EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

Proofs of non-stomatal limitations of potato photosynthesis during drought by using in-situ eddy covariance data

Quentin Beauclaire, Louis Gourlez de la Motte, Heinesch Bernard, and Longdoz Bernard
Quentin Beauclaire et al.
  • ULiege, Gembloux AgroBioTech, TERRA, Belgium (

Water stress in one of the main limiting factors in agro-systems, causing a reduction in gross primary production (GPP) and by extend, yields. However, it is still unclear to attribute whether the limitations of photosynthesis originate from a strict stomatal control (SOL) or from other non-stomatal limitations (NSOL). In this study, we investigated the effects of drought on potato crop by using eddy covariance data at the Lonzée Terrestrial Observatory during three consecutive cultivation periods (2010, 2014 and 2018). Regardless the years and the timing of the drought appearance, the maximum carboxylation rate Vcmax (one of the NSOL) was reduced with decreasing REW, while the stomatal sensitivity to GPP parameter in the Medlyn et al. model (G1-SOL) remained constant. We showed that below the REW threshold of 0.55 ± 0.05, the non-consideration of NSOL in the ecosystem CO2 model led to an overestimation of the modelled GPP, which was about three times higher than its unstressed corresponding value. As a result, decreasing Vcmax while maintaining G1 constant was sufficient to reproduce GPP and canopy conductance dynamics during drought. At a sub-daily scale, the intrinsic water-use efficiency did not vary during drought, neither its dependence on VPD nor its hourly dynamics. This reinforced the hypothesis of direct and feedback effects of NSOL on canopy conductance and photosynthesis, which was supported by the uniform coupling between carbon and water fluxes. We recommend the implementation of NSOL in ecosystem CO2 models since non-stomatal factors were responsible for the decrease in potato crop GPP during drought.

How to cite: Beauclaire, Q., Gourlez de la Motte, L., Bernard, H., and Bernard, L.: Proofs of non-stomatal limitations of potato photosynthesis during drought by using in-situ eddy covariance data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5183,, 2020

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Display material version 1 – uploaded on 30 Apr 2020
  • CC1: Comment on EGU2020-5183, Martin De Kauwe, 04 May 2020

    Hi Quentin,

    How do you estimate your Ci for eqn 2 to derive your Vcmax, sorry I can't follow that from your slides. Presumably, Ci drives the decline in Vcmax you show in Fig 2 and yet it is seemingly not apparent in the change in g1. However, looking at your eqn 1, you rely on Ca, not Ci...



    • AC1: Reply to CC1, Quentin Beauclaire, 04 May 2020

      Hi Martin,

      I used a classical Fickian formulation to derive Ci from Cs, GPP and Gc. It is not implicit in Eq1 in the poster (which is the general Medlyn et al. model), but in the calculation process, g1 actually depends on Ci by combining the USO general formulation and the Fick law: g1=(Ci*sqrt(VPD)) / (Cs-Ci). This is a formulation that can also be found in the Medlyn et al. paper (Medlyn et al., 2017) which compares leaf and ecosystem WuE. However, Eq1 mentions Ca meanwhile I actually used Cs, which I derived from Ca, net carbon flux and aerodynamic conductance in all calculations. It’s a clerical error.
      I can post an updated version of the poster with more precise explanations about g1 calculation if you think that’s necessary.

      I hope I answered your question.
      Thanks for your comment,


  • CC2: Comment on EGU2020-5183, Martin De Kauwe, 05 May 2020

    Thanks for the explanation.

    I think the likely explanation of your gs vs Vcmax plot is therefore that you have no direct feedback of soil water limitation on your estimation of g1 (?), only via a change in D. It is usual to reduce g1 as soil moisture seem to assuming there is no interaction?

    • AC2: Reply to CC2, Quentin Beauclaire, 05 May 2020

      The constant g1 evolution with REW does not necessarily mean that g1 and REW don’t have any interactions. It also depend on how g1 is modelled. The non consideration of NSOL in the USO model might explain why discrepancies in g1 relationship to soil moisture have been noticed (Gourlez de la Motte et al., 2020; and others), and might be a call for a change in g1 modelling, especially by taking into account NSOL or soil/leaf water status for example.(as suggested by Dewar et al., 2018). This would probably need a reformulation of the cost  associated with the opening of the stomata in some specific situations, depending on how the ecosystem reacts to drought. For example, it has been shown that stomatal closure can be induced by an inhibition of the Calvin cycle activity, suggesting that stomatal closure is not always the first adaptative response of an ecosystem to drought, leading to various responses of g1 to soil moisture / leaf water potential. A deeper knowledge of the dependance of gs/gm/real Vcmax to REW would be helpful to understand how an ecosystem reacts to water depletion in the soil, and to know what triggers what. I think we should follow this path in our future studies.  


      • CC3: Reply to AC2, Martin De Kauwe, 06 May 2020


        If I'm following - the Medlyn model *assumes* that gs saturates at high D, i.e. it doesn't go to zero. Given that you're not allowing g1 to decline directly with soil moisture (as is standard in models), you're essentially seeing what you're assuming. As a result, the relative interpretation of changes in stomatal vs non-stomatal responses needs careful interpretation.

        Best wishes,