Circumnutations of Sunflower Hypocotyls in Satellite Orbit

The principal objective of the research reported here was to determine whether a plant's periodic growth oscillations, called circumnutations, would persist in the absence of a significant gravitational or inertial force. The definitive experiment was made possible by access to the condition of protracted near weightlessness in an earth satellite. The experiment, performed during the first flight of Spacelab on the National Aeronautics and Space Administration shuttle, Columbia, in November and December, 1983, tested a biophysical model, proposed in 1967, that might account for circumnutation as a gravity-dependent growth response. However, circumnutations were observed in microgravity. They continued for many hours without stimulation by a significant g-force. Therefore, neither a gravitational nor an inertial g-force was an absolute requirement for initation or continuation of circumnutation. On average, circumnutation was significantly more vigorous in satellite orbit than on earth-based clinostats. Therefore, at least for sunflower (Helianthus annuus L.) circumnutation, clinostatting is not the functional equivalent of weightlessness.     

Limitation on the use of the horizontal clinostat as a gravity compensator

If the horizontal clinostat effectively compensates for the influence of the gravity vector on the rotating plant, it should make the plant unresponsive to whatever chronic acceleration may be applied transverse to the axis of clinostat rotation. This was tested by centrifuging plants while they were growing on clinostats. For a number of morphological end-points of development the results depended on the magnitude of the applied g-force. Therefore, gravity compensation by the clinostat was incomplete. This conclusion is in agreement with results of satellite experiments which are reviewed.     

Purification of the Trehalase GMTRE1 from Soybean Nodules and Cloning of Its cDNA. GMTRE1 Is Expressed at a Low Level in Multiple Tissues

The effects of ultraviolet-B (UV-B) radiation on stomatal conductance (g_s) in pea (Pisum sativum L.), commelina (Commelina communis L.), and oilseed rape (Brassica napus L.) plants were investigated. Plants were grown in a greenhouse either with three different high ratios of UV-B to photosynthetically active radiation or with no UV-B radiation. Pea plants grown in the highest UV-B radiation (0.63 W m⁻²) exhibited a substantial decrease of adaxial and abaxial g_s (approximately 80% and 40%, respectively). With growth in 0.30 W m⁻² of UV-B adaxial g_s was decreased by 23%, with no effect on abaxial g_s, and lower UV-B irradiance of 0.21 W m⁻² had no effect on either surface. Although abaxial g_s increased when leaves were turned over in control plants, it did not in plants grown with the highest UV-B. Adaxial g_s in commelina and oilseed rape also decreased on exposure to high UV-B (0.63 W m⁻²). For previously unexposed pea plants the time course of the effect of UV-B on g_s was slow, with a lag of approximately 4 h, and a time constant of approximately 3 h. We conclude that there is a direct effect of UV-B on stomata in addition to that caused by changes in mesophyll photosynthesis.     

Microgravity effects on leaf morphology, cell structure, carbon metabolism and mRNA expression of dwarf wheat

The use of higher plants as the basis for a biological life support system that regenerates the atmosphere, purifies water, and produces food has been proposed for long duration space missions. The objective of these experiments was to determine what effects microgravity (μg) had on chloroplast development, carbohydrate metabolism and gene expression in developing leaves of Triticum aestivum L. cv. USU Apogee. Gravity naive wheat plants were sampled from a series of seven 21-day experiments conducted during Increment IV of the International Space Station. These samples were fixed in either 3% glutaraldehyde or RNAlater^™ or frozen at −25°C for subsequent analysis. In addition, leaf samples were collected from 24- and 14-day-old plants during the mission that were returned to Earth for analysis. Plants grown under identical light, temperature, relative humidity, photoperiod, CO₂, and planting density were used as ground controls. At the morphological level, there was little difference in the development of cells of wheat under μg conditions. Leaves developed in μg have thinner cross-sectional area than the 1 g grown plants. Ultrastructurally, the chloroplasts of μg grown plants were more ovoid than those developed at 1 g, and the thylakoid membranes had a trend to greater packing density. No differences were observed in the starch, soluble sugar, or lignin content of the leaves grown in μg or 1 g conditions. Furthermore, no differences in gene expression were detected leaf samples collected at μg from 24-day-old leaves, suggesting that the spaceflight environment had minimal impact on wheat metabolism.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

ACC deaminase containing diazotrophic endophytic bacteria ameliorate salt stress in Catharanthus roseus through reduced ethylene levels and induction of antioxidative defense systems

Among a total of 27 cultivable salt tolerant endophytic bacteria isolated from Catharanthus roseus grown in highly salt affected coastal region of cuddalore district, Tamilnadu, India four isolates were found to be positive for nitrogenase activity. The isolates were evaluated for their stress tolerance efficiency and screened for different PGP traits. Based on the above studied parameters, and ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase (4.24???mol ??-ketobutyrate mg^_1 protein h^_1) the salt tolerant diazotrophic isolate AUM54 was selected for further investigation and identified as Achromobacter xylosoxidans by 16S rRNA gene sequencing. The ability of this isolate to ameliorate salt stress in C. roseus was evaluated under gnotobiotic and pot culture conditions. At 150?mM NaCl level A. xylosoxidans AUM54 treated plants recorded ethylene level of 394.1 p mol ethylene g^?1 FW h^?1 compared to the ethylene level of 516.0 p mol ethylene g^?1 FW h^?1 recorded in the un inoculated control. A. xylosoxidans AUM54 inoculated plants recorded the maximum germination percentage of 98.3, vigor index of 2231.4, plant height of 120.4?cm, root dry weight of 53.24?g Plant^_1 and ajmalicine content of 1.60?mg?g^?1, compared to the germination percentage of 91.6%, vigour index of 1511.5, plant height of 105.8, root weight of 47.2?g Plant^?1, and ajmalicine content of 1.23?mg?g^?1 in uninoculated plants grown without NaCl treatment. This isolate also decreased plant ethylene levels by 11?C23% and increased the antioxidative enzyme content of inoculated C. roseus plants to the tune of 19?C32% for ascorbate peroxidase (APX) activity, 20?C30% for superoxide dismutase (SOD) activity and 4?C16% for catalase (CAT) under normal and salt affected conditions.     

Phytochromes influence stomatal conductance plasticity in Arabidopsis thaliana

The ability to sense and respond effectively to drought stress can be important for plant fitness. Here, phytochrome loss-of-function mutants were grown in dry and moist conditions to examine the role of three photoreceptors (phyA, phyB, and phyE) in stomatal conductance (g _ST) and abscisic acid (ABA) concentration. Overall, drought treatment plants had lower g _ST than moist treatment plants. However, the wild-type Landsberg erecta line had a less pronounced conductance response to drought treatment than the phytochrome mutants, suggesting a role for phytochrome in suppressing drought tolerance. Phytochrome gene effects were potentially additive for g _ST; however, PHYB and PHYE effects were nonadditive for ABA concentration.     

Comparison of vetiver root essential oils from cleansed (bacteria- and fungus-free) vs. non-cleansed (normal) vetiver plants

‘Karnataka’ and ‘Malaysia’ cultivars of vetiver (Vetiveria zizanioides (L.) Nash, =Chrysopogon zizanioides (L.) Roberty) were subjected to meristem tissue culture in order to produce plants that were bacteria- and fungi-free. Tissue cultured (“cleansed” or phytosanitary) vetiver was grown for five months in sterilized soil contained in pots, and the oil content of plants grown on the medium was compared to that of non-cleansed (normal) vetiver plants grown in unsterilized soil under the same conditions. Statistical analysis of 49 of the major oil components revealed numerous significant differences between tissue culture derived and natural plants for both genotypes.     

The effect of combined salinity and waterlogging on the halophyte Suaeda maritima: The role of antioxidants

Suaeda maritima is a halophytic plant and its habitat is salt marsh. In order to adapt to saline or waterlogged conditions, plants have evolved mechanisms that include antioxidant protection. However, the combined effect of salinity and waterlogging on antioxidants in S. maritima is unknown. The aim of this study was therefore to investigate the effect of saline-flooding on levels of glutathione and phenolic compounds (antioxidants) and the correlation between their concentration and activity in S. maritima shoots grown in their natural habitat and in a glasshouse.Shoots were collected from two different elevations (and so different degrees of flooding) of a salt marsh while other plants were grown in half strength seawater in the controlled conditions of a glasshouse for 8 weeks (drained and flooded). Shoot samples were used to measure dry weight, glutathione and its reduction state, malonyldialdehyde content (MDA), polyphenol content, superoxide anion and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) scavenging activity in the shoots of S. maritima.Growth of S. maritima was greater in plants growing on the high marsh than at a lower elevation and in drained medium as opposed to waterlogged conditions in the glasshouse. Waterlogging caused an increase in glutathione and its reduction state. The glutathione half-cell redox potential (E_GSSG/2GSH) was more negative in plant shoots grown under waterlogged conditions than in plants grown under normal conditions. Higher DPPH and superoxide anion scavenging activity was associated with high antioxidant concentrations (glutathione and polyphenols).Conclusions. Under saline-flooded conditions in the field and in the glasshouse, plants produced a higher concentration of antioxidants than under drained conditions. These result indicated that antioxidant molecules play an important role in S. maritima plants under combined salinity and waterlogging stress.     

Thermal imaging and carbon isotope composition indicate variation amongst strawberry (Fragaria × ananassa) cultivars in stomatal conductance and water use efficiency

Cultivars with low stomatal conductance (g_s) may show high water use efficiency (WUE) under drought conditions, but under optimal conditions low g_s may result in low vigour. A combination of thermal imaging and carbon isotope composition (δ¹³C) analysis offers potential for screening simultaneously for both high g_s and high WUE. Ten cultivars of strawberry (Fragaria × ananassa Duch.) were grown in well watered or water limited conditions. Thermal images were taken of the plants, with various approaches to determine the optimal protocol for detecting variation in g_s, including use of reference leaves, grids to maintain leaves horizontal, and collection of meteorological data in synchrony with thermal images. δ¹³C of leaves, fruit, and crowns was determined. An index of g_s derived from the temperature of horizontal leaves and the temperature of wet and dry references showed significant differences between cultivars and between well watered and water limited plants, as did g_s estimated from leaf temperature, the temperature of a dry reference, and humidity. Thermal imaging indicated low g_s in ‘Elsanta’ and ‘Totem’ and relatively high g_s in well watered ‘Elvira’, ‘Florence’ and ‘Cambridge Favourite’. δ¹³C of all plant material was higher in water limited than well watered plants and showed significant differences between cultivars. In one experiment leaf δ¹³C indicated lowest WUE in ‘Elvira’ and highest WUE in ‘Totem’. δ¹³C was inversely correlated with an index of g_s derived from thermal imaging. Although the results indicate substantial variation in g_s and WUE between cultivars, generally all cultivars responded to water deficit by lowering g_s and hence increasing WUE.     

Acclimation of tobacco plantlets to ex vitro conditions as affected by application of abscisic acid

Plantlets of Nicotiana tabacum L. cv. Petit Havana SR1 were grown in vitro on Murashige and Skoog medium containing 2% saccharose, and then transplanted ex vitro into pots with coarse sand and Hewitt nutrient solution. In the first day after transplantation, the anti-transpirant abscisic acid (ABA; 0.01, 0.05 or 0.10 mM) was added to the substrate. Leaf stomatal conductance (gs), which was high in plants during the first days after transplantation similarly as in plantlets grown in vitro, was considerably decreased by ABA-treatment. However, in the further days gs decreased more quickly in control than in ABA-treated plants, and after 2 or 3 weeks gs was significantly lower than that of plantlets grown in vitro but similar in control and ABA-treated plants. Two weeks after transplantation, net photosynthetic rate, chlorophyll a + b content, maximal photochemical efficiency, and actual quantum yield of photosystem II in plant leaves were higher in comparison with those in plantlets grown in vitro. ABA-treatment had slight positive or insignificant effect on photosynthetic parameters and enhanced plant growth. Thus ABA application can alleviate 'transplant shock' and speed up acclimation of plantlets to ex vitro conditions.     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: I. Influence of Nitrogen Nutrition, Phosphorus Nutrition, Photon Flux Density, and Ambient Partial Pressure of CO(2) during Ontogeny

Plants of Zea mays were grown with different concentrations of nitrate (0.6, 4, 12, and 24 millimolar) and phosphate (0.04, 0.13, 0.53, and 1.33 millimolar) supplied to the roots, photon flux densities (0.12, 0.5, and 2 millimoles per square meter per second), and ambient partial pressures of CO₂ (305 and 610 microbars). Differences in mineral nutrition and irradiance led to a large variation in rate of CO₂ assimilation per unit leaf area (A, 11 to 58 micromoles per square meter per second) when measured under standard conditions. The variation was shown, with the plants that had received different amounts of nitrate, to be related to variations in the nitrogen and chlorophyll contents, and phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylase activities per unit leaf area. Irrespective of growth treatment, A and leaf conductance to CO₂ transfer (g), measured under standard conditions were in almost constant proportion, implying that intercellular partial pressure of CO₂ (p_i), was almost constant at 95 microbars. The same proportionality was maintained as A and g increased in an initially nitrogen-deficient plant that had been supplied with abundant nitrate. It was shown that p_i measured at a given ambient partial pressure was not affected by the ambient partial pressure at which the plants had been grown, although it was different when measured at different ambient partial pressures. This suggests that the close coupling between A and g in these experiments is not associated with sensitivity of stomata to change in p_i.

Similar, though less comprehensive, experiments were done with Gossypium hirsutum, and yielded similar conclusions, except that the proportionality between A and g at normal ambient partial pressure of CO₂ implied P_i ≈ 200 microbars.

     

Growth and photosynthesis of Chinese cabbage plants grown under light-emitting diode-based light source

We compared growth and the content of sugar, protein, and photosynthetic pigments, as well as chlorophyll fluorescence parameters in 15- and 27-day-old Chinese cabbage (Brassica chinensis L.) plants grown under a high-pressure sodium (HPS) lamps or a light source built on the basis of red (650 nm) and blue (470 nm) light-emitting diodes (LEDs) with a red to blue photon ratio of 7: 1. One group of plants was grown at a photosynthetic photon flux (PPF) level of 391 ± 24 μ mol/(m² s) (normal level); the other, at a PPF level of 107 ± 9 μ mol/(m² s) (low light). Plants of the third group were firstly grown at the low light and then (on the 12th day) transferred to the normal level. When grown at the normal PPF level, the plants grown under LEDs didn’t differ from plants grown under HPS lamps in shoot fresh weight, but they showed a lower root fresh and dry weights and the lower content of total sugar and sugar reserves in the leaves. No differences in the pigment content and photosystem II quantum yield were found; however, a higher Chl a/b ratio in plants grown under LEDs indicates a different proportion of functional complexes in thylakoid membranes. The response to low light conditions was mostly the same in plants grown under HPS lamps and LEDs; however, LED plants showed a lower growth rate and a higher nonphotochemical fluorescence quenching. In the case of the altered PPF level during growth, the plant photosynthetic apparatus adapted to new conditions of illumination within three days. Plants grown under HPS lamps at a constant normal PPF level and those transferred to the normal PPF level on the 12th day, on the 27th day didn’t differ in shoot fresh weight, but in plants grown under LEDs, the differences were considerable. Our results show that LED-based light sources can be used for plant growing. At the same time, some specific properties of plant photosynthesis and growth under these conditions of illumination were found.     

Hypergravity affects cell traction forces of fibroblasts

Cells sense and react on changes of the mechanical properties of their environment and, likewise, respond to external mechanical stress applied to them. However, whether the gravitational field as overall body force modulates cellular behavior is unclear. Different studies demonstrated that micro- and hypergravity influences the shape and elasticity of cells, initiate cytoskeleton reorganization, and influence cell motility. All these cellular properties are interconnected and contribute to forces that cells apply on their surrounding microenvironment. Yet, studies that investigated changes of cell traction forces under hypergravity conditions are scarce. Here, we performed hypergravity experiments on 3T3 fibroblast cells using the large-diameter centrifuge at the European Space Agency - European Space Research and Technology Centre. Cells were exposed to hypergravity of up to 19.5 g for 16 h in both the upright and the inverted orientation with respect to the g-force vector. We observed a decrease in cellular traction forces when the gravitational field was increased up to 5.4 g, followed by an increase of traction forces for higher gravity fields up to 19.5 g independent of the orientation of the gravity vector. We attribute the switch in cellular response to shear thinning at low g-forces, followed by significant rearrangement and enforcement of the cytoskeleton at high g-forces.     

Evidence for the role of wheat eukaryotic translation initiation factor 3 subunit g (TaeIF3g) in abiotic stress tolerance

The gene encoding eIF3g (TaeIF3g), one of the 11 subunits of eukaryotic translation initiation factor 3 (eIF3), was cloned from wheat for carrying out its functional analysis. Transgenic expression of TaeIF3g enhanced the tolerance of TaeIF3g-overexpressing parental yeast cells and Arabidopsis plants under different abiotic stress conditions. Compared to untransformed plants, TaeIF3g-overexpressing Arabidopsis thaliana plants exhibited significantly higher survival rate, soluble proteins and photosynthetic efficiency, and enhanced protection against photooxidative stress under drought conditions. This study provides first evidence that TaeIF3g imparts stress tolerance and could be a potential candidate gene for developing crop plants tolerant to abiotic stress.     

Root morphology, hydraulic conductivity and plant water relations of high-yielding rice grown under aerobic conditions

Background and Aims

Increasing physical water scarcity is a major constraint for irrigated rice (Oryza sativa) production. ‘Aerobic rice culture’ aims to maximize yield per unit water input by growing plants in aerobic soil without flooding or puddling. The objective was to determine (a) the effect of water management on root morphology and hydraulic conductance, and (b) their roles in plant–water relationships and stomatal conductance in aerobic culture.

Methods

Root system development, stomatal conductance (g_s) and leaf water potential (Ψ_leaf) were monitored in a high-yielding rice cultivar (‘Takanari’) under flooded and aerobic conditions at two soil moisture levels [nearly saturated (> –10 kPa) and mildly dry (> –30 kPa)] over 2 years. In an ancillary pot experiment, whole-plant hydraulic conductivity (soil-leaf hydraulic conductance; K_pa) was measured under flooded and aerobic conditions.

Key Results

Adventitious root emergence and lateral root proliferation were restricted even under nearly saturated conditions, resulting in a 72–85 % reduction in total root length under aerobic culture conditions. Because of their reduced rooting size, plants grown under aerobic conditions tended to have lower K_pa than plants grown under flooded conditions. Ψ_leaf was always significantly lower in aerobic culture than in flooded culture, while g_s was unchanged when the soil moisture was at around field capacity. g_s was inevitably reduced when the soil water potential at 20-cm depth reached –20 kPa.

Conclusions

Unstable performance of rice in water-saving cultivations is often associated with reduction in Ψ_leaf. Ψ_leaf may reduce even if K_pa is not significantly changed, but the lower Ψ_leaf would certainly occur in case K_pa reduces as a result of lower water-uptake capacity under aerobic conditions. Rice performance in aerobic culture might be improved through genetic manipulation that promotes lateral root branching and rhizogenesis as well as deep rooting.     

Evidence of crussulacean acid metabolism in two North American isoetids

The internal acidity levels of four common North American isoetids, or rosette-form aquatic macrophytes, were monitored under field conditions to determine the presence of crassulacean acid metabolism (CAM). The leaves of Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Durieu were found to have a diurnal fluctuation in titratable acidity characteristic of CAM plants, 124 and 142 μeq g^?1 fresh wt., respectively. No variations were detected in the acidity of stems and leaves of Myriophyllum tenellum Bigel. and Lobelia dortmanna L. Litorella and Isoetes were then grown in the laboratory where the diurnal acid rhythm was shown to be due to the accumulation and disappearance of malic acid.Based on the magnitude of the diurnal acid rhythm and existing information on the productivity of these plants, it appears that the carbon assimilated via crassulacean acid metabolism may contribute substantially to their net annual productivity. This appears to be a case for which CAM has been selected directly as a response to carbon stress.     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: II. Effects of Short-Term Exposures to Different Photon Flux Densities

When photon flux density incident on attached leaves of Zea mays L. was varied from the equivalent of 0.12 of full sunlight to full sunlight, leaf conductance to CO₂ transfer, g, changed in proportion to the change in rate of CO₂, assimilation, A, with the result that intercellular partial pressure of CO₂ remained almost constant. The proportionality was the same as that previously found in g and A measured at one photon flux density in plants of Zea mays L. grown at different levels of mineral nutrition, light intensities, and ambient partial pressures of CO₂. In shade-grown Phaseolus vulgaris L. plants, A as photon flux density was increased from about 0.12 up to about 0.5 full sunlight, the proportionality being almost the same in plants grown at low and at high light intensity.

When photon flux density incident on the adaxial and abaxial surfaces of the isolateral leaves of Eucalyptus pauciflora Sieb. ex Spreng was varied, g and A also varied proportionally. The leaf conductance in a particular surface was affected by the photon flux density at the opposite surface to a greater extent than was expected on the basis of transmittance. The results indicated that stomata may, in some way, be sensitive to the photon flux absorbed within the leaf as a whole.

     

Sorbitol-6-Phosphate Dehydrogenase Expression in Transgenic Tobacco : High Amounts of Sorbitol Lead to Necrotic Lesions

We analyzed transgenic tobacco (Nicotiana tabacum L.) expressing Stpd1, a cDNA encoding sorbitol-6-phosphate dehydrogenase from apple, under the control of a cauliflower mosaic virus 35S promoter. In 125 independent transformants variable amounts of sorbitol ranging from 0.2 to 130 μmol g⁻¹ fresh weight were found. Plants that accumulated up to 2 to 3 μmol g⁻¹ fresh weight sorbitol were phenotypically normal, with successively slower growth as sorbitol amounts increased. Plants accumulating sorbitol at 3 to 5 μmol g⁻¹ fresh weight occasionally showed regions in which chlorophyll was partially lost, but at higher sorbitol amounts young leaves of all plants lost chlorophyll in irregular spots that developed into necrotic lesions. When sorbitol exceeded 15 to 20 μmol g⁻¹ fresh weight, plants were infertile, and at even higher sorbitol concentrations the primary regenerants were incapable of forming roots in culture or soil. In mature plants sorbitol amounts varied with age, leaf position, and growth conditions. The appearance of lesions was correlated with high sorbitol, glucose, fructose, and starch, and low myo-inositol. Supplementing myo-inositol in seedlings and young plants prevented lesion formation. Hyperaccumulation of sorbitol, which interferes with inositol biosynthesis, seems to lead to osmotic imbalance, possibly acting as a signal affecting carbohydrate allocation and transport.     

The influence of mycorrhizal inoculation and paclobutrazol on water and nutritional status of Arbutus unedo L.

The purpose of this study was to analyze morphological and physiological aspects of Arbutus unedo L. plants treated with paclobutrazol (PAC), compounds characterized by their double activity as plant growth regulators and fungicides, and the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker and Couch, which forms a special type of mycorrhizal colonization called arbutoid mycorrhiza. Native A. unedo L. seedlings were grown in a greenhouse and subjected to four treatments for 4 months: 0 or 60 mg of PAC and inoculated or not with P. tinctorius (Pers.). The arbutoid mycorrhizal inoculation increased in plants treated with PAC. Paclobutrazol reduced shoot and root biomass, plant height, internode length, stem diameter, leaf area, total root length and number of tips. P. tinctorius increased plant height and had a beneficial effect on the root system (increasing root diameter and the number of tips). PAC treatment led to an increase in ion levels in the leaf tissue, while mycorrhizal inoculation induced lower K and higher P contents in the roots. Leaf water potentials (at predawn and at midday) increased with the combined treatment. The absence of water deficit conditions meant there was no osmotic adjustment. Higher photosynthesis (P_n) values were associated with higher stomatal conductance (g_s) values in the mycorrhizal plants, which influenced water uptake from the roots. However, g_s decreased in the PAC-treated plants, reducing photosynthesis and, as a consequence, growth. The higher hydraulic conductivity (L_p) in the plants treated with PAC may have induced a better water energy status and good water transport. The combined treatment produced beneficial effects in the plants, improving their water and nutritional status.     

A Temperature-Sensitive Chlorophyll b-Deficient Mutant of Sweetclover (Melilotus alba)

The ch4 mutant of sweetclover (Melilotus alba) has previously been demonstrated to be partially deficient in chlorophyll and to have a higher ratio of chlorophyll a to b than normal plants. We were able to substantiate these findings when plants were grown at 23°C and lower (permissive temperatures). However, when grown at 26°C (nonpermissive temperature) the plants produced small yellow leaves which exhibited one-twentieth the chlorophyll content of normal plants. Affected leaves did not increase their chlorophyll content when plants were incubated at permissive temperatures, but leaves which developed at the lower temperature contained increased amounts of chlorophyll. Similarly, only new leaves, not previously grown leaves, exhibited the yellow phenotype when the mutant plant was shifted from the permissive temperature to the nonpermissive temperature. Ribulose 1,5-bisphosphate carboxylase activity was decreased by half, relative to normal plants, in the mutant plants grown at the nonpermissive temperature, indicating that general protein synthesis was not greatly impaired and that the effect of the mutation was perhaps specific for chlorophyll content. HPLC analysis indicated that carotenoid content was not diminished to the same extent as chlorophyll and we have determined that the thylakoid protein kinase is not altered, as is the case for other chlorophyll b-deficient mutants. Experiments suggest that changes in photoperiod may be able to modulate the effect of temperature.