Induction of CO2-gas exchange and electron transport: comparison of dynamic and steady-state responses in Fagus sylvatica leaves

Abstract

Photosynthetic induction was followed in a juvenile understorey beech (Fagus sylvatica L.) in a shaded habitat which was temporarily exposed to direct sunlight passing through a gap in the canopy. Simultaneous in situ measurements of leaf gas exchange and chlorophyll fluorescence were carried out and a steady-state light response curve of photosynthesis was recorded. The measured dynamic carbon gain was compared to the predicted carbon gain, calculated from the steady-state light response curve and the ambient photon flux density (PPFD) incident during the sunfleck event. Integration over the first 20 min, during which induction took place, resulted in a deviation of 27% if any induction effects are disregarded. The carbon gain was overestimated by 11% when the carbon gain was integrated over the whole measuring period of about 1 h including a 40-min period of full induction. In situ gas exchange measurements under constant saturating light, following dark adaptation, revealed an induction error of 21%. About 20 min was required to reach the final steady-state level of photosynthesis. The data show that the prediction of the carbon gain by steady-state models leads to a distinct overestimation of the CO2 uptake, irrespective of whether the induction state rises concurrently with the incident radiation or saturating light induces photosynthesis. From chlorophyll fluorescence and absorptance data, rates of linear electron transport (ETR) were calculated. Uninduced leaves exposed to saturating light of constant PPFD show an initial fast down-regulation of ETR, due to excessive light, and a subsequent increase in electron transport attributable to the increasing energy demand during induction of carbon fixation. No difference in the ETR to PPFD relationship between data sets sampled during induction and in the fully induced state was found when both the incident light and the induction state of carbon fixation increased concomitantly. The proportion of electrons fed into alternative pathways besides carbon fixation was higher during induction as compared to the fully induced state. Thus, electrons were used for carbon fixation with a higher efficiency when full induction was reached.