experiment where many different responses were measured, it not so often you come across a study that has so many response variables! In the end, the communities essentially metabolized whatever we introduced, derived from terrestrial or aquatic sources. The community composition, on the other hand, was not affected by the treatment, but the community did shift when both carbon sources were present.
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This paper is based on a project in Norway working with people all over Europe. Rainer went up and took tree cores from to sites in southern Norway where spruce dieback is occurring. He took cores from pairs of trees: one healthy the other showing dieback symptoms. This paper tries to go back in the tree ring archive to see what happened: why does one tree show symptoms while the other does not when they are just about right next to each other?
We compared Sabine's anatomical measurements with the isotopic record that the ZALF group produced. Surprisingly, they told different stories that is worth the read. Our findings do suggest, however, that there are two different strategies exhibited by the spruce trees we measured: water spending and water saving, each of which has certain advantageous for a tree.
In chronological order:
1. Drought and plant physiology
Large shifts in precipitation patterns and rain events are predicted to occur with the continued global increase of CO2 concentrations and climate change. (See simulations of what that might look like in the future (link)). How plants tolerate these changes, particularly drought conditions, is an ongoing research topic that is important for our environment, food supply and quality of life.
Our article focused on forest understory plants and juvenile trees. In particular, we were interested in the role mesophyll conductance plays in regulating the carbon and water balance under drought stress.
Our article focused on forest understory plants and juvenile trees. In particular, we were interested in the role mesophyll conductance plays in regulating the carbon and water balance under drought stress.
A CO2 molecule has a long way to travel from the atmosphere to leaf chloroplasts, where photosynthesis takes place. We often measure stomatal conductance, the rate at which water or CO2 moves between the stomatal aperture and the atmosphere, to understand rates of photosynthesis.
But, this only describes a part of the path a CO2 molecule travels! It must go from the gaseous phase, diffuse into an aqueous environment, and then travel through cell walls and membranes before it arrives in the chloroplast. Here is a nice summary of this process (link). Mesophyll conductance, simplified, is the ease to which a CO2 molecule travels from the stomatal cavity to the chloroplast.
3. In our study, we exposed seedlings of trees (Acer platanoides and Fraxinus excelsior) and herbaceous forest understory species (Impatiens noli tangere, Allium ursinum and Mercurialis annua) to moderate drought. We measured leaf water loss (transpiration rate), photosynthesis, stomatal and mesophyll conductance, water use efficiency and intrinsic water-use efficiency (WUEi = A/gs).
We hypothesized:
1) water use efficiency would directly correlate with changes in rates of mesophyll conductance,
and
2) the leaf effective path length, the tortuous path water makes through a leaf (quantified by stable isotope of water), would inversely scale with mesophyll conductance.
Interestingly, we found assimilation rate decreased before stomatal conductance was negatively affected. This has been found before and one thought is that ribulose bisphosphate regeneration and ATP production are reduced as a response to drought, therefore reducing assimilation. However, for only two species did we see WUE increase and gm increase. Furthermore, we did not see a clear response of gm to the effective leaf path length.
All in all, I think it is fair to say that the drought we imposed was not strong enough to push the plants out of their "comfort" zone, physiologically speaking. I address this in an upcoming conceptual paper, until this is out you can read about climate extremes research. Yet, this study does contribute to the our growing understanding of how flexible plants are in managing the crucial balance of carbon gain and water loss.
_______________________________________________________________________
Contributions of terrestrially derived dissolved organic carbon (DOC) is increasing with climate change and land-use management. This is a significant because: 1) terrestrial DOC sources represent a big increase in available energy to heterotrophic microbes, and 2) the bacterial community composition may change, both of which may have feedbacks on flows of energy and nutrients through aquatic systems.
To test the influence of DOC from terrestrial sources on the bacterial metabolism and community composition we used labeled (13C) leaf water extracts, and unlabeled lysates from phytoplankton in an incubation experiment. Katrin designed her experiment where many different responses were measured, it not so often you come across a study that has so many response variables! In the end, the communities essentially metabolized whatever we introduced, derived from terrestrial or aquatic sources. The community composition, on the other hand, was not affected by the treatment, but the community did shift when both carbon sources were present.
_______________________________________________________________________
This paper is based on a project in Norway working with people all over Europe. Rainer went up and took tree cores from to sites in southern Norway where spruce dieback is occurring. He took cores from pairs of trees: one healthy the other showing dieback symptoms. This paper tries to go back in the tree ring archive to see what happened: why does one tree show symptoms while the other does not when they are just about right next to each other?
We compared Sabine's anatomical measurements with the isotopic record that the ZALF group produced. Surprisingly, they told different stories that is worth the read. Our findings do suggest, however, that there are two different strategies exhibited by the spruce trees we measured: water spending and water saving, each of which has certain advantageous for a tree.
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