Gas
exchange and chlorophyll fluorescence techniques
were used to evaluate the acclimation capacity
of the schlerophyll shrub Heteromeles arbutifolia
M. Roem. to the multiple co-occurring summer
stresses of the California chaparral. We examined
the influence of water, heat and high light
stresses on the carbon gain and survival of
sun and shade seedlings via a factorial experiment
involving a slow drying cycle applied to plants
grown outdoors during the summer. The photochemical
efficiency of PSII exhibited a diurnal, transient
decrease ( AF/Fm') and a chronic decrease or
photoinhibition (Fv/Fm) in plants exposed to
full sunlight. Water stress enhanced both transient
decreases of AF/Fm' and photoinhibition. Effects
of decreased AF/Fm' and Fv/Fm on carbon gain
were observed only in well-watered plants since
in waterstressed plants they were overidden
by stomatal closure. Reductions in photochemical
efficiency and stomatal conductance were observed
in all plants exposed to full sunlight, even
in those that were well-watered. This suggested
that H. arbutifolia sacrificed carbon gain for
water conservation and photoprotection (both
structurally via shoot architecture and physiologically
via down-regulation) and that this response
was triggered by a hot and dry atmosphere together
with high PFD, before severe water, heat or
high PFD stresses occur. We found fast adaptive
adjustments of the thermal stability of PSII
(diurnal changes) and a superimposed long-term
acclimation (days to weeks) to high leaf temperatures.
Water stress enhanced resistance of PSII to
high temperatures both in the dark and over
a wide range of PFD. Low PFD protected photochemical
activity against inactivation by heat while
high PFD exacerbated damage of PSII by heat.
The greater interception of radiation by horizontally
restrained leaves relative to the steep leaves
of sun-acclimated plants caused photoinhibition
and increased leaf temperature. When transpirational
cooling was decreased by water stress, leaf
temperature surpassed the limits of chloroplast
thermostability. The remarkable acclimation
of water-stressed plants to high leaf temperatures
proved insufficient for the semi-natural environmental
conditions of the experiment. Summer stresses
characteristic of Mediterranean-type climates
(high leaf temperatures in particular) are a
potential limiting factor for seedling survival
in H. arbutifolia, especially for shade seedlings
lacking the crucial structural photoprotection
provided by steep leaf angles. |