Basically, the amount of light that a plant can use depends on many other factors... including but not limited to plant species, water, nutrient levels, CO2 levels, acclimatization, etc.
Some research on the subject
..........A prolonged exposure of plants or organisms to excessive radiation may result in the
photodestruction of the photosynthetic pigments, since the discoloration (bleaching) of these
pigments depends on oxygen and light; this phenomenon is normally called “photooxidation,”
and it may cause the death of the cell or the organism (Powles, 1984; Hendrey et al., 1987).
In most cases, photooxidation is a secondary phenomenon, occurring after a slow phase
during which there is already a decrease of the photosynthetic activity dependent on light
intensity and exposure time, but without any changes in the pigment pool (Powles, 1984; Long
et al., 1994). Therefore, photoinhibition of photosynthesis does not appear after the destruction
of the pool of pigments; on the contrary, the bleaching of pigments occurs when a certain
degree of photoinhibition has already occurred (Hendrey et al., 1987).
As a rule, plants adapted to full sunlight are able to acclimate and grow in shady conditions,
whereas shade-grown plants may not bear full sunlight (Smith, 1982). Furthermore, sun-acclimated
plants show a higher capacity not only for the use of light in photosynthesis but also for
xanthophyll cycle–dependent energy dissipation (Demmig-Adams et al., 1995). The transfer
of a sunlight plant cultivated under low radiance conditions to high radiance produces an
enhancement in the photosynthetic capacity as the plant adapts itself to the increase of irradiance.
However, leaves from these plants may show photoinhibition, with a decline in photosynthetic
activity and in the quantum yield, if this transfer is abrupt. In this case, mature leaves
that at first suffered photoinhibition may suffer discoloration of the photosynthesizing pigments,
leading to cellular death. Young leaves, developed after transfer to high irradiance, do
not exhibit photoinhibition. Plants acclimated to low irradiance and exposed to high irradiance
are more severely photoinhibited than are those primarily adapted to high irradiance (Long et
al., 1994). This difference shows that the photosynthetic capacity influences susceptibility to
photoinhibition.
The light-harvesting complexes (antennae) must have pigments of such shape and size that
they can transfer energy to reaction centers efficiently. Thus, when the plant develops in shade,
there is an increase in the ratio between antenna pigments and reaction centers (Anderson &
Osmond, 1987; Osmond & Chow, 1988; Horton & Ruban, 1992). The result of this adaptation
is that photosynthesis saturates at low irradiances. As a consequence, under high irradiance,
the absorption rate exceeds the rate that can be used for photosynthesis, predisposing the plan
to damage induced by the excessive radiation (Horton & Ruban, 1992). Cleland and Melis
(1987) proved that a mutant of Secale cereale without the light-harvesting chlorophyll complex
a/b was less affected by high irradiance than was the wild variety.
Under normal conditions, a considerable amount of photons is intercepted by the photosynthetic
apparatus, funnel shaped to the reaction centers and transferred, via electron transport
chain, to production of NADPH2 and ATP (Powles, 1984). According to the accepted electrontransport
scheme, there should be 8 mols of photons for a reduction of 2 mols of NADPH+,
which is linked to the synthesis of 2.66 mols of ATP. In C3 plants, 2 NADPH and 3 ATP are
necessary to assimilate one CO2 in carbohydrate (Krause, 1988). The main drains of this chemical
energy are the cycles of photosynthetic reduction of CO2 (PCR) and the photorespiratory carbon
oxidation (PCO). In this way, a large fraction of intercepted photons is transferred to
propel carbon metabolism. When this metabolism is lacking, the use of excitation energy is
insignificant, even though radiation absorption remains constant; this can result in
photoinhibition (Powles, 1984). Actually, photoinhibition also depends on the rate of light
absorption through the leaf (Anderson & Osmond, 1987).
............Several works demonstrate the photoinhibitory effect of the interaction between light and
drought. Working with ecotypes of sun and shade of Solanum dulcamara, Gauhl (1979) noticed
that some shade ecotypes grew and photosynthesized very well under conditions of full
sun when they were normally irrigated, but with a simultaneous imposition of moderate water
stress they quickly showed signs of photoinhibition. For other ecotypes, which suffered
photoinhibition after being transferred to full sun, imposition of water stress emphasized the
adverse effects of light. According to Osmond (1983), these results may have been consequence
of nutritional stress, which interacted with the water stress.
Wong et al. (1985) pointed out that water stress and photoinhibitory treatment reduced the
photosynthetic metabolism of Eucalyptus pauciflora leaves, with significant alterations in the
partial pressure of intercellular CO2. Munne-Bosch and Alegre (2000) showed that the efficiency
of PS II photochemistry decreased to approximately 65% in plants exposed to the interaction
of high light and drought.
The Phenomenon of Photoinhibition of Photosynthesis
and Its Importance in Reforestation
PEDRO LUÍS DA C. A. ALVES
Department of Applied Biology
FCAVJ/UNESP
Jaboticabal, São Paulo, CEP 14884–900, Brazil
ANTÔNIO C. N. MAGALHÃES
Department of Plant Physiology
IB/UNICAMP
Campinas, São Paulo, CEP 13083–970, Brazil
AND
PAULO ROXO BARJA
Institute for Research & Development
Universidade do Vale do Paraíba
IP&D/UNIVAP
São José dos Campos, São Paulo, CEP 12244–000, Brazil
