No evidence for photoinhibition of photosynthesis in alpine Caltha leptosepala DC

Alpine plants experience high levels of insolation, as well as cold nighttime temperatures throughout the summer growth period. These two stress factors in combination are now recognized as potentially important limitations to photosynthetic carbon gain. Although likely candidates, the possible occurrence of photoinhibition in alpine plants has been reported infrequently. We measured photoinhibitory stress under natural field conditions and after high-light treatments in an herbaceous species (Caltha leptosepala DC) with structural traits that appeared especially susceptible to photoinhibition, i.e., large, broad, laminar leaves with a near-horizontal leaf orientation. Although photosynthesis declined gradually during the afternoon under natural field conditions, no evidence was found for photoinhibition of photosynthesis, despite incident sunlight levels of (Formula presented.). Also, values of (Formula presented.) (an indicator of dynamic photoinhibition) changed little (<10 %) from early morning to late afternoon values. Moreover, an experimental test of photoinhibition was conducted in the field using artificially applied low (250 µmol m⁻² s⁻¹) followed by unnaturally high (3500 µmol m⁻² s⁻¹) light levels, led to only small reductions in (Formula presented.) (20 % maximum). Also, afternoon declines in photosynthesis and other gas exchange parameters were associated with significant decreases in xylem water potentials. Thus, accumulating daily water stress appeared to be a possible, greater physiological limitation than photoinhibition, even in this common, hypothetically susceptible alpine species.