Photosynthesis Research 33: 63-71, 1992. © 1992 Kluwer Academic Publishers. Printed in the Netherlands.
Regular paper
On the significance of photoinhibition of photosynthesis in the field and its generality among species Erling Ogren & Eva Rosenqvist Department of Plant Physiology, University of Ume&, S-901 87 Ume&, Sweden Received 16 December 199l; accepted in revised form 8 May 1992
Key words: fluorescence, PS II, quenching, Salix Abstract Photoinhibition was examined in naturally exposed willow leaves in the field. In the afternoon on clear and warm days, the quantum yield of electron transport, derived from gas exchange data, was decreased by 28%. Besides this photoinhibition, decreases in the photosynthetic capacity and in the stomatal conductance were also observed. Of these three limitations of carbon assimilation, photoinhibition was the major one at limiting light only. To investigate the generality of photoinhibition, shade- and sun-acclimated leaves of fourteen different species were compared in a laboratory study. Photoinhibition was quantified by fluorescence measurements following exposure to moderate and high light for 30 min. The extent of photoinhibition was inversely related to the photochemical quenching, qp, during exposure (the proportion of open PS II traps). This relationship was the same independent of the species, the light-acclimation state of the leaf and the light intensity. However, sun- and shade-acclimated leaves occupied opposite sides of the relationship: the sun-leaves showed lower photoinhibition and higher qp. The sun-leaves were also more competent than shade-leaves by showing faster recovery from a given level of photoinhibition.
Abbreviations: F0, Fv, FM, F s -minimal, variable, maximal and steady-state fluorescence; qN, qi -- total and photoinhibitory non-photochemical quenching of variable fluorescence; q p - photochemical quenching
Introduction The light response curve of photosynthesis is not a static property of the leaf but depends on the light pre-history by processes of acclimation and photoinhibition. Photoinhibition is the least categorized of these, although its symptoms are well recognized. They are, parallel and sustained decreases in the maximal quantum yields of photosynthesis and of PSII photochemistry (e.g., Demmig and Bj6rkman 1987). The latter effect is conveniently probed by the fluorescence ratio of Fv/F M. Recently, a flattening of the photosynthetic light response curve has also been
reported, which shows up as a decreased convexity parameter in the currently used model of the curve (Leverenz et al. 1990, Ogren and Sj6str6m 1990). Although the light-saturated rate of photosynthesis is not directly affected by this, it is shifted to higher light levels. There are a growing number of reports demonstrating the occurrence of photoinhibition under natural conditions. The majority of these are dealing with photoinhibition under conditions of stress, for instance low temperatures (Oquist et al. 1987, Farage and Long, 1991). Evidently, the photosynthetic machinery is sensitized to photoinhibition by concomitant stress.
64 Recently, evidence have also appeared indicating that photoinhibition occurs more frequently. For instance, a regular midday-depression in the efficiency of PS II photochemistry has been demonstrated in a number of plants species (Adams et al. 1988, Ogren 1988, Bolhar-Nordenkampf et al. 1991), which was paralleled by decreased efficiency of 0 2 evolution (Ogren 1988, Ogren and Evans 1992). Although this diurnal type of photoinhibition is relatively m i l d - it rarely exceeds 30% and recovers overnight - it can still be important since it is induced by light levels below the saturation point for photosynthesis (Ogren and Sj6str6m 1990, Rosenqvist et al. 1991). Can the great variability in photoinhibition in response to biological and environmental variation be accounted for by a single mechanism? A strong candidate for such a mechanism was presented in a previous study (Ogren 1991). It was found that the overall variation in photoinhibition was not correlated with the incident light intensity. However, it was increased with decreased photochemical and non-photochemical quenching of the fast-relaxing type as described by a multiple and linear regression model. This relationship was established using a single species, although the biological variation was enhanced by the use of different environmental conditions and certain chemicals. One objective of the present study was to find out whether this relationship holds true for plants in general. Another objective is concerned with the significance of photoinhibition in terms of carbon assimilation. In a simulation study we have estimated that diurnal photoinhibition causes about one-tenth loss of the potential carbon gain of peripheral willow shoots on clear days (Ogren and Sj6str6m 1990). However, data from in situ CO 2 measurements are still lacking. Such data would also be useful in order to compare photoinhibition with other possible limitations of carbon assimilation under photoinhibitory conditions.
Materials and methods
Plant material Wild plants growing outside the laboratory in
Ume~ were used, representing taxonomically diverse species of herbs, grasses and trees: Alnus incana (L.) Moench., Aquilegia vulgaris L., Betula verrucosa Ehrh., Calamangrostis purpurea Trin., Lamium galeobdolon (L.) L., Melandrium rubrum (Weig.) Garcke, Potentilla erecta (L.) Rfiusch, Ranunculus repens L., Ribes rubrum L., Salix sp., Silene vulgaris (Moench) Garcke, Taraxacum sp., Trifolium pratense L. and Tussilago farfara L. Each species was represented by sun-acclimated plants growing in an open plot, and shade-acclimated plants growing in the shade of a hedge where the light averaged 215% of that on the open plot. Fully developed leaves were used. In the field study, attached leaves of Salix were used, situated on upper parts of main shoots, and oriented within the sector SE-SW. The leaf angle varied from 20° above to 30° below the horizontal plane. In the laboratory study (Figs. 1 and 2), leaves were excised which had not received direct sunlight on the particular day. After petioles had been recut and placed in vials of water, the leaves were kept in a growth room under a Q of approx. 30/xmol m -2 s -1 for no longer than 2 h before use.
Treatment In the field study, leaves were exposed to full daylight at their natural positions until measurements started at some point during 12:0017:30 h. The days were clear with a maximum incident Q of 2000/xmolm -2 s -1 (on a plane facing the sun at noon). The control leaves had been wrapped in white muslin in the previous evening which reduced the incident Q to max. 300 tzmol m-2 s- 1. Following unwrapping of the leaf, a disc (2 cm 2) was punched for the fluorescence assay. Before the leaf was enclosed in the gas exchange chamber, photosynthesis was allowed to come to steady-state in full daylight. Except for a few measurements of quantum yields of controls, all measurements were done on clear days with air temperatures within 1829 °C, as measured with thin thermocouples and a datalogger as described elsewhere (Ogren and Sj6str6m 1990). On a typical day the minimum water potentials of study leaves varied within
65 - 1 . 0 to -1.1 MPa, as measured by a pressure bomb (5 replicate leaves). In the laboratory study, the leaf was enclosed in a water-jacketed clamp-on cuvette maintained at 25°C and flushed with a humidified stream of air of 40 Pa CO 2. It was kept under these conditions throughout treatment and measurement. Illumination was provided by a modified projector lamp and directed by the fibre optics of the fluorometer onto a spot (1 cm 2) on the adaxial surface. The Q at the position of the leaf was 150 txmol m -2 s -~ during the first 15 min to allow light-activation to occur. It was then increased in one step to the final level and held constant for 30 min.
Measurement of gas exchange Exchange of CO 2 and H20 was measured in the field using a portable system (LCA-3 with broadleaf chamber PLC3, ADC Ltd, Hoddesdon, UK) that was calibrated against a CO 2 standard of 355vpm. The light sensor of the chamber was calibrated against a quantum sensor (Li-189, LiCor Inc., Lincoln, Neb., USA) over the whole natural light range, using Schott neutral density filters for the attenuation. The humidity sensors were calibrated against gas streams of known water vapour concentrations as set by equilibration with FeSO 4 • 7H20 of known temperatures. A series of measurements were done by stepwise decreasing the prevailing light using screens of white muslin. Finally, the rate of dark-respiration was measured. The vpd during measurements was ~
Regular paper
On the significance of photoinhibition of photosynthesis in the field and its generality among species Erling Ogren & Eva Rosenqvist Department of Plant Physiology, University of Ume&, S-901 87 Ume&, Sweden Received 16 December 199l; accepted in revised form 8 May 1992
Key words: fluorescence, PS II, quenching, Salix Abstract Photoinhibition was examined in naturally exposed willow leaves in the field. In the afternoon on clear and warm days, the quantum yield of electron transport, derived from gas exchange data, was decreased by 28%. Besides this photoinhibition, decreases in the photosynthetic capacity and in the stomatal conductance were also observed. Of these three limitations of carbon assimilation, photoinhibition was the major one at limiting light only. To investigate the generality of photoinhibition, shade- and sun-acclimated leaves of fourteen different species were compared in a laboratory study. Photoinhibition was quantified by fluorescence measurements following exposure to moderate and high light for 30 min. The extent of photoinhibition was inversely related to the photochemical quenching, qp, during exposure (the proportion of open PS II traps). This relationship was the same independent of the species, the light-acclimation state of the leaf and the light intensity. However, sun- and shade-acclimated leaves occupied opposite sides of the relationship: the sun-leaves showed lower photoinhibition and higher qp. The sun-leaves were also more competent than shade-leaves by showing faster recovery from a given level of photoinhibition.
Abbreviations: F0, Fv, FM, F s -minimal, variable, maximal and steady-state fluorescence; qN, qi -- total and photoinhibitory non-photochemical quenching of variable fluorescence; q p - photochemical quenching
Introduction The light response curve of photosynthesis is not a static property of the leaf but depends on the light pre-history by processes of acclimation and photoinhibition. Photoinhibition is the least categorized of these, although its symptoms are well recognized. They are, parallel and sustained decreases in the maximal quantum yields of photosynthesis and of PSII photochemistry (e.g., Demmig and Bj6rkman 1987). The latter effect is conveniently probed by the fluorescence ratio of Fv/F M. Recently, a flattening of the photosynthetic light response curve has also been
reported, which shows up as a decreased convexity parameter in the currently used model of the curve (Leverenz et al. 1990, Ogren and Sj6str6m 1990). Although the light-saturated rate of photosynthesis is not directly affected by this, it is shifted to higher light levels. There are a growing number of reports demonstrating the occurrence of photoinhibition under natural conditions. The majority of these are dealing with photoinhibition under conditions of stress, for instance low temperatures (Oquist et al. 1987, Farage and Long, 1991). Evidently, the photosynthetic machinery is sensitized to photoinhibition by concomitant stress.
64 Recently, evidence have also appeared indicating that photoinhibition occurs more frequently. For instance, a regular midday-depression in the efficiency of PS II photochemistry has been demonstrated in a number of plants species (Adams et al. 1988, Ogren 1988, Bolhar-Nordenkampf et al. 1991), which was paralleled by decreased efficiency of 0 2 evolution (Ogren 1988, Ogren and Evans 1992). Although this diurnal type of photoinhibition is relatively m i l d - it rarely exceeds 30% and recovers overnight - it can still be important since it is induced by light levels below the saturation point for photosynthesis (Ogren and Sj6str6m 1990, Rosenqvist et al. 1991). Can the great variability in photoinhibition in response to biological and environmental variation be accounted for by a single mechanism? A strong candidate for such a mechanism was presented in a previous study (Ogren 1991). It was found that the overall variation in photoinhibition was not correlated with the incident light intensity. However, it was increased with decreased photochemical and non-photochemical quenching of the fast-relaxing type as described by a multiple and linear regression model. This relationship was established using a single species, although the biological variation was enhanced by the use of different environmental conditions and certain chemicals. One objective of the present study was to find out whether this relationship holds true for plants in general. Another objective is concerned with the significance of photoinhibition in terms of carbon assimilation. In a simulation study we have estimated that diurnal photoinhibition causes about one-tenth loss of the potential carbon gain of peripheral willow shoots on clear days (Ogren and Sj6str6m 1990). However, data from in situ CO 2 measurements are still lacking. Such data would also be useful in order to compare photoinhibition with other possible limitations of carbon assimilation under photoinhibitory conditions.
Materials and methods
Plant material Wild plants growing outside the laboratory in
Ume~ were used, representing taxonomically diverse species of herbs, grasses and trees: Alnus incana (L.) Moench., Aquilegia vulgaris L., Betula verrucosa Ehrh., Calamangrostis purpurea Trin., Lamium galeobdolon (L.) L., Melandrium rubrum (Weig.) Garcke, Potentilla erecta (L.) Rfiusch, Ranunculus repens L., Ribes rubrum L., Salix sp., Silene vulgaris (Moench) Garcke, Taraxacum sp., Trifolium pratense L. and Tussilago farfara L. Each species was represented by sun-acclimated plants growing in an open plot, and shade-acclimated plants growing in the shade of a hedge where the light averaged 215% of that on the open plot. Fully developed leaves were used. In the field study, attached leaves of Salix were used, situated on upper parts of main shoots, and oriented within the sector SE-SW. The leaf angle varied from 20° above to 30° below the horizontal plane. In the laboratory study (Figs. 1 and 2), leaves were excised which had not received direct sunlight on the particular day. After petioles had been recut and placed in vials of water, the leaves were kept in a growth room under a Q of approx. 30/xmol m -2 s -1 for no longer than 2 h before use.
Treatment In the field study, leaves were exposed to full daylight at their natural positions until measurements started at some point during 12:0017:30 h. The days were clear with a maximum incident Q of 2000/xmolm -2 s -1 (on a plane facing the sun at noon). The control leaves had been wrapped in white muslin in the previous evening which reduced the incident Q to max. 300 tzmol m-2 s- 1. Following unwrapping of the leaf, a disc (2 cm 2) was punched for the fluorescence assay. Before the leaf was enclosed in the gas exchange chamber, photosynthesis was allowed to come to steady-state in full daylight. Except for a few measurements of quantum yields of controls, all measurements were done on clear days with air temperatures within 1829 °C, as measured with thin thermocouples and a datalogger as described elsewhere (Ogren and Sj6str6m 1990). On a typical day the minimum water potentials of study leaves varied within
65 - 1 . 0 to -1.1 MPa, as measured by a pressure bomb (5 replicate leaves). In the laboratory study, the leaf was enclosed in a water-jacketed clamp-on cuvette maintained at 25°C and flushed with a humidified stream of air of 40 Pa CO 2. It was kept under these conditions throughout treatment and measurement. Illumination was provided by a modified projector lamp and directed by the fibre optics of the fluorometer onto a spot (1 cm 2) on the adaxial surface. The Q at the position of the leaf was 150 txmol m -2 s -~ during the first 15 min to allow light-activation to occur. It was then increased in one step to the final level and held constant for 30 min.
Measurement of gas exchange Exchange of CO 2 and H20 was measured in the field using a portable system (LCA-3 with broadleaf chamber PLC3, ADC Ltd, Hoddesdon, UK) that was calibrated against a CO 2 standard of 355vpm. The light sensor of the chamber was calibrated against a quantum sensor (Li-189, LiCor Inc., Lincoln, Neb., USA) over the whole natural light range, using Schott neutral density filters for the attenuation. The humidity sensors were calibrated against gas streams of known water vapour concentrations as set by equilibration with FeSO 4 • 7H20 of known temperatures. A series of measurements were done by stepwise decreasing the prevailing light using screens of white muslin. Finally, the rate of dark-respiration was measured. The vpd during measurements was ~
