Taking the long-held view that photoinhibition embraces several processes leading to a reduction in the efficiency of light utilization in photosynthesis, and that photorespiration embraces several processes associated with O2 uptake in the light, photoinhibition and photorespiration now can be considered as inevitable, but essential inefficiencies of photosynthesis which help preserve photosynthetic competence in bright light. Photorespiratory O2 uptake via Rubisco, and O2 uptake via the Mehler reaction, both promote non-assimilatory electron transport, and stimulate photon utilization during CO2-limited photosynthesis in bright light. Although fluorescence studies show that the proportion of total photon use via oxygenase photorespiration in air may decline to only about 10% in full sunlight, mass spectrometer studies show that O2 uptake in Mehler reaction photorespiration in C3 and CAM plants can still account for up to 50% of electron flow in saturating CO2 and light. The Mehler-ascorbate peroxidase reaction has an additional role in sustaining membrane energization which promotes dynamic photoinhibition and photon protection (rapidly reversible decrease in PSII efficiency involving dissipation of the energy of excess photons in the antennae). Net CO2 and O2 exchange studies evidently underestimate the extent of total electron transport, and hence overestimate the extent of photon excess in bright light, leading to overestimates of the role of energy-dependent photon dissipation through dynamic photo-inhibition. Nevertheless, in C3 plants in air all of these processes help to mitigate chronic photoinhibition and photon damage (slowly reversible decrease in PSII efficiency involving loss of reaction centre function). The possibility remains that residual electron transport to O2 from intermediates in the vicinity of PSII may also lead to reactive O2 species that potentiate this photon damage.

Water stress experiments were performed with grapevines (Vitis vinifera L.) and other C3 plants in the field, in potted plants in the laboratory, and with detached leaves. It was found that, in all cases, the ratio of steady state chlorophyll fluorescence (Fs) normalized to dark-adapted intrinsic fluorescence (Fo) inversely correlated with non-photochemical quenching (NPQ). Also, at high irradiance, the ratio Fs/Fo was positively correlated with CO2 assimilation in air, with electron transport rate calculated from fluorescence, and with stomatal conductance, but no clear correlation was observed with qP. The significance of these relationships is discussed. The ratio Fs/Fo, measured with a portable instrument (PAM-2000) or with a remote sensing FIPAM system, provides a good method for the early detection of water stress, and may become a useful guide to irrigation requirements.

Photosynthetic 02 production and photorespiratory 02 uptake were measured using isotopic techniques, in the C3 species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO2 and normal 02,02 production increased linearly with light intensity. At low 02 or low C02, 02 production was suppressed, indicating that increased concentrations of both 02 and CO2 can stimulate 02 production. At the CO2 compensation point, 02 uptake equaled 02 production over a wide range of 02 concentrations. 02 uptake increased with light intensity and 02 concentration. At low light intensities, 02 uptake was suppressed by increased CO2 concentrations so that 02 uptake at 1,000 microliters per liter CO2 was 28 to 35% of the uptake at the CO2 compensation point. At high light intensities, 02 uptake was stimulated by low concentrations of CO2 and suppressed by higher concentrations of C02. 02 uptake at high light intensity and 1000 microliters per liter CO2 was 75% or more of the rate of 02 uptake at the compensation point. The response of 02 uptake to light intensity extrapolated to zero in darkness, suggesting that 02 uptake via dark respiration may be suppressed in the light. The response of 02 uptake to 02 concentration saturated at about 30% 02 in high light and at a lower 02 concentration in low light. 02 uptake was also observed with the C4 plant Amaranthus edulis, the rate of uptake at the CO2 compensation point was 20% of that observed at the same light intensity with the C3 species, and this rate was not influenced by the CO2 concentration. The results are discussed and interpreted in terms of the ribulose-1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of 02.Both 02 evolution and 02 uptake take place in leaves of C3 and C4 plants in the light (4,8,17,21,23,24,27,28). 02 evolution is derived entirely from the water-splitting reaction of PSII, but three principal 02 uptake processes are presently recognized. These are: the oxygenase reaction of ribulose bisP carboxylase-oxygenase and the associated metabolism of P-glycolate (2,4,5,18,20) Norway) was injected into the system. The system was closed and the gas was circulated over the leaf with a metal bellows pump. Mass 32, mass 36, and mass 40 were monitored continuously with a GD 150/4 mass spectrometer.02 uptake and 02 evolution were calculated using the methods previously described (25,27). CO2 concentration was measured with an IRGA analyzer (UNOR-2, Maihak, Hamburg, Germany) included in the gas circuit, and CO2 concentration during illumination could be controlled by varying the pressure of CO2 on a capillary that bled pure CO2 into the closed system. CO2 uptake, at constant CO2 concentration in the system, was calculated from the rate of CO2 addition. Each measurement was averaged over an 8-to 10-min period of gas exchange after the rate of CO2 uptake had reached a steady rate at each CO2 concentration. The total gas pressure in the small system increa...

Several xanthophyll cycles have been described in photosynthetic organisms. Among them, only two are present in higher plants: the ubiquitous violaxanthin (V) cycle, and the taxonomically restricted lutein epoxide (Lx) cycle, whereas four cycles seem to occur in algae. Although V is synthesised through the β-branch of the carotenoid biosynthetic pathway and Lx is the product of the α-branch; both are co-located in the same sites of the photosynthetic pigment-protein complexes isolated from thylakoids. Both xanthophylls are also de-epoxidised upon light exposure by the same enzyme, violaxanthin de-epoxidase (VDE) leading to the formation of zeaxanthin (Z) and lutein (L) at comparable rates. In contrast with VDE, the reverse reaction presumably catalysed by zeaxanthin epoxidase (ZE), is much slower (or even inactive) with L than with antheraxanthin (A) or Z. Consequently many species lack Lx altogether, and although the presence of Lx shows an irregular taxonomical distribution in unrelated taxa, it has a high fidelity at family level. In those plants which accumulate Lx, variations in ZE activity in vivo mean that a complete Lx-cycle occurs in some (with Lx pools being restored overnight), whereas in others a truncated cycle is observed in which VDE converts Lx into L, but regeneration of Lx by ZE is extremely slow. Accumulation of Lx to high concentrations is found most commonly in old leaves in deeply shaded canopies, and the Lx cycle in these leaves is usually truncated. This seemingly anomalous situation presumably arises because ZE has a low but finite affinity for L, and because deeply shaded leaves are not often exposed to light intensities strong enough to activate VDE. Notably, both in vitro and in vivo studies have recently shown that accumulation of Lx can increase the light harvesting efficiency in the antennae of PSII. We propose a model for the truncated Lx cycle in strong light in which VDE converts Lx to L which then occupies sites L2 and V1 in the light-harvesting antenna complex of PSII (Lhcb), displacing V and Z. There is correlative evidence that this photoconverted L facilitates energy dissipation via non-photochemical quenching and thereby converts a highly efficient light harvesting system to an energy dissipating system with improved capacity to engage photoprotection. Operation of the α-and β-xanthophyll cycles with different L and Z epoxidation kinetics thus allows a combination of rapidly and slowly reversible modulation of light harvesting and photoprotection, with each cycle having distinct effects. Based on the patchy taxonomical distribution of Lx, we propose that the presence of Lx (and the Lx cycle) could be the result of a recurrent mutation in the epoxidase gene that increases its affinity for L, which is conserved whenever it confers an evolutionary advantage.

Disturbance or rainforest is often followed by mass mortality of understorey seedlings. Transitions of shade grown plants to full sunlight can cause reductions in the efficiency with which light is used in photosynthesis, called photoinhibition. In order to assess the influence of photoinhibition on mortality and growth after rainforest disturbance this study examined photoinhibition in both simulated and real forest disturbances in northern Papua New Guinea. In an experiment simulating rainforest disturbance, exposure of shade-grown plants to full sunlight resulted in abrupt decreases in the chlorophyll fluorescence parameter F /F that is characteristic of photoinhibition. However, in the well-watered plants used in these experiments there were no fatalities during 3 weeks after exposure to full sunlight. Thus, it is unlikely that photoinhibition, alone, is responsible for seedling fatalities after rainforest disturbances, but more likely that fatalities are due to photoinhibition in conjunction with other environmental stress. There were differences between the response of species to the simulated disturbance that concurred with their preferred habitats. For example, species form the genus Barringtonia, which is commonly found in shaded understorey environments, underwent greater reductions in F /F and were slower to recover than species that usually inhabit high solar radiation environments. The extent of photoinhibition and the rate of recovery appeared to be dependent on avoidance of direct solar radiation by altering leaf angles and on increasing maximum photosynthetic rates. A field survey of photoinhibition in man-made rainforest gaps corroborated the findings of the simulated disturbance experiment showing that plant species commonly found in shaded environments showed a greater degree of photoinhibition in forest gaps at midday than those species which are classified as species that benefit from gaps or specialist gap inhabitors.