Effects of light acclimation on the redox state of the PS II electron acceptor in Quercus alba L. seedlings

William L. Bauerle
Department of Horticulture, Clemson University, Clemson, SC, USA
and
G. Geoff Wang
Department of Forest and Natural Resources, Clemson University, Clemson, SC, USA

Introduction
Light is a critical factor affecting the early survival and growth of trees in metropolitan areas, but our knowledge on how white oaks respond to light is rather limited. White oak is generally described as a species of intermediate shade-tolerance (Teskey and Shrestha 1985). However, it can persist under a forest canopy for more than 90 years and responds well to release (Rogers 1990, Orwig and Abrams 1995; Rentch et al. 2003). It is critical to understand the light condition under which oak seedlings survive, if we are to place them in potentially stressful metropolitan conditions.

The shift between photochemical and non-photochemical quenching balances energy input with utilization at the leaf chloroplast level. The transition from photochemical quenching at low light to non-photochemical quenching at high light reduces the quantum yield pf PS II, fPS II, as light increases. Modulated chlorophyll fluorescence can readily measure the adjustment of light harvesting to energy utilization over a short time perspective (Schreiber et al. 1994) and characterizes light acclimation of photosynthesis. To our knowledge, there are no quantitative and/or experimental studies on how white oak photochemical and non-photochemical mechanisms balance each other in response to light acclimation, and the same can be said for most oak species. In general, there is a lack of fundamental knowledge of the physiological processes controlling early growth and development of oaks compared to many commercially important deciduous and coniferous trees in temperate metropolitan conditions.

The objective of the study was to investigate light acclimation in white oak growing under a wide range of light intensities (5, 15, 30, and 100% of full sunlight). Specifically, we examined (1) how growth dynamics and biomass allocation change with light levels and (2) to what extent acclimation influences the rate of electron transport by affecting photochemical and non-photochemical mechanisms.

Materials and Methods
Plant material and growth conditions:
Before the initiation of aboveground growth, 270 rooted seeds were carefully excavated from a mature white oak tree. These rooted seedlings were transplanted into 1.5 L plastic pots containing standard greenhouse potting substrate and irrigated with a 1:100 Hydro Sol 5-11-26 N-P-K. White oak seedlings were grown either in full sunlight or under one of three shading screen regimes (30%, 15%, and 5% of full sunlight) in a rainout shelter. Six plastic pots (24 seedlings) were randomly assigned into each light level.

Modulated chlorophyll fluorescence measurements: Randomly selected plants from each treatment combination were chosen for repeated sampling of modulated chlorophyll fluorescence (n = 6). The light response of modulated chlorophyll fluorescence was measured on individual leaves using a portable modulated fluorometer (OS5-FL; Opti-Sciences, Tyngsboro, Mass.) at room temperature. When designing the actinic light intensity protocol, it was established that no significant photosynthetic increase occurred above 800 µmol m-2 s-1, therefore, a slightly lower but yet still saturated value of 725 µmol m-2 s-1 was used as the highest actinic intensity (~37% of full sunlight) to prevent effects of photoinhibition. Prior to the first measurement, leaves were equilibrated for a minimum of 30 min using dark-adapting leaf clips to determine the dark-adapted initial fluorescence, Fo, and maximum fluorescence (Fms) and the maximal fluorescence (F'm). Before the light level was changed, the minimum fluorescence in the light (F'o) was determined using a far-red light source.

Results
Irradiance Treatments: The plants were grown under four different irradiance conditions: full light (FL), light shade (LS), medium shade (MS), and high shade (HS). The LS, MS, and HS treatments resulted in the gradient of light environments that are characteristic of understory conditions in gap, moderate, and dense vegetation cover.

Photosynthetic light acclimation: Oak seedlings showed light acclimation of photosynthetic performance (Figure 1A-D). As growth irradiance declined, qP decreased in response to shade at higher values of PPFD across all treatments (Figure 1A). Non-photochemical quenching (NPQ), a relative estimate of thermal dissipation of excitation energy, was also affected as shade intensity increased. In response to lower light levels, NPQ versus actinic light increased (Figure 1B). Figure 1B further illustrates that NPQ and PS II reaction centers are affected in response to light acclimation. Figure 1C illustrates a decrease in Φpsπ as actinic PPFD values increase. Oak seedlings also showed a slight decrease of Φpsπ as irradiance growth conditions decreased, but the main affect was due to differences in actinic light. The electron transport rate (ETR), illustrated in Figure 3D, followed a more pronounced pattern as opposed to Φpsπ, where the light saturated ETR was 46% lower in HS as opposed to seedlings grown under FS conditions.

Figure 2 depicts the relationship between ETR and the driving force of electron transport to the reduction of the redox state of the primary electron acceptor of PS II (1-qP). Figure 2 also illustrates an estimate of the flux of electrons through PS II via the ETR. There was a clear separation of irradiance response due to growth irradiance acclimation among the seedlings. The reduction in yield of ETR was lowest in HS and highest under FS conditions.

Figure 3 is an indication that the photosynthetic apparatus has a lower conductance as actinic light increases. The conductance estimate is derived by dividing ETR by 1-qP to obtain an approximate estimate of the conductance of the photosynthetic apparatus (Rosenqvist 2001). The large drop in the ratio seen at low PPFD in FL plants and slightly evident in HS plants, was a result of a large reduction in the redox state at the lowest PPFD levels. The lower response of HS plants was indicative of an absent or possibly lower PPFD redox state reduction.

Figure 4A illustrates that at low irradiance, the (1-qP)/NPQ level peaked in FS and LS conditions, whereas MS and HS presented no clear relationships. A clear difference between treatments is evident in Figure 4B, where the difference between the NPQ/qP ratio versus irradiance level increases with increasing growth irradiance. Figure 4B further illustrates the shift from photochemical to non-photochemical quenching with increasing actinic light. The slope of this relationship gives a relative value of the ‘quenching shift rate’ (Rosenqvist 2001). As growth irradiance declined, the ‘quenching shift rate’ declined as well.

Discussion To characterize light acclimation of photosynthesis, we used modulated chlorophyll a fluorescence. The absence of light acclimation of light-saturated NPQ in white oak seedlings is consistent with findings in Digitalis (Johnson et al. 1994) and Hibiscus (Rosenqvist 2001). The level of total NPQ at saturating PPFD of 725 µmol m-2 s-1 in white oak is consistent with values found in obligate shade species and sun species (Johnson et al. 1994). Alternatively, Osmond et al. (1993) found that some species acclimate, where plants exposed to high light have a higher NPQ capacity than plants grown in low light. The conflicting results emphasize the light acclimation strategies that occur and the variation in thermal dissipation of excitation energy among species.

Photochemical quenching (qP) is often used as an indicator of PS II primary electron acceptor oxidation (e.g. Rosenqvist 2001), where the Φpsπ is the product of qP and the excitation energy transfer efficiency from the light harvesting complex of PS II to the PS II reaction center (Genty et al. 1989). White oak showed a larger decline in qP versus light with decreasing growth irradiance as compared to Φpsπ. The oxidation of the primary electron acceptor of PS II indicated by qP was more pronounced than the loss of Φpsπ, indicated by an increase in non-photochemical quenching. The flux of electrons through PS II can also be estimated as ETR. The relationship between ETR and 1- qP illustrated a clear differentiation of light acclimation within the species, where the apparent reduction yield of elctron transport through PS II was lowest in HS and highest in FS treatments.

To estimate the amount of excess photons reaching PS II, we used an index related to light acclimation, namely the ratio (1- qP)/NPQ. The ratio is also an index of the susceptibility of PS II to light stress. The white oak seedlings did not show a constant ratio on (1- qP)/NPQ. Although the highest ratio was found under low light conditions in FL seedlings, other light levels did not follow suit. Other than this one instance, the results do not indicate a clear difference in response to light acclimation. The light response of the NPQ/qP ratio on the other hand did present a clear light treatment effect. The shift from photochemical to non-photochemical quenching occurs as actinic light increases and is a relative indication of the quenching shift rate. The non-linear curves show the demand for electron flux through PS II, where LS, MS, and HS had a lower flux of electrons through a reduced PS II.

In conclusion, white oak seedlings demonstrated clear photosynthesis acclimation and strong growth and allometric responses to light intensity. Using a steady-state irradiance during chlorophyll fluorescence measurements of light response, we were able to obtain steady-state results of quenching and provide insight into physiological constraints that manifest themselves in growth and survival responses that may influence oak survival. A lack of studies prevents us from directly comparing our greenhouse study with field experiments. Further work is needed to investigate how white oak seedlings respond to light intensity under different overstory shading regimes in a dynamic light environment.

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Acknowledgements
We thank Joe Toler for statistical advice and Joe Bowden and Gavin Wiggins for measurement assistance. Clemson University and the State of South Carolina Research and Experiment Station funded this research.