Beschreibung:
<jats:title>ABSTRACT</jats:title><jats:p>Leaf intrinsic water‐use efficiency (WUE), the ratio of photosynthetic rate to stomatal conductance (<jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub>), is a key plant trait linking terrestrial carbon and water cycles. A rapid, integrative proxy for <jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub> is of benefit to crop breeding programmes aiming to improve WUE, but also for ecologists interested in plant carbon‐water balance in natural systems. We hypothesize that the carbon isotope composition of leaf‐respired CO<jats:sub>2</jats:sub> (<jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub>), two hours after leaves are transferred to the dark, records photosynthetic carbon isotope discrimination and so provides a proxy for <jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub>. To test this hypothesis, <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub> was measured in four barley cultivars grown in the field at two levels of water availability and compared to leaf‐level gas exchange (the ratio of leaf intercellular to ambient CO<jats:sub>2</jats:sub> partial pressure, <jats:italic>C</jats:italic><jats:sub>i</jats:sub>/<jats:italic>C</jats:italic><jats:sub>a</jats:sub>, and <jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub>). Leaf‐respired CO<jats:sub>2</jats:sub> was more <jats:sup>13</jats:sup>C‐depleted in plants grown at higher water availability, varied between days as environmental conditions changed, and was significantly different between cultivars. A strong relationship between <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub> and <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C of sucrose was observed. <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub> was converted into apparent photosynthetic discrimination (Δ<jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub>) revealing strong relationships between Δ<jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub> and <jats:italic>C</jats:italic><jats:sub>i</jats:sub>/<jats:italic>C</jats:italic><jats:sub>a</jats:sub> and <jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub> during the vegetative stage of growth. We therefore conclude that <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C<jats:sub>Rl</jats:sub> may provide a rapid, integrative proxy for <jats:italic>A</jats:italic>/<jats:italic>g</jats:italic><jats:sub>s</jats:sub> in barley.</jats:p>