The kinetics of P515 in relation to the lipid composition of the thylakoid membrane

Flash-induced P515 absorbance changes have been studied in dark-adapted chloroplasts isolated from spinach plants grown under two different light intensities. The slow component (reaction 2), normally present in the P515 response of chloroplasts isolated from plants grown at an intensity of 60 W · m^–2, was largely reduced in chloroplasts isolated from plants grown at an intensity of 6 W · m^–2. This reduction of the slow component in the P515 response appeared to be coincident with an alteration in the lipid composition of the thylakoid membrane. Mainly the ratio monogalactosyldiacylglycerol to digalactosyldiacylglycerol appeared to be altered. In thylakoids from plants grown at 6 W · m^–2, the ratio was approximately 35% lower than that of plants grown at 60 W · m^–2. The amount of both cytochromeb ₅₆₃ and cytochromef was largely reduced in chloroplasts isolated from plants grown at low light intensity. These results may indicate a possible correlation between structural organization of the thylakoid membrane and the kinetics of the flash-induced P515 response.     

Fine Chloroplast Structure in Cucumber and Pea Leaves Developed under Red Light

Photosynthetic activity, the content of various photosynthetic pigments, and the chloroplast ultrastructure were examined in the leaves of cucumber (Cucumis sativus L.) and pea (Pisum sativum L.) plants of different ages grown under red light (600–700 nm, 100 W/m²). In pea leaves tolerant to red-light irradiation, chloroplast ultrastructure did not essentially change. In the first true leaves of cucumber plants susceptible to red-light irradiation, we observed a considerable increase in the number and size of plastoglobules, the appearance of chloroplasts lacking grana or containing only infrequent grana, and stromal thylakoids. In the upper leaves of 22-day-old cucumber plants, the chloroplast structure was essentially similar to that of the control chloroplasts in white light, and we therefore suppose that these plants have acclimated to red light.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

High Irradiance Induced Pigment Degradation and Loss of Photochemical Activity of Wheat Chloroplasts

Loss of chlorophyll (Chl) and carotenoids (Car) of leaves and changes in Chl fluorescence emission and polarisation, malondialdehyde (MDA) accumulation, and 2,6-dichlorophenol indophenol (DCPIP) photoreduction in chloroplasts of wheat seedlings grown under different irradiance and subsequently exposed to high irradiance stress (HIS; 250 W m^–2) were studied in mature and senescent primary wheat leaves. Faster rate of pigment loss was observed in leaves of moderate irradiance (MI; 15 W m^–2) grown plants, compared to high irradiance (HI-1 and HI-2; 30 and 45 W m^–2) ones when exposed to HIS. A relatively lower loss of Car in the plants grown in HI-1 and HI-2 exposed to HIS suggests HI adaptation of these seedlings. The slower rate of increase in the ratio of Chl fluorescence emission (F₆₈₅/F₇₃₅) also may suggest photoprotective strategy of HI grown seedlings. There was a positive correlation between MDA accumulation and Chl fluorescence polarisation. The DCPIP photoreduction activity in chloroplasts isolated from HI-1 and HI-2 grown plants exposed to HIS showed slower loss of electron transport activity compared to MI grown plants. These observations suggest that plants grown under higher irradiance have capacity to manage the excess quanta better than those grown under lower irradiance.     

A study of factors which regulate the membrane appression of lettuce thylakoids in relation to irradiance

Factors that may influence the extent of thylakoid membrane appression have been examined using lettuce (Lactuca sativa cv. Celtuce) grown under different irradiances. Electron microscopy and salt-induced chlorophyll fluorescence suggest that the percentage of membrane appression is increased in plants grown in low light (20 Wm^–2) compared with those grown in high light (150 Wm^–2). In high light plants surface charge, as measured by 9-aminoacridine, was found to be twice that measured in low light plants. There was a similar difference in ATPase activity of CF₁ and in light saturated photophosphorylation. The chlorophyll content of LHC-2 as a proportion of the total chlorophyll was greatest in thylakoids of low light plants. Measurement of non-cyclic photophosphorylation rates suggested that membrane appression has a stimulatory role in the photophosphorylation process. The importance of these inter-related factors for the mechanism of thylakoid appression is discussed.Abbreviations PS photosystem - chl chlorophyll - LHC-2 light harvesting chlorophyll-protein complex serving PS 2 - CF₁ coupling factor 1 - NADP nicotinamide-adenine dinucleotide phosphate     

Comparisons of primary production and nutrients absorption by aMiscanthus sinensis community in different soils

Soil properties, primary production, nitrogen and phosphorus uptake in aMiscanthus sinensis community on serpentine gangue area were compared with that on nonserpentine area. Soil water content, soil pH and nitrogen content were quite different between the serpentine gangue area and nonserpentine area; but phosphorus content of the soil was similar between the two sites. The maximum above-ground net production in the serpentine gangue and nonserpentine areas was 4.5±0.2 kg m^–2 yr^–1 and 7.8±0.2 kg m^–2 yr^–1, respectively. The total maximum standing biomass in the serpentine gangue and nonserpentine areas was 8.5±0.8 kg m^–2 and 11.9±0.4 kg m^–2, respectively. Nitrogen uptake by plants in the nonserpentine area was 2.4 times greater than that in the serpentine gangue area. Phosphorus uptake by plants were similar for the two sites. The most probable reasons for the small biomass produced by theMiscanthus sinensis community in this serpentine gangue area are the low levels of nitrogen and water availability in the soil.     

Photosynthetic and leaf morphological characteristics in Leucaena leucocephala as affected by growth under different neutral shade levels

Morphological and physiological measurements on individual leaves of Leucaena leucocephala seedlings were used to study acclimation to neutral shading. The light-saturated photosynthetic rate (P_{n max}) ranged from 19.6 to 6.5 mol CO₂ m^–2 s^–1 as photosynthetic photon flux density (PPFD) during growth decreased from 27 to 1.6 mol m^–2 s^–1. Stomatal density varied from 144 mm^–2 in plants grown in high PPFD to 84 mm^–2 in plants grown in low PPFD. Average maximal stomatal conductance for H₂O was 1.1 in plants grown in high PPFD and 0.3 for plants grown in low PPFD. Plants grown in low PPFD had a greater total chlorophyll content than plants grown in high PPFD (7.2 vs 2.9 mg g^–1 on a unit fresh weight basis, and 4.3 vs 3.7 mg dm^–2 on a unit leaf area basis). Leaf area was largest when plants were grown under the intermediate PPFDs. Leaf density thickness was largest when plants were grown under the largest PPFDs. It is concluded that L. leucocephala shows extensive ability to acclimate to neutral shade, and could be considered a facultative shade plant.Abbreviations the initial slope of the photosynthesis vs PPFD curve - P_{n max} the light-saturated photosynthetic rate - PPFD photosynthetic photon flux density     

Compartmental nitrate concentrations in barley root cells measured with nitrate-selective microelectrodes and by single-cell sap sampling

Nitrate-selective microelectrodes were used to measure intracellular nitrate concentrations (as activities) in epidermal and cortical cells of roots of 5-d-old barley (Hordeum vulgare L.) seedlings grown in nutrient solution containing 10 mol · m^–3 nitrate. Measurements in each cell type grouped into two populations with mean (±SE) values of 5.4 ± 0.5 mol · m^–3 (n=19) and 41.8 ± 2.6 mol · m^–3 (n = 35) in epidermal cells, and 3.2 ± 1.2 mol · m^–3 (n = 4) and 72.8 ± 8.4 mol · m^–3 (n = 13) in cortical cells. These could represent the cytoplasmic and vacuolar nitrate concentrations, respectively, in each cell type. To test this hypothesis, a single-cell sampling procedure was used to withdraw a vacuolar sap sample from individual epidermal and cortical cells. Measurement of the nitrate concentration in these samples by a fluorometric nitrate-reductase assay confirmed a mean vacuolar nitrate concentration of 52.6 ± 5.3 mol · m^–3 (n = 10) in epidermal cells and 101.2 ± 4.8 mol · m^–3 (n = 44) in cortical cells. The nitrate-reductase assay gave only a single population of measurements in each cell type, supporting the hypothesis that the higher of the two populations of electrode measurements in each cell type are vacuolar in origin. Differences in the absolute values obtained by these methods are probably related to the fact that the nitrate electrodes were calibrated against nitrate activity but the enzymic assay against concentration. Furthermore, a 28-h time course for the accumulation of nitrate measured with electrodes in epidermal cells showed the apparent cytoplasmic measurements remained constant at 5.0 ± 0.7 mol · m^–3, while the vacuole accumulated nitrate to 30–50 mol · m^–3. The implications of the data for mechanisms of nitrate transport at the plasma membrane and tonoplast are discussed.Symbol _n ² Chi-squared with n degrees of freedom R.-G.Z. was awarded a Sino-British Friendship Scholarship sponsored by the British Council and H.-W.K. was supported by an AFRC Linked Research Grant to A.D.T for collaboration with R.A.L. We wish to thank Dr. K. Goulding for advice on ion chromatography, Dr. K. Moore for assistance with statistical analysis and Dr. J.H. Williams for advice on the microsample analysis.     

Changes in distribution of nucleoids in developing and dividing chloroplasts and etioplasts ofArena sativa

Summary Nucleoid distribution in chloroplasts and etioplasts at the different developmental stages was examined with the first leaves ofAvena sativa by using a DNA-specific fluorescent probe, 46-diamidino-2-phenylindole (DAPI). In light-grown first leaves, three types of plastid nucleoid distribution were recognized. 1. Peripheral distribution in undeveloped chloroplasts which contain only a few thylakoids in the middle region of the leaf sheath. 2. Ring-like arrangement along the rim of developing and dividing young chloroplasts, of which grana were composed of four to eight layers of thylakoids, at the base of the leaf blade. The plane of the nucleoids' ring is in parallel with the face of the thylakoids. 3. Scattered distribution of 10 to 20 discrete spherular nucleoids in the stroma of fully developed chloroplasts, of which grana were composed of up to 20 thylakoids, in the regions of the middle and the tip of the leaf blade. In dark-grown first leaves two types were recognized. 1. Peripheral distribution in developing and dividing young etioplasts in the leaf sheath and the base of the leaf blade. 2. Scattered distribution of 10 or more discrete spherular nucleoids in fully developed etioplasts, containing extended prothylakoids, in the regions of the middle and the tip of the leaf blade. Ring-like arrangement of nucleoids was not observed in any etioplasts. The results indicates that spatial arrangement of plastid nucleoids dynamically changes in close relationship with the development of the inner membrane systems of plastids.     

Effects of nitrogen supply and elevated carbon dioxide on construction cost in leaves of Pinus taeda (L.) seedlings

Seedlings of loblolly pine (Pinus taeda L.) were grown under varying conditions of soil nitrogen and atmospheric carbon dioxide availability to investigate the interactive effects of these resources on the energetic requirements for leaf growth. Increasing the ambient CO₂ partial pressure from 35 to 65 Pa increased seedling growth only when soil nitrogen was high. Biomass increased by 55% and photosynthesis increased by 13% after 100 days of CO₂ enrichment. Leaves from seedlings grown in high soil nitrogen were 7.0% more expensive on a g glucose g^–1 dry mass basis to produce than those grown in low nitrogen, while elevated CO₂ decreased leaf cost by 3.5%. Nitrogen and CO₂ availability had an interactive effect on leaf construction cost expressed on an area basis, reflecting source-sink interactions. When both resources were abundant, leaf construction cost on an area basis was relatively high (81.8±3.0 g glucose m^–2) compared to leaves from high nitrogen, low CO₂ seedlings (56.3±3.0 g glucose m^–2) and low nitrogen, low CO₂ seedlings (67.1±2.7 g glucose m^–2). Leaf construction cost appears to respond to alterations in the utilization of photoassimilates mediated by resource availability.     

Biomass and litter dynamics in a Melaleuca forest on a seasonally inundated floodplain in tropical,northern Australia

Litterfall from a Melaleuca forest was investigated as part of chemical cycling studies on the Magela Creek floodplain in tropical, northern Australia. The forest contained two species of tree, Melaleuca cajaputi and Melaleuca viridiflora, with a combined average density of 294 trees ha^–1. The M. viridiflora trees had diameter breast height measurements ranging from 11.8 to 62.0 cm, median class 25.1–30.0cm and a mean value of 29.2±1.0 cm, compared to 13.0 to 66.3 cm, 30.1–35.0cm and 33.5±1.0cm for M. cajaputi trees. A regression model between tree height, diameter breast height and fresh weight was determined and used to calculate average tree weights of 775±1.6kg for M. viridiflora and 1009±1.6kg for M. cajaputi, and a total above-ground fresh weight of 263±0.3t ha^–1. The weight of litter recorded each month on the ground beneath the tree canopy ranged from 582±103 to 2176±376 g m^–2 with a monthly mean value of 1105±51 g m^–2. The coefficient of variation of 52% on this mean indicates the large spatial and temporal variability in litter distribution over the study site. This variability was greatly affected by the pattern of water flow and litter transport during the Wet season. Litterfall from the trees was evaluated using two techniques - nets and trays. The results from these techniques were not significantly different with annual litterfall collected in the nets being 705 ± 25 g m^–2 and in the trays 716±49 g m^–2. The maximum monthly amount of litterfall, 108 ±55g m^–2, occurred during the Dry season months of June–July. Leaf material comprised 70% of the total annual weight of litter, 480±29 g m^–2 in the nets and 495 ± 21 g m^–2 in the trays. The tree density and weight of litter suggest that the Melaleuca forests are highly productive and contribute a large amount of material to the detrital/debris turnover cycle on the floodplain.     

THE EFFECT OF LIGHT INTENSITY AND TEMPERATURE ON PLANT GROWTH AND CHLOROPLAST ULTRASTRUCTURE IN SOYBEAN

Soybean plants grown in controlled environment cabinets under light intensities of 220 w/m² or 90 w/m² (400–700 nm) and day to night temperatures of 27.5–22.5 C or 20.0–12.5 C in all combinations, exhibited differences in growth rate, leaf anatomy, chloroplast ultrastructure, and leaf starch, chlorophyll, and chloroplast lipid contents. Leaves grown under the lower light intensity at both temperatures had palisade mesophyll chloroplasts containing well-formed grana. The corresponding leaves developed under the higher light intensity had very rudimentary grana. Chloroplasts from high temperature and high light had grana consisting of two or three appressed thylakoids, while grana from the low temperature were confined to occasional thylakoid overlap. Spongy mesophyll chloroplasts were less sensitive to growth conditions. Transfer experiments showed that the ultrastructure of chloroplasts from mature leaves could be modified by changing the conditions, though the effect was less marked than when the leaf was growing.     

Life cycle and growth ofTrichostegia minor (Curtis) in temporary woodland pools (Trichoptera: Phryganeidae)

Larvae of the caddisTrichostegia minor (Curtis) were collected from four woodland pools in The Netherlands, three of which are temporary, from August 1986 till June 1987. Eggs and larvae of this species proved to be very well adapted to drought, freezing, strongly fluctuating pH and alkalinity levels and prolonged oxygen deficit. The life cycle ofT.minor in a small woodland marsh overgrown byCalla palustris took one year. Adult flight period started at the end of May. Oviposition took place independent of water. Hatching of the eggs started in September and was probably induced by immersion. During the larval stage from September until May, 5 instars could be distinguished by the size of the head capsule. Growth of instars I, II and III during autumn was moderate. Most larvae overwintered as instar III or IV. Possibly there was a larval diapause during winter. In spring rapid growth to instar V took place prior to pupation. Growth rate, expressed as the increase of mean individual dry weight was highest from March to April (2.05±0.75% DW.m^–2.d^–1). In extremely shallow water growth in spring was initially more rapid compared to growth in deeper water. During winter the growth rate decreased to 0.038±0.071% DW.m^–2.d^–1. Net annual production based on the changes of momentary biomass was 183.2±31.7 mg DW.m^–2.y^–1 or 177.2±31.3 mg AFDW.m^–2.y^–1. Production loss during the winter season was 75.1±10.8 mg DW.m^–2.y^–1 or 72.3±10.6 mg AFDW.m^–2.y^–1.     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

Pinoresinol and matairesinol accumulation in a Forsythia × intermedia cell suspension culture

A newly established Forsythia × intermedia cell suspension culture was shown to accumulate (+)- and (–)-pinoresinol as well as matairesinol. The influence of the sucrose content of the culture medium and of the cultivation time on pinoresinol and matairesinol accumulation was evaluated. The highest pinoresinol yield was achieved from cells grown in medium containing 6% sucrose for 12 ± 2 days with levels of 0.6–0.8 mg g^–1 dry weight and an average enantiomeric composition of 75 ± 5% (+)-pinoresinol. The highest matairesinol amount was reached in the same medium at the 14th ± 2 culture day with levels of 1.0–2.7 mg g^–1 dry weight. To our knowledge, this is the first report on pinoresinol accumulation in Forsythia × intermedia plants or cell suspension cultures.     

Superoxide production by thylakoids during chilling and its implication in the susceptibility of plants to chilling-induced photoinhibition

Factors influencing the rate of superoxide (O ₂ ^- ) production by thylakoids were investigated to determine if increased production of the radical was related to injury induced by chilling at a moderate photon flux density (PFD). Plants used were Spinacia oleracea L., Cucumis sativus L. and Nerium oleander L. grown at either 200° C or 45° C. Superoxide production was determined by electron-spin-resonance spectroscopy of the (O ₂ ^- )-dependent rate of oxidation of 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidine (OXANOH) to the corresponding oxazolidinoxyl radical, OXANO ·. For all plants, the steady-state rate of O ₂ ^- production by thylakoids, incubated at 25° C and 350 mol photon · m^–2 · s^–1 (moderate PFD) with added ferredoxin and NADP, was between 7.5 and 12.5 mol · (mg chlorophyll)^–1 · h^–1. Incubation at 5° C and a moderate PFD, decreased the rate of O ₂ ^- production 40% and 15% by thylakoids from S. oleracea and 20° C-grown N. oleander, chillinginsensitive plants, but increased the rate by 56% and 5% by thylakoids from C. sativus and 45° C-grown N. oleander, chilling-sensitive plants. For all plants, the addition of either ferredoxin or methyl viologen increased the rate of O ₂ ^- -production at 25° C by 75–100%. With these electron acceptors, lowering the temperature to 5° C caused only a slight decrease in O ₂ ^- production. In the absence of added electron acceptors, thylakoids produced O ₂ ^- at a rate which was about 45% greater than that when ferredoxin and NADP were present. The addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea reduced O ₂ ^- production under all conditions tested. The results show that the rate of O ₂ ^- production increases in thylakoids when the rate of electron transfer to NADP is reduced. This could explain differences in the susceptibility of thylakoids from chilling-sensitive and chilling-insensitive plants to chilling at a moderate PFD, and is consistent with the proposal that O ₂ ^- production is involved in the injury leading to the inhibition of photosynthesis induced under these conditions.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophen-yl)-1,1-dimethylurea - Fd ferredoxin - MV methyl viologen - 20°oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - OXANOH 2-ethyl-1-hydroxy-2,5,5-tri-methyl-3-oxazolidine - PFD photon flux density (photon fluence rate) - TEMED tetramethyl ethylenediamine We would like to thank R.T. Furbank, R.S.B.S., Australian National University, Canberra, A.C.T., and C.B. Osmond, now of Duke University, Durham, N.C., USA, for the gift of ferredoxin, R.A.J.H. was supported by a Commonwealth Postgraduate Research Award.     

Invertase production by Aspergillus niger in submerged and solid-state fermentation

Three Aspergillus nigerstrains were grown in submerged and solid state fermentation systems with sucrose at 100 g l^–1. Average measurements of all strains, liquid vs solid were: final biomass (g l^–1), 11 ± 0.3 vs 34 ± 5; maximal enzyme titres (U l^–1) 1180 ± 138 vs 3663 ± 732; enzyme productivity (U l^–1h^–1) 20 ± 2 vs 87 ± 33 and enzyme yields (U/gX) 128 ± 24 vs 138 ± 72. Hence, better productivity in solid-state was due to a better mould growth.     

A culture unit system for the study of responses of mycorrhizal and non-mycorrhizal seedling to treatments

The time-dependence of Mn accumulation was confirmed in potato foliage (Solanum tuberosum. L.cv. Norland) grown in solution culture. Older leaves grown at 0.61 mM Mn had substantially higher Mn concentrations than younger leaves and stem samples. Levels of Mn in older leaves increased steadily from 4000 µg g^–1 at one week to 8–10,000 µg g^–1 at 6 weeks, but were relatively constant in the emerging leaves. Even foliage grown at low Mn levels (0.01 mM Mn) had 4 fold gradients in Mn concentration from younger (40 µg g^–1) to older leaves (180 µg g^–1).At 0.61 mM Mn, concentrations of 3–4000 µg g^–1 in the youngest fully-developed leaves did not bring about any decline in yield, and levels of up to 5000 µg g^–1 occurred in individual potato leaves before Mn toxicity symptoms were observed. Potato foliage grown at the high Mn had similar leaf numbers, but showed an increased stem length and smaller leaves than foliage grown at 0.01 mM Mn. In particular, the leaf area of the middle and lower leaf fractions were affected by the high Mn level.The ability of rapidly growing plants to withstand high concentrations of Mn is discussed in relation to the pattern of dry matter and Mn accumulation shown by potato foliage.     

Comparison of the growth rates of dinitrogen fixing subterranean clover swards with those assimilating nitrate ions

Summary Subterranean clover plants were grown as swards (about 2000 plants/m²) under controlled conditions with N provided either by N₂-fixation (NO ₃ ^– withheld) or by assimilation of NO ₃ ^– (NO ₃ ^– supplied). Crop growth rates were measured by dry matter sampling over periods of up to 70 days at PPFD values of 400–1000 mole quanta/m²/s. When NO ₃ ^– was supplied from sowing the swards grew more rapidly than when the swards were not supplied with NO ₃ ^– and plants had to establish an N₂-fixing apparatus. When inter-plant competition was reduced within the sward, a difference in growth rate in favour of NO ₃ ^– -fed plants continued for at least 50 days. When however, a closed canopy was allowed to form, the NO ₃ ^– -fed swards had more dry weight than the N₂-fed swards at the time of canopy closure but thereafter the two swards grew at similar rates at light flux densities of above about 800 mole quanta/m²/s. At light flux densities of about 400 mole quanta/m²/s N₂-fed swards had a growth rate 70–80% of that of NO ₃ ^– -fed plants. NO ₃ ^– -fed plants had a higher organic N content than did N₂-fed plants under all conditions.     

The role of chloroplast-encoded protein biosynthesis on the rate of D1 protein degradation in Dunaliella salina

Mechanistic aspects of the Photosystem II (PS II) damage and repair cycle in Dunaliella salina were investigated. The work addressed the role of chloroplast-encoded protein biosynthesis on the rate of the D1 protein (chloroplast psbA gene product) degradation, following photoinhibition of PS II under in vivo conditions. Cells were grown under different light-intensities and the rate of D1 photodamage and degradation was measured via pulse-chase measurements with (³⁵S)sulfate. It is shown that no detectable difference exists in the rate of D1 degradation in D. salina, measured in the presence or absence of lincomycin, a chloroplast protein biosynthesis inhibitor. The results suggest that de novo D1 biosynthesis does not play a role in the regulation of D1 degradation. In low-light (100 mol photons m^–2 s^–1) grown cells, the rate of photodamage to D1 did not exceed the rate of its degradation and replacement. In high-light (2200 mol photons m^–1 s^–1) grown cells, the rate of D1 photodamage was faster than the rate of its degradation, resulting in a significant accumulation of photoinactivated PS II centers in the chloroplast thylakoids (chronic photoinhibition). The latter was coincident with the appearance of a 160 kD complex that contained photodamaged D1. Electron micrographs of D. salina thylakoids revealed extensive grana stacks in the thylakoid membrane of low-light grown cells. Only rudimentary appressions consisting of simple membrane pairings were found in the high-light grown cells. The results are discussed in terms of the regulation of D1 degradation in chloroplasts under in vivo conditions.Abbreviations Chl chlorophyll - D1 the 32 kD reaction center protein of PS II, encoded by the chloroplast psbA gene - D2 the 34 kD reaction center protein of PS II, encoded by the chloroplast psbD gene - HL high light - LL low light - Linc lincomycin