Photoregulation of the Light-Harvesting Chlorophyll Protein Complex Associated with Photosystem II in Dunaliella tertiolecta: Evidence that Apoprotein Abundance but Not Stability Requires Chlorophyll Synthesis

The marine chlorophyte Dunaliella tertiolecta Butcher responds to a one-step transition from a high growth irradiance level (700 micromoles quanta per square meter per second) to a low growth irradiance level (70 micromoles quanta per square meter per second) by increasing the total amount of light-harvesting chlorophyll (Chl) a/b binding protein associated with photosystem II (LHC II), and by modifying the relative abundance of individual LHC II apoproteins. When high light-adapted cells were incubated with gabaculine, which inhibits Chl synthesis, and transferred to low light, the LHC II apoproteins were still synthesized and the ³⁵S-labeled LHC II apoproteins remained stable after a 24 hour chase. These results suggest that Chl synthesis is not required for stability of the LHC II apoproteins in this alga. However, when the control cells are transferred from high light to low light, the amount of the four LHC II apoproteins per cell increases, whereas it does not in the presence of gabaculine. These results suggest that Chl synthesis is required for a photoadaptive increase in the cellular level of LHC II.     

Acclimation of Ribulose Bisphosphate Carboxylase and mRNAs to Changing Irradiance in Adult Tobacco Leaves: Differential Expression in LSU And SSU mRNA

The transfer of Nicotiana tabacum plants grown in low light (60 micromoles quanta per square meter per second) to higher light (360 micromoles quanta per square meter per second) was previously shown to induce adaptive stimulation of photosynthetic capacities. The variations of ribulose bisphosphate carboxylase/oxygenase (RubisCo) expression in mature leaves was examined as a result of this acclimation. Maximum or initial activities increased markedly after low- to high-light transfer with a maximum effect after 2 to 3 days. The higher activity is mainly explained by RubisCo protein synthesis as shown by immunorocket technique. Small subunits of RubisCo (SSU) mRNA relative content determined by hybridization of total RNA with DNA probe by Dot-blot method, followed the same pattern as RubisCo quantity. The magnitude of this response was amplified when more contrasting light conditions (25 versus 360 micromoles per square meter per second) were established on the same leaf: RubisCo activity, RubisCo protein, and SSU mRNA contents decreased in the shaded zone and increased in the high-light zone within 1 day. After 2 days the shade/light ratio was 1 to 3 for RubisCo protein and 1 to 4 for SSU-RNA, whereas the ratios remained equal to one in controls. Hybridization of the same RNA extracts with large subunits of RubisCo (LSU) probe showed no variation in LSU-RNA content. So in green adult leaves, the expression of SSU and LSU genes is regulated differently. The observed white light quantitative effect on RubisCo expression was not dependent on the photosynthetic rate or assimilate content since low CO₂ concentration around the leaf after the light shift did not modify the response.     

Protein-bound ribulose bisphosphate correlates with deactivation of ribulose bisphosphate carboxylase in leaves

Previous reports indicate that ribulose 1,5-bisphosphate (RuBP) binds very tightly to inactive ribulose bisphosphate carboxylase (rubisco) in vitro. Therefore, we decided to investigate whether there was evidence for tight binding of RuBP associated with deactivation of rubisco in vivo. We modified a technique for rapidly separating `free' metabolites from those bound to high molecular compounds. Arabidopsis thaliana plants were illuminated at various irradiances before freezing the leaves in liquid N₂ and assaying rubisco activity and RuBP. The percentage activation of rubisco varied from 37% at low irradiance (45 micromoles quanta per square meter per second) to 100% at high irradiance (800 micromoles quanta per square meter per second). The total amount of RuBP did not vary much with irradiance, but bound RuBP changed from 36% of the total at low irradiance to none at high irradiance. Bound RuBP was significantly correlated with the estimated number of inactive rubisco sites, with a ratio of about 1:1. After a step increase in irradiance, rubisco activation increased and total RuBP increased transiently, but steady levels of both occurred by 10 minutes. The amount of bound RuBP decreased with a similar time course to the estimated decrease in inactive rubisco sites. After a step decrease in irradiance, rubisco deactivated slowly for at least 25 minutes. Bound RuBP increased gradually but did so more slowly than the estimated increase in inactive rubisco sites.     

Growth and Tuberization of Potato (Solanum tuberosum L.) under Continuous Light

The growth and tuberization of potatoes (Solanum tuberosum L.) maintained for 6 weeks under four different regimes of continuous irradiance were compared to plants given 12 hours light and 12 hours dark. Treatments included: (a) continuous photosynthetic photon flux of 200 micromoles per square meter per second cool-white fluorescent (CWF); (b) continuous 400 micromoles per square meter per second CWF; (c) 12 hours 400 micromoles per square meter per second CWF plus 12 hours dim CWF at 5 micromoles per square meter per second; (d) 12 hours micromoles per square meter per second CWF plus 12 hours dim incandescent (INC) at 5 micromoles per square meter per second and a control treatment of 12 hours light at 400 micromoles per square meter per second CWF and 12 hours dark. The study included five cultivars ranging from early- to late-season types: `Norland,' `Superior,' `Norchip,' `Russet Burbank,' and `Kennebec.' Tuber development progressed well under continuous irradiation at 400 micromoles per square meter per second and under 12 hours irradiance and 12 hours dark, while tuber development was suppressed in all other light treatments. Continuous irradiation at 200 or 400 micromoles per square meter per second resulted in severe stunting and leaf malformation on `Superior' and `Kennebec' plants, but little or no injury and vigorous shoot growth in the other cultivars. No injury or stunting were apparent under 12-dim light or 12-dark treatments. Plants given 12 hours dim INC showed significantly greater stem elongation but less total biomass than plants in other treatments. The continuous light encouraged shoot growth over tuber growth but this trend was overridden by providing a high irradiance level. The variation among cultivars for tolerance to continuous lighting indicates that potato may be a useful species for photoinhibition studies.     

Participation of Photosynthesis in Floral Induction of the Long Day Plant Anagallis arvensis L

The saturating photon flux density (400 to 700 nanometers) for induction of flowering of the long day plant Anagallis arvensis L. was 1,900 micromoles per square meter per second (6,000 foot-candles) when an 8-hour daylength was extended to 24 hours by a single period of supplementary irradiation. The saturating photon flux density for photosynthetic CO₂ uptake during the same single supplementary light period was lower, at about 1,000 to 650 micromoles per square meter per second (3,000 to 2,000 foot-candles).

The per cent flowering and mean number of floral buds per plant were significantly reduced when the light extension treatment was given in CO₂-free air, and glucose (10 kilograms per cubic meter in water) relieved this effect. Glucose solution also significantly increased flowering of plants given supplementary light treatment in atmospheric air under a photon flux density of 80 micromoles per square meter per second. Increasing the CO₂ concentration to 1.27 grams per cubic meter of CO₂ in air during the supplementary light period did not increase flowering.

It is concluded that high photon flux densities promote flowering of Anagallis through both increased photosynthesis and the photomorphogenic action of high irradiance.

     

Limitation of CO(2) Assimilation and Regulation of Benson-Calvin Cycle Activity in Barley Leaves in Response to Changes in Irradiance, Photoinhibition, and Recovery

The response of the Benson-Calvin cycle to changes in irradiance and photoinhibition was measured in low-light grown barley (Hordeum vulgare) leaves. Upon the transition from the growth irradiance (280 micromoles per square meter per second) to a high photoinhibitory irradiance (1400 micromoles per square meter per second), the CO₂ assimilation rate of the leaves doubled within minutes but high irradiance rapidly caused a reduction in quantum efficiency. Following exposure to high light the activities of NADP-malate dehydrogenase and fructose-1,6-bisphosphatase obtained near maximum values and the activation state of ribulose-1,5-bisphosphate carboxylase increased. The activity of the latter remained constant throughout the period of photoinhibitory irradiance, but the increase in the activities of fructose-1,6-bisphosphatase and NADP-malate dehydrogenase was transient decreasing once more to much lower values. This suggests that immediately following the transition to high light reduction and activation of redox-modulated enzymes occurred, but then the stroma became relatively oxidized as a result of photoinhibition. The leaf contents of glucose 6-phosphate and fructose 6-phosphate increased following exposure to high light but subsequently decreased, suggesting that following photoinhibition sucrose synthesis exceeded the rate of carbon assimilation. The ATP content attained a constant value much higher than that in low light. During photoinhibition the glycerate 3-phosphate content greatly increased while ribulose-1,5-bisphosphate decreased. The fructose-1,6-bisphosphate and triose phosphate contents increased initially and then remained constant. During photoinhibition CO₂ assimilation was not limited by ribulose-1,5-bisphosphate carboxylase activity but rather by the regeneration of the substrate, ribulose-1,5-bisphosphate, related to a restriction on the supply of reducing equivalents.     

Light-dependent kinetics of 2-carboxyarabinitol 1-phosphate metabolism and ribulose-1,5-bisphosphate carboxylase activity in vivo

The light-dependent kinetics of the apparent in vivo synthesis and degradation of 2-carboxyarabinitol 1-phosphate (CA1P) were studied in three species of higher plants which differ in the extent to which this compound is involved in the light-dependent regulation of ribulose-1,5-bisphosphate carboxylase (Rubisco) activity. Detailed studies with Phaseolus vulgaris indicate that both the degradation and synthesis of this compound are light-stimulated, although light is absolutely required only for CA1P degradation. We hypothesize that the steady state level of CAIP at any particular photon flux density (PFD) represents a pseudo-steady state balance between ongoing synthesis and degradation of this compound. The rate of CA1P synthesis in P. vulgaris and the resultant reduction in the total catalytic constant of Rubisco were maximal at 200 micromoles quanta per square meter per second following a step decrease from a saturating PFD, and substantially faster than the rate of synthesis in the dark. Under these conditions an amount of CA1P equivalent to approximately 25% of the Rubisco catalytic site content was synthesized in less than 1 minute. The rate of synthesis was reduced at higher or lower PFDs. In Beta vulgaris, the rate of CA1P synthesis at 200 micromoles quanta per square meter per second was substantially slower than in P. vulgaris. In Spinacea oleracea, an apparent noncatalytic tight-binding of RuBP to deactivated sites on the enzyme was found to occur following a step decrease in PFD. When dark acclimated leaves of P. vulgaris were exposed to a step increase in PFD, the initial rate of CA1P degradation was also found to be dependent on PFD up to a maximum of approximately 300 to 400 micromoles quanta per square meter per second. The rate of degradation of this compound was similar in B. vulgaris. In S. oleracea, a step increase in PFD resulted in noncatalytic RuBP binding to Rubisco followed by an apparent release of RuBP and activation of the enzyme. The in vivo rate of change of Rubisco activity in response to an increase or decrease in PFD was similar between species despite the differences between species in the mechanisms used for the regulation of this enzyme's activity.     

The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O₂ evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.     

Sugar Concentrations in Guard Cells of Vicia faba Illuminated with Red or Blue Light : Analysis by High Performance Liquid Chromatography

Concentrations of soluble sugars in guard cells in detached, sonicated epidermis from Vicia faba leaves were analyzed quantitatively by high performance liquid chromatography to determine the extent to which sugars could contribute to changes in the osmotic potentials of guard cells during stomatal opening. Stomata were illuminated over a period of 4 hours with saturating levels of red or blue light, or a combination of red and blue light. When stomata were irradiated for 3 hours with red light (50 micromoles per square meter per second) in a solution of 5 millimolar KCl and 0.1 millimolar CaCl₂, stomatal apertures increased a net maximum of 6.7 micrometers and the concentration of total soluble sugar was 289 femtomoles per guard cell (70% sucrose, 30% fructose). In an identical solution, 2.5 hours of irradiation with 25 micromoles per square meter per second of blue light caused a maximum net increase of 7.1 micrometers in stomatal aperture and the total soluble sugar concentration was 550 femtomoles per guard cell (91% sucrose, 9% fructose). Illumination with blue light at 25 micromoles per square meter per second in a solution lacking KCl caused a maximum net increase in stomatal aperture of 3.5 micrometers and the sugar concentration was 382 femtomoles per guard cell (82% sucrose, 18% fructose). In dual beam experiments, stomata irradiated with 50 micromoles per square meter per second of red light opened steadily with a concomitant increase in sugar production. Addition of 25 micromoles per square meter per second of blue light caused a further net gain of 3.7 micrometers in stomatal aperture and, after 2 hours, sugar concentrations had increased by an additional 138 femtomoles per guard cell. Experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) were performed with epidermis illuminated with 50 micromoles per square meter per second of red light or with 25 micromoles per square meter per second of blue light in solutions containing or lacking KCl. DCMU completely inhibited sugar production under red light, had no effect on guard cell sugar production under blue light when KCl was present, and inhibited sugar production by about 50% when guard cells were illuminated with blue light in solutions lacking KCl. We conclude that soluble sugars can contribute significantly to the osmoregulation of guard cells in detached leaf epidermis of V. faba. These results are consistent with the operation of two different sugar-producing pathways in guard cells: a photosynthetic carbon reduction pathway and a pathway of blue light-induced starch degradation.     

Photosynthetic Responses to Irradiance by the Grey Mangrove, Avicennia marina, Grown under Different Light Regimes

Photosynthetic responses to irradiance by the grey mangrove, Avicennia marina (Forstk.) Vierh var. australasica (Walp.) Moldenke, were studied using seedlings grown under natural understory shade and exposed conditions as well as in the laboratory under high and low light regimes, i.e. 100% and 6% sunlight, respectively. Leaves in exposed locations were subjected to daylight quantum flux densities greater than 1,000 microeinsteins per square meter per second from 0900 to 1700 hours, whereas those in understory shade experienced only 30 to 120 microeinsteins per square meter per second, interrupted for brief periods by sunflecks ranging in quantum flux density from 800 to 1,500 microeinsteins per square meter per second. The low light regime was similar in light intensity to that of the understory environment, but lacked sunflecks.

Leaves from the understory environment showed several properties of `shade' leaves; i.e. they contained more chlorophyll on both a leaf area and fresh weight basis but had a lower specific weight and greater area than exposed leaves, and were enriched in chlorophyll b relative to a. However, there were no significant differences in either the gas exchange or leaf chlorophyll fluorescence characteristics of the two populations, both being typical of `sun' leaves.

Leaves grown in the laboratory under low and high light regimes had similar properties. However, light saturated assimilation rates in the leaves from the low light treatment were 20% less and became light saturated at a lower quantum flux density than those of leaves grown under the high light regime. The ecological significance of these results is discussed.

     

Photosynthetic Genes of Petunia (Mitchell) Are Differentially Expressed during the Diurnal Cycle

The petunia (Petunia [Mitchell]) chloroplast proteins, the chlorophyll a/b-binding (Cab) proteins, and the small subunit of ribulose bisphosphate carboxylase (RbcS) are encoded by nuclear genes that are expressed in a light-dependent manner. The steady-state concentrations of five cab mRNAs vary with a dramatic circadian rhythm in plants grown under a constant diurnal cycle (10 hours light, 14 hours dark). cab mRNA levels reach their maximum during the light period, but begin to drop prior to the dark period. These RNAs fall to their minimum concentration during the dark period and then begin to increase again in anticipation of the light. Within this general pattern, there are variations in expression among specific classes of cab genes. The light harvesting complex of photosystem II LHCII-type 1 cab mRNAs rise to a well-defined maximum at 2 hours prior to the dark period. All but one of these genes are expressed in anticipation of the light period. The LHCII type 2 cab mRNA and the LHC of photosystem I cab mRNA are expressed at more constant levels throughout the light period. The expression of these genes anticipates the light more than does the expression of the LHCII type 1 genes. The steady state mRNA levels for the petunia rbcS genes show no significant diurnal fluctuation.     

Light intensity regulates the accumulation of the major light-harvesting chlorophyll-protein in greening seedlings

Etiolated pea (Pisum sativum [L.] cv Progress 9) and barley (Hordeum vulgare [L.] cv Boone) seedlings greened under either low (40 microeinsteins per square meter per second) or high (550 microeinsteins per square meter per second) intensity light were analyzed for chlorophyll (Chl) content and the levels of mRNA and protein for the major light-harvesting chlorophyll (Chl)-protein of photosystem II (LHC-II). Low intensity plants accumulated Chl more rapidly than high intensity plants. Both single radial immunodiffusion analysis and mild sodium dodecyl sulfate-polyacrylamide gel electrophoresis green gels showed that low intensity plants also accumulated LHC-II protein more rapidly than high intensity plants, following a kinetic pattern similar to the total Chl data. In contrast, LHC-II mRNA levels appeared to be independent of LHC-II protein levels although pea and barley LHC-II mRNA exhibited different light intensity responses. The absence of coordination between LHC-II mRNA and protein levels suggested that the biosynthesis of LHC-II in greening seedlings is not limited by mRNA. A correlation (better than the 0.01 significance level) between LHC-II protein accumulation and Chl accumulation was found for both pea and barley. The accumulation of LHC-II protein was not linked to the development of photosynthetic electron transport. These results and the similar effect of light intensity on Chl content and LHC-II protein levels suggested that the availability of Chl may limit LHC-II protein accumulation in greening seedlings.     

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Light Intensity and CO(2) in the C(3) Annuals Chenopodium album L. and Phaseolus vulgaris L

The light and CO₂ response of (a) photosynthesis, (b) the activation state and total catalytic efficiency (k_cat) of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) the pool sizes of ribulose 1,5-bisphosphate, (RuBP), ATP, and ADP were studied in the C₃ annuals Chenopodium album and Phaseolus vulgaris at 25°C. The initial slope of the photosynthetic CO₂ response curve was dependent on light intensity at reduced light levels only (less than 450 micromoles per square meter per second in C. album and below 200 micromoles per square meter per second in P. vulgaris). Modeled simulations indicated that the initial slope of the CO₂ response of photosynthesis exhibited light dependency when the rate of RuBP regeneration limited photosynthesis, but not when rubisco capacity limited photosynthesis. Measured observations closely matched modeled simulations. The activation state of rubisco was measured at three light intensities in C. album (1750, 550, and 150 micromoles per square meter per second) and at intercellular CO₂ partial pressures (C₁) between the CO₂ compensation point and 500 microbars. Above a C₁ of 120 microbars, the activation state of rubisco was light dependent. At light intensities of 550 and 1750 micromoles per square meter per second, it was also dependent on C₁, decreasing as the C₁ was elevated above 120 microbars at 550 micromoles per square meter per second and above 300 microbars at 1750 micromoles per square meter per second. The pool size of RuBP was independent of C₁ only under conditions when the activation state of rubisco was dependent on C₁. Otherwise, RuBP pool sizes increased as C₁ was reduced. ATP pools in C. album tended to increase as C₁ was reduced. In P. vulgaris, decreasing C₁ at a subsaturating light intensity of 190 micromoles per square meter per second increased the activation state of rubisco but had little effect on the k_cat. These results support modelled simulations of the rubisco response to light and CO₂, where rubisco is assumed to be down-regulated when photosynthesis is limited by the rate of RuBP regeneration.     

Resistance to low temperature photoinhibition is not associated with isolated thylakoid membranes of winter rye

In vivo measurements of chlorophyll a fluorescence indicate that cold-hardened winter rye (Secale cereale L. cv Musketeer) develops a resistance to low temperature-induced photoinhibition compared with nonhardened rye. After 7.2 hours at 5°C and 1550 micromoles per square meter per second, the ratio of variable fluorescence/maximum fluorescence was depressed by only 23% in cold-hardened rye compared with 46% in nonhardened rye. We have tested the hypothesis that the principal site of this resistance to photoinhibition resides at the level of rye thylakoid membranes. Thylakoids were isolated from cold-hardened and nonhardened rye and exposed to high irradiance (1000-2600 micromoles per square meter per second) at either 5 or 20°C. The photoinhibitory response measured by room temperature fluorescence induction, photosystem II electron transport, photoacoustic spectroscopy, or [¹⁴C]atrazine binding indicates that the differential resistance to low temperature-induced photoinhibition in vivo is not observed in isolated thylakoids. Similar results were obtained whether isolated rye thylakoids were photoinhibited or thylakoids were isolated from rye leaves preexposed to a photoinhibitory treatment. Thus, we conclude that increased resistance to low temperature-induced photoinhibition is not a property of thylakoid membranes but is associated with a higher level of cellular organization.     

Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a three-fold reduction in photosystems I and II (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P₇₀₀) and PSII (chlorophyll/Q_A) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in β-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.     

Blue-Light Regulation of Epicotyl Elongation in Pisum sativum

Blue light is known to induce suppression of stem elongation. To avoid the complication of blue-light-induced transformation of phytochrome we have adapted the procedure of measuring blue-light-induced suppression of stem elongation in Pisum sativum L. var Alaska grown under continuous red light. The resulting fluence-response curve for suppression of epicotyl elongation measured twenty-four hours after a blue-light treatment is bell-shaped, with the peak of suppression between 10⁰ and 10¹ micromoles per square meter, and no suppression at 10⁴ micromoles per square meter. Suppression is first observed 5 and 11 hours after the blue-light treatment for the fourth and third internodes, respectively. No significant differences in elongation rates were noted for the 10⁴ micromoles per square meter treated seedlings throughout the 24 hour period. Reciprocity holds for both third and fourth internodes in response to 10¹ and 10⁴ micromoles per square meter of blue light over the range of irradiation times tested (10⁰ to 10⁴ seconds, 10¹ micromoles per square meter; 10⁰ to 10³ seconds, 10⁴ micromoles per square meter). In contrast to the bell-shaped fluence-response obtained for epicotyl elongation, measurements of chlorophyll and carotenoid accumulation indicate increasing accumulation with increasing fluence.     

Novel light-regulated chloroplast thylakoid membrane protein

A 64 kilodalton chloroplast membrane polypeptide was dependent on growth irradiance with 10-fold greater quantities of the protein present in barley (Hordeum vulgare) grown under 500 micromoles of photons per square meter per second compared with growth at 50 micromoles per square meter per second. The concentration of the protein was sensitive to changes in irradiance, with a slow time course for the response (days) similar to other reported light acclimation processes. The polypeptide also was observed in maize (Zea mays), oats (Avena sativa), and wheat (Triticum aestivum), but not in soybean (Glycine max Merr). The 64 kilodalton polypeptide did not correspond to any thylakoid membrane protein with an assigned function, so its structural or regulatory role is not known.     

Light Quality and Irradiance Level Interaction in the Control of Expression of Light-Harvesting Complex of Photosystem II: Pigments, Pigment-Proteins, and mRNA Accumulation

Effects of red and blue light at irradiances from 1.6 to 28.3 micromolar per square meter per second on chloroplast pigments, light-harvesting pigment-proteins associated with photosystem II, and the corresponding mRNA were evaluated in maize (Zea mays L.) plants (OP Golden Bantum) grown for 14 days under 14 hours light/10 hours dark cycles. Accumulation of pigments, pigment-proteins, and mRNA was less in blue than in red light of equal irradiance. The difference between blue and red light, however, varied as a function of irradiance level, and the pattern of this variation suggests irradiance-controlled activation/deactivation (switching) of blue-light receptor. The maximum reduction in blue light of mRNA and proteins associated with light-harvesting complex occurs at lower irradiance levels than the maximum reduction of chlorophylls a and b.