Diurnal courses of leaf water
potential (wp), gas exchange and chlorophyll
fluorescence were measured in natural sunand
shade populations of Heteromeles arbutifolia
throughout the seasons of an unusually dry El
Niño year in Central California. The
onset of drought resulted in decreased stomatal
conductance and net photosynthesis in both sun
and shade plants. However, the decline in wp
was much greater and carbon gain was much more
strongly limited by the development of drought
stress in the shade than in the sun. Photorespiratory
energy dissipation was significantly higher
in the sun than in the shade in spring and autumn,
but not during the summer. Pre-dawn photochemical
efficiency (Fv/Fm) was significantly higher
in the shade than in the sun during the spring
but the differences disappeared during the summer
and autumn. The strong irradiance in the open
field site studied led to a chronic but only
mild reduction in Fv/Fm, with values around
0·79. Summer sunflecks led to a sustained
photo-inhibition in shade plants, which exhibited
a significant reduction in pre-dawn Fv/Fm of
10% with the onset of drought. Photoinhibition
became relatively more important for carbon
gain in the shade than in the sun due to the
low photochemical efficiency under the low light
that follows sunflecks. Sun plantsof H. arbutifolia
exhibited a rather efficient photoprotection
against strong irradiance conferred by both
the architecture of the crown and the physiology
of the leaves. There is evidence that El Niño
events and the associated droughts have become
more frequent and severe. Counterintuitively,
the effects on plant performance of such extreme
droughts could be more critical in the shade
than in the sun. |