Steady-state and dynamic photosynthetic response of Adenocaulon bicolor (Asteraceae) in its redwood forest habitat

The gas exchange characteristics under steadystate and transient light conditions were determined for a redwood forest understory herb Adenocaulon bicolor, that depends on use of sunflecks for a large fraction of its daily carbon gain. Measurements under steady-state conditions indicated that this species has photosynthetic characteristics that are typical for understory plants. The mean light-saturated assimilation rate was 5.26 μmol CO₂ m^-2 s^-1; the light saturation and compensation occurred at 243 and 2 μmol photons m^-2 s^-1, respectively. This light compensation point was much less than the photon flux density under diffuse light in the understory so that positive assimilation could be maintained throughout the day. When leaves that had been in diffuse light for at least 2 h were exposed to a sudden increase in PFD to saturating levels, 10-30 min were required for both assimilation and stomatal conductance to reach maximum values. Calculation of intercellular CO₂ pressures, however, suggest that for the first 10 min after the light increase, biochemical factors were responsible for most of the increase in assimilation. Thereafter stomatal opening caused a further increase in assimilation that was no more than 25% of the total. When fully induced leaves were returned to low light, induction was rapidly lost even though stomatal conductance decreased only slowly. This rapid loss of induction limited the capacity of A. bicolor to use sunflecks after low light periods that lasted longer than 1-2 min. However, during periods when sunflecks are more frequent there is probably little loss of induction. Under these conditions, sunflecks are used with high efficiency for assimilation.