Cotton plant size and fiber developmental responses to FR/R ratio reflected from the soil surface

Cotton ( Gossypium hirsutum L.) plants were grown in irrigated field plots over red, green or white soil covers (mulches). The far-red (FR) to red (R) light ratios were higher in upwardly reflected light over the red and green surfaces than in incoming sunlight. Plants that grew over the white mulches received higher photosynhetic photon flux (PPF), but the reflected FR/R ratio did not differ significantly from that in incoming sunlight. At five weeks after emergence, seedling stern lengths were significantly longer over red and green than over white surfaces. At maturity, plants that had grown over the red and green surfaces had longer stems, larger shoots, more bolls (fruit), more seed cotton, and longer fibers than plants grown over the white surfaces even though those grown over the white surfaces had received more reflected photosynthetic light during growth and development.     

Row orientation effects on FR/R light ratio, growth and development of field-grown bush bean

Bush bean ( Phaseolus vulgaris L. cv. Tenderette) plants were grown in north-south (N-S) and east-west (E-W) rows under field conditions to test effects of row orientation on reflected far-red (FR) light patterns and on shoot size and edible bean productivity. Soil water and nutrients were adequate. Plants in N-S rows received slightly higher ratios of FR relative to red (R) light, because of heliotropic movement of the leaves. Plants in N-S rows partitioned more dry matter to shoots and edible green beans than those in E-W rows. We conclude that row orientation of broadleaf plants can affect the FR/R light ratio and the phytochrome-mediated regulation of plant development under field conditions.     

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.     

Application of a Chlorophyll Fluorescence Sensor to Detect Chelate-Induced Metal Stress in Zea mays

Metal stress was induced in maize (Zea mays L.) by the addition to the soil of a range of concentrations of either ethylene-diamine-tetra-acetate (EDTA) or citric acid (CA) as chelating agents. Measurements were taken using a recently-developed sensor capable of plant fluorescence detection at wavelengths of 762 and 688 nm. Atmospheric oxygen absorbs radiation at these wavelengths. As such, measured fluorescence can be attributed to the plants under observation. Red/far-red (690/760 nm, R/FR) chlorophyll (Chl) fluorescence ratios were measured before addition of the chelating agents and during the month following. Significant differences were seen in the fluorescence responses of those plants for which high concentrations [ 30 mmol kg^–1(d.m. soil)] of EDTA were added to the pots compared to those for which CA or no chelating agent was added. The plants for which high concentrations of EDTA were added also exhibited higher tissue metal concentrations and demonstrated visible signs of stress. Before signs of visual stress became apparent, R/FR Chl fluorescence ratios for metal-stressed plants were significantly different to those observed for unstressed plants. These results support the use of plant fluorescence as a potential tool for early indication of phytotoxic metal stress.     

Chloroplast Response in Dunaliella salina to Irradiance Stress (Effect on Thylakoid Membrane Protein Assembly and Function)

The chloroplast response in the green alga Dunaliella salina to irradiance stress was investigated. Cells were grown under low light (LL) at 100 [mu]mol photons m-2 s-1 or high light (HL) at 2000 [mu]mol photons m-2 s-1 incident intensity. LL-grown cells had a low chlorophyll (Chl) a/b ratio, an abundance of light-harvesting complex II proteins (LHC-II), and a large Chl antenna size. HL-grown cells had a higher Chl a/b ratio, relatively fewer LHC-II, and a small Chl antenna size. The more abundant higher molecular mass subunits of the LHC-II (approximately 31 kD) were selectively depleted from the thylakoid membrane of HL-grown cells. Light-shift experiments defined the kinetics of change in the subunit composition of the LHC-II and suggested distinct mechanisms in the acclimation of thylakoids to HL or LL conditions. The results showed that irradiance exerts a differential regulation on the expression of various Lhcb genes. The specific polyclonal antibodies used in this work, raised against the purified LHC-II, cross-reacted with a polypeptide of approximately 20 kD in HL-grown samples. In this work we examined the dynamics of induction of this novel protein and discuss its function in terms of a chloroplast response to the level of irradiance.     

Organization and Stability of Polypeptides Associated with the Chlorophyll a-b Light-Harvesting Complex of Photosystem-II

In the oxygen-evolving photosystem-II (PSII) of higher plantchioroplasts and green algae, most of the light-harvesting functionis performed by the chlorophyll (Chl) a-b-protein complex (LHC-II).On the average, the LHC-II contains about 210 Chl (a+b) moleculesper PSII reaction center. The polypeptide composition, copynumber and organization of assembly in the LHC-II complex arenot fully understood at present. This work utilized the chlorinaf2 mutant of barley (lacking Chl b and having a LHC-II antennaof only 13 Chl a molecules) to determine the organization andstability of assembly of proteins in the LHC-II. High-resolutionSDS-PAGE and immunoblot analysis showed the presence of fourmain constitutive polypeptides in the wild-type LHC-II (termedhere subunits a, b, c and d) with molecular masses in the range30–25 kDa. Of those, only subunit d (a 25 kDa polypeptide)was found to occur at an equal copy number per PSII reactioncenter in both wild-type and in the Chl b-less chlorina f2 mutant.All other subunits were either absent or existed in much loweramounts in the mutant. Subunit d is a polypeptide constituentof the major Chl-protein subcomplex (CPII) of the LHC-II. Itis stably incorporated in the thylakoid membrane in the absenceof Chl b and probably binds the 13 Chl a molecules in the residualLHC-II antenna of the chlorina f2 mutant. We propose that, ofall LHC-II polypeptides, subunit d is most proximal to the PSIIcore and may serve as a linker in the process of excitationenergy transfer from the bulk LHC-II to the PSII reaction centerin chloroplasts. (Received February 25, 1992; Accepted May 12, 1992)     

Phenotypic plasticity of sun and shade ecotypes of Stellaria longipes in response to light quality signaling: Cytokinins

From two distinct ecotypes of Stellaria longipes, one genotype was chosen from each of two very different locations, an alpine (sun) and a prairie (shade) habitat. Plants were clonally propagated and grown in controlled environment chambers under low and moderate red to far-red (R/FR) ratios. The prairie ecotype plants exhibited increased stem elongation, leaf expansion and flowering (6-fold) in response to a low R/FR ratio, relative to plants grown under the moderate R/FR ratio. In contrast, plants of the alpine ecotype showed no increased growth in response to a low R/FR ratio and their flowering was reduced, all relative to the plants grown under the moderate R/FR ratio. These different phenotypic responses to the reduction in R/FR ratio were associated with very different profiles and concentrations of endogenous cytokinins (CKs) assessed in growing tissues of the upper shoots. Specifically, increased total CKs were associated with the rapid growth of plants of the prairie ecotype under a low R/FR ratio. In particular, concentrations of bioactive trans-zeatin and dihydrozeatin, were increased during the period of most rapid shoot growth by 2- to 4- fold for these prairie ecotype plants grown under the low R/FR ratio treatment. In contrast, changes in CK levels for the alpine ecotype plants grown under low R/FR ratios were muted. Of especial interest, plants of the alpine ecotype had a predominance of cis-pathway CKs, whereas the low elevation, prairie ecotype plants accumulated predominantly trans-pathway CKs. Speculatively, the pattern emphasizing trans-pathway CKs may be explained by increased LONELY GUY enzyme activity. This enzyme converts and activates nucleotide CKs to free base CKs (bypassing riboside CKs). It could thus explain, in part, the prairie ecotype's ability to respond to shade light with such a high degree of plasticity if one assumes that high trans-CKs levels are causal for the increased shoot growth seen under a low R/FR ratio.     

Nuclear, Virescent Mutants of Zea mays L. with High Levels of Chlorophyll (a/b) Light-Harvesting Complex during Thylakoid Assembly

We have found nuclear, recessive mutants in Zea mays L. where assembly of the major chlorophyll (a/b) light-harvesting complex (LHC) was not delayed relative to most other thylakoid protein complexes during thylakoid biogenesis. This contrasts with the normal development of maize chloroplasts (NR Baker, R Leech 1977 Plant Physiol 60: 640-644). All four mutants examined were allelic and virescent, and displayed visibly higher yields of leaf Chl fluorescence during greening. Fully greened mutants had normal leaf Chl fluorescence yield and normal levels of LHC, and grew to maturity under field conditions. Therefore, delayed LHC assembly is not an obligate feature of thylakoid differentiation.

Assigning the molecular basis for the mutation should provide information concerning reguation of LHC assembly. Several possibilities are discussed. The pleiotropic mutant phenotype is not attributable to defects in thylakoid glycerolipid synthesis. Thylakoids isolated from greening mutant leaf sections had elevated glycerolipid/Chl ratios. In addition, both the molar distribution and acyl composition of four major glycerolipids were normal for developing mutant thylakoids.

     

Photomorphogenic and Chlorophyll Studies in the Bryophyte Marchantia polymorpha

Besides the standard rod (R) and far-red (FK) irradiations, a graded series of different R/FR ratios were tested as 10 min terminal exposures at the end of the daily 8-hour photoperiod of white fluorescent light. Water filtered incandescent light of 3780 lux during 10 min caused A rather weak hut reproducible effect. A superposition to the water layer of different filter combinations shifting the initial transmittance more towards the FR region, and thus gradually lowering the R/FB ratio, resulted in a parallel increase in orthotropic growth and a decrease in chlorophyll content. Our data show growth similarities with the results of other authors on light grown seedlings of higher plants. Rather high levels of the P_FR form of phytochrome seem to he required to maintain horizontal growth and optimal chlorophyll content in Marchantia thalli.     

Control of ascorbic acid synthesis and accumulation and glutathione by the incident light red/far red ratio in Phaseolus vulgaris leaves

The effects of red/far red (R/FR) ratios on leaf ascorbate (AA) and glutathione (GSH) accumulation were examined in common bean (Phaseolus vulgaris L.). Growth under low R/FR ratios resulted in a “shade” phenotype and much lower leaf AA and GSH contents than high (R/FR) ratios. Photosynthesis rates were unaffected by changes in the R/FR ratio but leaf respiration rates, pyridine nucleotide pools and antioxidant enzyme activities were decreased under the low R/FR regime. The GSH pool changed slowly in response to altered R/FR ratios but leaf ascorbate acclimated over a single photoperiod. We conclude that light quality signals, particularly R/FR ratios, are important regulators of antioxidant synthesis and accumulation. These acclimatory changes are an early response to changing light environment.     

Elimination of the Far-Red-Induced Lag Phase of Chlorophyll a Accumulation by a Brief Light in Pharbitis nil Cotyledons

In dark-grown Pharbitis nil seedlings, far-red light (FR) irradiationof 48 h or less promotes Chl a accumulation in the first 2-hof a subsequent white light (WL) period, without a lag phaseof Chl a accumulation. However, continuous FR irradiation of72 h or more, causes the so-called FR-induced lag phase. A 5-minWL given 4 h before the onset of the continuous WL promotesChl a accumulation irrespective of the length of the precedingFR irradiation period, if a 4-h dark period is inserted betweenthe 5-min WL and continuous WL. This suggests that the effectof the brief WL is independent of and additive to the effectof the preceding FR irradiation, although the effect of theFR irradiation changes from promotive to inhibitory with anincrease in the irradiation period. Red light (R) is more active than blue light (B) in this brieflight effect. The R effect is reversed by subsequent exposureto FR when the period of the preceding FR irradiation is 24h, but not when it is 72 h. The relative effectiveness of Bto R increases after prolonged FR irradiation. (Received August 6, 1986; Accepted March 12, 1987)     

Shoot/root assimilate allocation and nodulation ofVigna unguiculata seedlings as influenced by shoot light environment

Spectral balance of light received by southern pea [Vigna unguiculata (L.) Walp.] seedling shoots affected photoassimilate allocation among leaves, stems and roots. A higher ratio of far-red (FR) relative to red (R) light resulted in longer stems, higher shoot/root biomass ratio, less massive roots and fewer nodules. The same response pattern to FR/R ratio was obtained in a controlled environment with artificial light sources, or in sunlight where the FR/R ratio was modified by reflection from different colored soil surfaces or by FR reflected from competing plants. The importance of early shoot/root photoassimilate allocation and nodulation may differ according to soil nitrogen availability and moisture content.     

Response of Plantago major seeds to the red/far-red ratio as influenced by other environmental factors

Seeds of Plantago major L. ssp. major were exposed to continuous light of various red/far-red ratios (R/FR). Germination was strongly inhibited at low R/FR. The quantitative response of the seeds to the R/FR appeared to depend on various environmental factors. A low R/FR was less inhibitory after a stratification pretreatment, at higher temperatures, in the presence of nitrate and at high osmotic potentials. This is discussed in relation to the concept of a variable requirement of P_fr for germination, which depends on state of dormancy as influenced by pretreatment and on conditions during the germination test. The data are also discussed in relation to field emergence. The species is known to form a persistent seed bank, among other things due to its almost absolute light requirement. It is predicted that germination is effectively inhibited under a leaf canopy in summer after dispersal, and perhaps even in spring to some extent when R/FR is not so low but temperatures are low.     

Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the Chlorophyll d-containing oxyphotobacterium Acaryochloris marina

Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM (Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O₂ electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206 ± 51 μmol m^− 2 s^− 1 (PAR) (Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109 ± 27 μmol m^− 2 s^− 1 (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73 ± 7 μmol mg Chl d^− 1 h^− 1. The gross photosynthesis/respiration ratio (P_g/R) of Acaryochloris under optimum conditions was about 4.02 ± 1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.     

Photosynthetic responses of sun- and shade-grown barley leaves to high light: is the lower PSII connectivity in shade leaves associated with protection against excess of light?

In this study, we have compared photosynthetic performance of barley leaves (Hordeum vulgare L.) grown under sun and shade light regimes during their entire growth period, under field conditions. Analyses were based on measurements of both slow and fast chlorophyll (Chl) a fluorescence kinetics, gas exchange, pigment composition; and of light incident on leaves during their growth. Both the shade and the sun barley leaves had similar Chl a/b and Chl/carotenoid ratios. The fluorescence induction analyses uncovered major functional differences between the sun and the shade leaves: lower connectivity among Photosystem II (PSII), decreased number of electron carriers, and limitations in electron transport between PSII and PSI in the shade leaves; but only low differences in the size of PSII antenna. We discuss the possible protective role of low connectivity between PSII units in shade leaves in keeping the excitation pressure at a lower, physiologically more acceptable level under high light conditions.     

Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis

Seeds of the wild type (WT) and of the phyA and phyB mutants of Arabidopsis thaliana were exposed to single red light (R)/far-red light (FR) pulses predicted to establish a series of calculated phytochrome photoequilibria (Pfr/P). WT and phyB seeds showed biphasic responses to Pfr/P. The first phase, i.e. the very-low-fluence response (VLFR), occurred below Pfr/P = 10-1%. The second phase, i.e. the low-fluence response, occurred above Pfr/P = 3%. The VLFR was similarly induced by either a FR pulse saturating photoconversion or a subsaturating R pulse predicted to establish the same Pfr/P. The VLFR was absent in phyA seeds, which showed a strong low-fluence response. In the field, even brief exposures to the very low fluences of canopy shade light (R/FR ratio < 0.05) promoted germination above dark controls in WT and phyB seeds but not in the phyA mutant. Seeds of the phyA mutant germinated normally under canopies providing higher R/FR ratios or under deep canopy shade light supplemented with R from light-emitting diodes. We propose that phytochrome A mediates VLFR of A. thaliana seeds.     

Maize leaves turn away from neighbors

In commercial crops, maize (Zea mays) plants are typically grown at a larger distance between rows (70 cm) than within the same row (16-23 cm). This rectangular arrangement creates a heterogeneous environment in which the plants receive higher red light (R) to far-red light (FR) ratios from the interrow spaces. In field crops, the hybrid Dekalb 696 (DK696) showed an increased proportion of leaves toward interrow spaces, whereas the experimental hybrid 980 (Exp980) retained random leaf orientation. Mirrors reflecting FR were placed close to isolated plants to simulate the presence of neighbors in the field. In addition, localized FR was applied to target leaves in a growth chamber. During their expansion, the leaves of DK696 turned away from the low R to FR ratio signals, whereas Exp980 leaves remained unaffected. On the contrary, tillering was reduced and plant height was increased by low R to FR ratios in Exp980 but not in DK696. Isolated plants preconditioned with low R/FR-simulating neighbors in a North-South row showed reduced mutual shading among leaves when the plants were actually grouped in North-South rows. These observations contradict the current view that phytochrome-mediated responses to low R/FR are a relic from wild conditions, detrimental for crop yield.     

Interaction of red to far red light ratio and ethylene in regulating stem elongation of <Emphasis Type="Italic">Helianthus annuus</Emphasis>

Sunflower (Helianthus annuus L.) stems showed increased elongation under two types of vegetative shade: canopy shade (low red to far red [R/FR] ratio) and neighbouring proximity shade (FR enrichment). Hypocotyls also elongated more under narrow-band FR light than under narrow-band R light. Ethylene levels were determined in actively elongating 7-day-old hypocotyls and 17-day-old internodes under three R/FR ratios. Ethylene levels were lower in both sunflower hypocotyls and internodes when the R/FR ratio was reduced. Both FR enrichment of normal R/FR ratio and narrow-band FR light with very low light irradiance resulted in reduction in ethylene levels in 7-day-old hypocotyls. Further, in application experiments, sunflower stems grown under low R/FR ratio were more sensitive to ethephon and less sensitive to aminoethoxyvinylglycine (AVG) than stems grown under high R/FR ratio. Low R/FR ratio appears to initiate reduction in ethylene levels in sunflower seedlings, allowing maximum stem elongation. These results, and findings of other authors, suggest that various plant species may have developed different ways of regulating stem elongation and ethylene levels in response to low R/FR ratio.     

Effects of light quality on leaf morphogenesis of a heterophyllous amphibious plant, Rotala hippuris

Background and Aims

For heterophyllous amphibious plants that experience fluctuating water levels, it is critical to control leaf development precisely in response to environmental cues that can serve as a quantitative index of water depth. Light quality can serve as such a cue because the ratio of red light relative to far-red light (R/FR) increases and blue-light intensity decreases with increasing water depth. Growth experiments were conducted to examine how R/FR and blue-light intensity alter leaf morphology of a heterophyllous amphibious plant, Rotala hippuris.

Methods

Using combinations of far red (730 nm), red (660 nm) and blue (470 nm) light-emitting diodes (LEDs), growth experiments were used to quantitatively evaluate the effects of the R/FR ratio and blue-light intensity on leaf morphology.

Key Results

Under the natural light regime in an outside growth garden, R. hippuris produced distinct leaves under submerged and aerial conditions. R/FR and blue-light intensity were found to markedly affect heterophyllous leaf formation. Higher and lower R/FR caused leaf characters more typical of submerged and aerial leaves, respectively, in both aerial and submerged conditions, in accordance with natural distribution of leaf types and light under water. High blue light caused a shift of trait values toward those of typical aerial leaves, and the response was most prominent under conditions of R/FR that were expected near the water surface.

Conclusions

R/FR and blue-light intensity provides quantitative cues for R. hippuris to detect water depth and determine the developmental fates of leaves, especially near the water surface. The utilization of these quantitative cues is expected to be important in habitats where plants experience water-level fluctuation.     

Effects of Short-term Red Radiation and Choline Compounds on Cytokinin Content, Chlorophyll Accumulation and Photomorphogenesis in Wheat Seedlings

Effects of choline compounds (2-chloroethyltrimethylammonium chloride and 2-ethyltrimethylammonium chloride) as well as red radiation (R) pulse on the dynamics of cytokinin changes, growth and chlorophyll (a + b) accumulation were studied during the growth and greening of etiolated wheat seedlings (Triticum aestivum L., var. Mironovskaya-808). The seedlings were grown for 120 h in the dark and then exposed for 72 h to white light. Pre-treatment of caryopses with cholines and pre-irradiation of etiolated seedlings with R inhibited elongation of both coleoptile and first leaf; but the same factors accelerated these growth responses when seedlings were exposed to white light. Chlorophyll (Chl) accumulation and the first leaf appearance from coleoptile were accelerated by the pre-treatments as well. Far-red radiation (FR) reversed all effects of R but choline pre-treatment eliminated partly R/FR photoreversibility. Two compounds with high cytokinin activity (tested on a fresh weight basis by the bioassay with Amaranthus caudatus L.) were found in shoots and first leaves. One of them had R_f, UV absorbance spectrum and the biological activity similar to N⁶-(Δ²-isopentenyl)adenosine. Another cytokinin-like substance was not identified with the used standards. Stimulation of greening by R pulse and cholines was accompanied with accelerated accumulation of both cytokinin-like substances. We conclude that the influence of R and cholines on the concentration of substances with cytokinin activities detected in the leaves might be involved in the stimulation of Chl accumulation.