Photosynthetic acclimation to light gradients in plant stands comes out of shade

Abstract

Dense plant populations or canopies exhibit a strong enrichment in far-red wavelengths which leads to unequal excitation of the two photosystems. In the long-term plants acclimate to changes in light quality by adjusting photosystem stoichiometry and antenna structure, a mechanism called here long-term response (LTR). Using an artificial light system it is possible to mimic such naturally occurring gradients in light quality under controlled laboratory conditions. By this means we recently demonstrated that the LTR is crucial for plant fitness and survival of Arabidopsis. We could also demonstrate that the chlorophyll fluorescence parameter Fs/Fm is a genuine non-invasive functional indicator for acclimatory changes during the LTR. Here we give supportive data that the Fs/Fm can be also used to monitor the LTR in field experiments in which Arabidopsis plants were grown either under canopies or wavelength-neutral shade. Furthermore our data support the notion that acclimation responses to light quality and light quantity are separate mechanisms. Thus, the long-term response to light quality represents an important and distinct acclimation strategy for improving plant survival under changing light quality conditions.

Photosynthetic Acclimation to Light Quality is Essential for Plant Survival

The sessile lifestyle of plants requires sophisticated acclimation programmes to cope with the steadily changing environment and to optimize growth and reproduction. Light as one essential environmental factor can vary in its composition and intensity in the order of magnitudes which often leads to disturbed and reduced photosynthetic efficiency. Such variations in illumination occur on different time scales. Fluctuating light evoked by passing clouds or leaf movements triggers short-term responses such as energy quenching (NPQ) or state transitions.^1–3 These processes act at the posttranslational level and are reversible within seconds to minutes.^4,5 Slow changes in the light environment, in contrast, need to be counterbalanced by more sustainable processes, so called long-term responses, which restructure the photosynthetic complexes within the thylakoid membrane in number and size. This involves changes in gene expression and re-modulations of metabolism.^6,7 So far, two distinct long-term light acclimation processes have been described (i) the long-term response to changes in light quantity whereupon an adjustment of PSII/LHCII antenna size occurs and (ii) the long-term response (LTR) to light quality gradients which acts under low light conditions and counteracts imbalances in excitation between the photosystems mainly by re-adjustment of PSI/PSII ratio (Fig. 1A).^8–11 Imbalances in photosynthetic electron transport due to changing light quality are sensed within the chloroplast via changes in the redox state of mobile electron carriers (i.e. PQ.) of the transport chain and this, in turn, activates an intraplastidial gene regulation network in order to adjust the stoichiometry of photosynthetic complexes. This mechanism assures an unhindered electron flow and prevents the photosynthetic apparatus from photo-oxidative damages. This regulation network extends also to photosynthetic and metabolic genes encoded in the nucleus and therefore constitutes an important retrograde (plastid-to-nucleus) flow of information.^12–14 The LTR is an evolutionary conserved feature that can be found in photosynthetic organisms ranging from cyanobacteria over unicellular algae up to plants.^15–19 To investigate this mechanism at molecular and physiological level we mimicked light quality gradients using a light system causing preferential excitation of either PSI or PSII.⁸ In a recent study we demonstrated that the LTR to light quality is an indispensable mechanism for plants to survive under imbalanced excitation. As a final measure for effectiveness of acclimation we determined seed production and found that plants lacking the LTR (e.g., in the stn7-mutant) produce 50% less seeds than WT under long-term alternating light qualities.²⁰ According to the Hardy-Weinberg rule of population genetics less fit individuals may become extinct in an exponential manner. This means that the homozygous mutant allele would be nearly extinct within 10 generations (1.6% allele frequency). Hence, it can be concluded that even small changes in the ability of plants to acclimate to different light situations may have huge effects on the survival of the species.

Figure 1.

Light gradients in a plant canopy. (A) Varying light intensities are depicted as grey triangle, light quality gradients are given as red triangle. Under saturating light the photosynthetic electron transport chain (including PQH₂ and TRX) is highly reduced and feedback-de-excitation mechanisms restore the redox-poise in the photosynthetic light reaction. Enrichment of farred wavelengths can be observed under light limiting conditions in plant canopies which preferentially excite PSI leading to a redox imbalance between the photosystems and oxidation of the PQ-pool. By this means a signal is generated which leads to an adjustment of photosystem stoichiometry, thylakoid structure and metabolism as a consequence of the LTR For further details see text. (B) Fs/Fm parameter as a marker of LTR under laboratory and field conditions. Plants acclimated to either PSI-light or shade of a canopy (can) were shifted for 2–3 days to PSII-light or a neutral (neut) shade and vice versa. The plants show a comparable acclimation pattern with similar high Fs/Fm values under PSI-light and canopy shade, respectively, whereas plants acclimated to PSII-light or neutral shade exhibit drastically lower values. PAR under all light conditions was set to ∼20 µmol photons*m⁻²*s⁻¹.

Using Chlorophyll Fluorescence to Monitor LTR

We developed and refined physiological markers for this photosynthetic acclimation. The ratio of chlorophyll (Chl) a over Chl b is a well known structural indicator for changes in PS-stoichiometry and/or LHC-antenna size.^7,10 As a second marker we established a functional indicator, the chlorophyll fluorescence parameter Fs/Fm. The steady state fluorescence yield (Fs) of PSI-light acclimated plants (PSI-plants) was found to be constantly higher than that of plants acclimated to PSII-light when determined under standard conditions. This rise in fluorescence can be best explained by the readjustment of PS stoichiometry leading to a higher PSII/PSI ratio in PSI-plants than in PSII-plants resulting in a less efficient transfer of electrons into photosynthetic products under the measuring light of the fluorometer.²¹ But are changes in Fs/Fm and Chl a/b independent from short-term responses such as state transitions and feedback-de-excitation mechanisms (NPQ)? To address this question, we shifted PSI-plants to PSII-light and recorded Fs/Fm at different time points after the shift. The kinetics demonstrated that the major changes in Fs/Fm occurred 12–24 h after the light shift indicating that state transitions had no or only little effect on this parameter.²⁰ Furthermore, we tested the ability to perform the LTR in an NPQ-deficient mutant and found a WT-like response indicating that the fast energy quenching via PsbS has no effect on the performance of the LTR.²⁰ We further tested whether the Fs/Fm parameter is affected by different light quantities. We determined Fs/Fm from plants acclimated to WL conditions ranging from 15–60 µmol photons*s⁻¹*m⁻² PAR and found no changes whereas plants shifted from PSI-light (24 µmol photons*s⁻¹*m⁻² PAR) to PSII-light (12 µmol photons*s⁻¹*m⁻² PAR) and vice versa exhibited significant differences of 40–50% in the Fs/Fm. Taken together, this clearly demonstrates that light quality and light quantity acclimation are functionally distinct although they occur within the same time range.²⁰

Occurrence of LTR in the Field

Here we give supportive data that the LTR is not only restricted to controlled lab conditions. In a field experiment we transferred Arabidopsis to different natural low light habitats. One set of seedlings was acclimated to shade under a canopy of beeches, the control group was left in light quality-neutral shade of identical flux density in photosynthetically active radiation (20 µmol photons*m⁻²*s⁻¹). The Fs/Fm value of plants acclimated 10 d to shade under a canopy was about 50% higher than that of plants grown in neutral shade. This corresponds to the results we obtained in laboratory experiments and strongly supports the notion that photosynthetic acclimation to different light qualities is a crucial mechanism for plant fitness in field conditions. Since the light intensity in the field experiments was set equally in neutral shade and canopy shade this indicates that the LTR is not overridden by other environmental factors such as illumination changes in the day night cycle and differences in temperature or humidity during the day. This provides further evidence that the Fs/Fm value is a robust photosynthetic parameter to indicate light quality acclimation. How this non-invasively measured parameter can be linked to molecular markers and signalling cascades is a challenging field for future research (Fig. 1B).

Material and Methods

Arabidopsis thaliana Col-0 plants for the field experiment were grown on soil for 5 weeks at an illumination of 80 µmol photons*m⁻²*s⁻¹ and 9 h-light 15 h-dark-cycle. The plants were then transferred to the Botanical Garden of the Friedrich-Schiller-University Jena under a beech canopy and under neutral shade at a place nearby. Light qualities were recorded at the sites with spectroradiometer (GER1500, Geophysical and Environmental Research Corp., Millbrook (NY), USA). PAR was determined using the Hansatech Quantitherm QRT1 light-meter (Norfolk, UK). Chlorophyll-fluorescence was measured with a PAM-2000 device (Walz, Effeltrich, Germany).²² After 7 days of acclimation half of the population was shifted between the sites for further 3 days. The experiment took place May 21^st to 31^st 2008. Weather data from Jena during the experiment can be found at www.bgc-jena.mpg.de/wetter/mpi_roof_2008a.zip

Abbreviations

LTR

long-term response to light quality

PS

photosystem

LHC

light harvesting complex

PQ

plastoquinone

TRX

thioredoxin

NPQ

non-photochemical quenching

PAR

photosynthetically active radiation

Addendum to: Wagner R, Dietzel L, Bräutigam K, Fischer W, Pfannschmidt T. The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of Arabidopsis thaliana under low light conditions. Planta. 2008;228:573–587. doi: 10.1007/s00425008-0760-y.