U. Schiefthaler et al., Photoregulation and photodamage in Schefflera arboricola leaves adapted todifferent light environments, AUST J PLAN, 26(5), 1999, pp. 485-494
Leaves of the subtropical understorey shrub Schefflera arboricola Hayata gr
owing in full sunlight had higher specific leaf weight, higher chlorophyll
a/b ratios, lower total chlorophyll content and a threefold higher xanthoph
yll cycle pigment content than leaves growing in a naturally shaded, but su
nfleck-punctuated, environment. A number of measurements, all made in situ
and during natural day/night cycles, were taken as follows: current photoch
emical capacity (F-v/F-m after 10 min dark-adaptation), size and epoxidatio
n state of the xanthophyll cycle, CO2 gas exchange and determination of the
D1 synthesis rate. In sun leaves the lowest daily F-v/F-m was found to be
approximately 0.6, the change from maximum correlating with an increase in
zeaxanthin. Daily changes in zeaxanthin were partly due to de novo synthesi
s and turnover. We suggest that sun leaves can dissipate most of the excess
light energy absorbed safely via the photoprotective xanthophyll cycle. D1
synthesis rates did not correlate with photosynthetic photon flux density
or F-v/F-m. The shade leaves had high F-v/F-m values and constant photosynt
hetic rates throughout the day except during sunflecks, when photosynthetic
rates increased and D1 synthesis accelerated, all without a substantial de
crease in F-v/F-m. It seems that leaves of S. arboricola adapted to natural
shade conditions can use sunflecks to contribute significantly to their pr
oductivity. The third leaf type investigated was from greenhouse-grown plan
ts of S. arboricola after exposure to full sunlight. These leaves showed a
rapid and large reduction in F-v/F-m (to 0.3), which neither correlated wit
h zeaxanthin formation nor recovered within the same day. From long-term ef
fects following full sunlight exposure of greenhouse-grown plants we sugges
t that this F-v/F-m reduction actually reflects photodestruction.