Quantifying Light Response of Leaf-Scale Water-Use Efficiency and Its Interrelationships With Photosynthesis and Stomatal Conductance in C 3 and C 4 Species.

Abstract

Light intensity ( I ) is the most dynamic and significant environmental variable affecting photosynthesis ( A n ), stomatal conductance ( g s ), transpiration ( T r ), and water-use efficiency (WUE). Currently, studies characterizing leaf-scale WUE- I responses are rare and key questions have not been answered. In particular, (1) What shape does the response function take? (2) Are there maximum intrinsic (WUE i ; WUE i-max ) and instantaneous WUE (WUE inst ; WUE inst-max ) at the corresponding saturation irradiances ( I i-sat and I inst-sat )? This study developed WUE i - I and WUE inst - I models sharing the same non-asymptotic function with previously published A n - I and g s - I models. Observation-modeling intercomparison was conducted for field-grown plants of soybean (C 3 ) and grain amaranth (C 4 ) to assess the robustness of our models versus the non-rectangular hyperbola models (NH models). Both types of models can reproduce WUE- I curves well over light-limited range. However, at light-saturated range, NH models overestimated WUE i-max and WUE inst-max and cannot return I i-sat and I inst-sat due to its asymptotic function. Moreover, NH models cannot describe the down-regulation of WUE induced by high light, on which our models described well. The results showed that WUE i and WUE inst increased rapidly within low range of I , driven by uncoupled photosynthesis and stomatal responsiveness. Initial response rapidity of WUE i was higher than WUE inst because the greatest increase of A n and T r occurred at low g s . C 4 species showed higher WUE i-max and WUE inst-max than C 3 species-at similar I i-sat and I inst-sat . Our intercomparison highlighted larger discrepancy between WUE i - I and WUE inst - I responses in C 3 than C 4 species, quantitatively characterizing an important advantage of C 4 photosynthetic pathway-higher A n gain but lower T r cost per unit of g s change. Our models can accurately return the wealth of key quantities defining species-specific WUE- I responses-besides A n - I and g s - I responses. The key advantage is its robustness in characterizing these entangled responses over a wide I range from light-limited to light-inhibitory light intensities, through adopting the same analytical framework and the explicit and consistent definitions on these responses. Our models are of significance for physiologists and modelers-and also for breeders screening for genotypes concurrently achieving maximized photosynthesis and optimized WUE.

Plant phenotyping relevance

葉スケールの水利用効率と光応答を定量化する新規モデルを開発し、観測値との比較および既存モデルとの頑健性検証を行っており、植物形質の取得・抽出法が研究の中心である。

Code and data availability

The paper's leaf gas-exchange light-response measurements (A_n, g_s, T_r, WUE for soybean and grain amaranth) are not publicly deposited; the data availability statement says they are available only on request to the corresponding author. The supplementary material link is provided but is described only as containing E

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