Acclimatory responses of Arabidopsis to fluctuating light environment: comparison of different sunfleck regimes and accessions

Abstract

Acclimation to fluctuating light environment with short (lasting 20 s, at 650 or 1,250 μmol photons m(-2) s(-1), every 6 or 12 min) or long (for 40 min at 650 μmol photons m(-2) s(-1), once a day at midday) sunflecks was studied in Arabidopsis thaliana. The sunfleck treatments were applied in the background daytime light intensity of 50 μmol photons m(-2) s(-1). In order to distinguish the effects of sunflecks from those of increased daily irradiance, constant light treatments at 85 and 120 μmol photons m(-2) s(-1), which gave the same photosynthetically active radiation (PAR) per day as the different sunfleck treatments, were also included in the experiments. The increased daily total PAR in the two higher constant light treatments enhanced photosystem II electron transport and starch accumulation in mature leaves and promoted expansion of young leaves in Columbia-0 plants during the 7-day treatments. Compared to the plants remaining under 50 μmol photons m(-2) s(-1), application of long sunflecks caused upregulation of electron transport without affecting carbon gain in the form of starch accumulation and leaf growth or the capacity of non-photochemical quenching (NPQ). Mature leaves showed marked enhancement of the NPQ capacity under the conditions with short sunflecks, which preceded recovery and upregulation of electron transport, demonstrating the initial priority of photoprotection. The distinct acclimatory responses to constant PAR, long sunflecks, and different combinations of short sunflecks are consistent with acclimatory adjustment of the processes in photoprotection and carbon gain, depending on the duration, frequency, and intensity of light fluctuations. While the responses of leaf expansion to short sunflecks differed among the seven Arabidopsis accessions examined, all plants showed NPQ upregulation, suggesting limited ability of this species to utilize short sunflecks. The increase in the NPQ capacity was accompanied by reduced chlorophyll contents, higher levels of the xanthophyll-cycle pigments, faster light-induced de-epoxidation of violaxanthin to zeaxanthin and antheraxanthin, increased amounts of PsbS protein, as well as enhanced activity of superoxide dismutase. These acclimatory mechanisms, involving reorganization of pigment-protein complexes and upregulation of other photoprotective reactions, are probably essential for Arabidopsis plants to cope with photo-oxidative stress induced by short sunflecks without suffering from severe photoinhibition and lipid peroxidation.

Fig. 1

Non-photochemical quenching (NPQ) measured in leaves of Col-0 plants during 7-day exposure to different light regimes. NPQ was induced by illumination at 1,000 μmol photons m⁻² s⁻¹ (indicated by a white bar above the x-axis) for 8 min and dark relaxation was monitored subsequently for 14 min. The different light regimes in the climate chamber were: constant PAR of a ca. 50 (C 50), b 85 (C 85) and f 120 μmol photons m⁻² s⁻¹ (C 120) with a photoperiod of 12 h/12 h day/night; c long sunflecks of 650 μmol photons m⁻² s⁻¹ once a day at around midday (LSF 650); short sunflecks of d 650 μmol photons m⁻² s⁻¹ applied every 6 min (SSF 650/6), or 1,250 μmol photons m⁻² s⁻¹ every e 12 (SSF 1250/12) or g 6 min (SSF 1250/6). The treatments with LSF and SSF were performed under the C 50 condition. The daily total PAR was about a 2.1, b–e 3.6 or f and g 5.1 mol photons m⁻² day⁻¹. Plants were grown under C 50 and the light treatments were started on day 0. The maximal PSII efficiency of dark-adapted leaves (F _v/F _m) at the beginning of the measurements was 0.79~0.82 for all plants throughout the 7-day experiment. Data are means of five plants (±SE)

Fig. 2

Reduction state of Q _A (1–qP) during light induction. The measurement protocol and the abbreviations of the light regimes are as described in the legend to Fig. 1. Data are means of five plants (±SE)

Fig. 3

Electron transport rate (ETR) during light induction. The values were calculated from the effective PSII efficiency measured under 1,000 μmol photons m⁻² s⁻¹ as described in the legend to Fig. 1. Data are means of five plants (±SE)

Fig. 4

Contents of a soluble sugars and b starch in leaves of Col-0 plants. a Sum of sucrose, glucose and fructose. b Starch concentrations measured as glucose. Leaf samples for carbohydrate assay were harvested after 10 h of illumination by different light regimes on day 2 (solid bars) and day 5 (striped bars). The daily total PAR of different light regimes was ca. 2.1 (black bars), 3.6 (gray bars) and 5.1 (white bars) mol photons m⁻² day⁻¹. Asterisks indicate significant differences (P < 0.05) compared to the C 50 samples of the same day. Data are means of three plants (±SE)

Fig. 5

Response of leaf growth in Col-0 plants to different light regimes. a Development of the projected total leaf area. Data of each treatment were fitted to an exponential growth function (r ² > 0.96 for all data sets) to obtain mean relative growth rates. b Relative growth rates ( % day⁻¹). The daily total PAR of different light regimes was ca. 2.1 (black symbols and bar), 3.6 (gray symbols and bars) and 5.1 (white symbols and bars) mol photons m⁻² day⁻¹. Asterisks in b indicate significant differences (P < 0.05) compared to C 50. Data are means of 20 plants (±SE). c A top-view and a side-view of leaves grown in C 50 (left) or SSF 1250/6 (right)

Fig. 6

Non-photochemical quenching (NPQ) measured in leaves of different Arabidopsis accessions during 7-day exposure to SSF 1250/6. The NPQ was induced by illumination at 1,000 μmol photons m⁻² s⁻¹ for 5 min. The maximal PSII efficiency of dark-adapted leaves at the beginning of the measurements was 0.78–0.82 for all plants during the 7-day experiment. Data are means of 10~12 plants for Col-0 and 3~4 plants for other accessions (±SE)

Fig. 7

Response of leaf growth to SSF 1250/6 in different Arabidopsis accessions. Relative growth rate was obtained by fitting the data of the projected total leaf area to an exponential function (r ² > 0.98 for all data sets), as illustrated in Fig. 5a. Asterisks indicate significant differences (***P < 0.001; *P < 0.05) between C 50 and SSF 1250/6 for each accession. Data are means of 11~15 plants (±SE)

Fig. 8

Changes in leaf pigment composition of Col-0, C24 and Eri. a Total chlorophyll content. b Chlorophyll a to chlorophyll b ratio. c Pool size of the xanthophyll-cycle pigments. Leaf samples for a–c were harvested at the end of the night period on day 0 (all plants under C 50) and day 7 (C 50 or SSF 1250/6). None of the leaves contained A or Z except a single SSF sample of Col-0 in which a small amount of A was detected on day 7. d De-epoxidation state (DPS) of the xanthophyll-cycle pigments after 5-min exposure to 1,000 μmol photons m⁻² s⁻¹. The DPS was calculated as (A + Z)/(V + A + Z). For each accession, asterisks indicate significant differences (**P < 0.01; *P < 0.05) between day 0 (C 50) and day 7 of SSF 1250/6; plus signs indicate significant differences (⁺⁺ P < 0.01; ⁺ P < 0.05) between C 50 and SSF 1250/6 on day 7. Data are means of 3~4 plants (±SE)

Fig. 9

Immunoblot analysis showing PsbS protein levels in mature leaves of Col-0, C24 and Eri acclimated to the C50 or SSF 1250/6 conditions. Extracts from three replicate leaves (from three replicate plants) were harvested on day 7 and pooled for each genotype and treatment

Fig. 10

Superoxide dismutase activity (a) and malondialdehyde content (b) in leaves of Col-0, C24 and Eri. Leaf samples were harvested on day 0 (black bars, all plants under C 50) and day 7 (gray bars, C 50; white bars, SSF 1250/6). For each accession, asterisks indicate significant differences (P < 0.05) between day 0 (C 50) and day 7 of SSF 1250/6; plus signs indicate significant differences (P < 0.05) between C 50 and SSF 1250/6 on day 7. Data are means of four plants (±SE)

Fig. 11

A diagram summarizing the responses of Arabidopsis (Col-0) during 7-day acclimation to constant (C 85, C 120) or fluctuating light environment with long (LSF 650) or short sunflecks (SSF 650/6, SSF 1250/12, SSF 1250/6). All plants were acclimated to the C 50 condition before starting the experiments on day 0