LIGHT AND NUTRIENT LIMITATION OF SEA-ICE MICROALGAE (HUDSON BAY,CANADIAN ARCTIC)1

Changes in the biochemical composition of sea-ice microalgae (southeastern Hudson Bay, Canadian Arctic) were used to assess the light and nutrient status of cells growing at the ire-water interface. These changes allowed us to test the hypothesis that ire algae are limited by light at the beginning of their growth season and become periodically limited by nutrients as the season progresses. During the vernal growth season, three patterns of variation in cellular components were found in response to changes in environmental conditions. 1) Chlorophylls a and c, ATP, carbohydrate, and carbon followed the seasonal increase in under-ice irradiance, which was mainly mused by melting of the snow cover. 2) Dissolved and biogenic silicon underwent periodic variations, which were coupled to the fortnightly neap-spring cycle of tidal mixing. 3) Cellular contents of free amino acids, protein, and total nitrogen remained relatively constant during the season. An early decrease in intracellular chlorophylls a and c suggests that ire algae did respond to small changes in solar irradiance by changing the pigment composition of their photosynthetic units. Seasonal increases in ATP, carbohydrate, and total carbon indicate light limitation in April, followed in May by a period of excess irradiance and/or nutrients in short supply. The seasonal increase in ATP and the high values of the ratio free amino acids: protein show that neither phosphorus nor nitrogen limited algal growth at the ire-water interface. In May, higher values of carbohydrate: protein, carbon: nitrogen, carbon: chlorophyll a, and also carbon: silicon and ATP: silicon indicate that the ice algae became silicon-deficient in their natural environment. Following a period of light limitation, at the beginning of the season, ice-algal growth became silicon-limited, when in situ irradiance and the accumulated algal biomass were high and the tidally-driven nutrient supply was not strong enough to satisfy algal nutrient requirements.     

Acid phosphatase activity in phosphorus-deficient white lupin roots

White lupin ( Lupinus albus L.) develops proteoid roots when grown in phosphorus (P)-deficient conditions. These short, lateral, densely clustered roots are adapted to increase P availability. Previous studies from our laboratory have shown proteoid roots have higher rates of non-photosynthetic carbon fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize P in the rhizosphere. The present work indicates that proteoid roots possess additional adaptations for increasing P availability and possibly for conserving P in the plant. Roots from P-deficient (–P) plants had significantly greater acid phosphatase activity in both root extracts and root exudates than comparable samples from P-sufficient (+P) plants beginning 10 d after emergence. The increase in activity in –P plants was most pronounced in the proteoid regions. In contrast, no induction of phytase activity was found in –P plants compared to +P plants. The number of proteoid roots present was not affected by the source of phosphorus supplied, whether organic or inorganic forms. Adding molybdate to the roots increased the number of proteoid roots in plants supplied with organic P, but not inorganic P. Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Native-polyacrylamide gel electrophoresis demonstrated that under P-deficient conditions, a unique isoform of acid phosphatase was induced between 10 and 12 d after emergence. This isoform was found not only within the root, but it comprised the major form exuded from proteoid roots of –P plants. The fact that exudation of proteoid-root-specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white lupin has several coordinated adaptive strategies to P-deficient conditions.     

Impaired mitochondrial oxidative phosphorylation in skeletal muscle of the dystrophin-deficient mdx mouse

The mdx mouse, an animal model of the Duchenne muscular dystrophy, was used for the investigation of changes in mitochondrial function associated with dystrophin deficiency. Enzymatic analysis of skeletal muscle showed an approximately 50% decrease in the activity of all respiratory chain-linked enzymes in musculus quadriceps of adult mdx mice as compared with controls, while in cardiac muscle no difference was observed. The activities of cytosolic and mitochondrial matrix enzymes were not significantly different from the control values in both cardiac and skeletal muscles. In saponin-permeabilized skeletal muscle fibers of mdx mice the maximal rates of mitochondrial respiration were about two times lower than those of controls. These changes were also demonstrated on the level of isolated mitochondria. Mdx muscle mitochondria had only 60% of maximal respiration activities of control mice skeletal muscle mitochondria and contained only about 60% of hemoproteins of mitochondrial inner membrane. Similar findings were observed in a skeletal muscle biopsy of a Duchenne muscular dystrophy patient. These data strongly suggest that a specific decrease in the amount of all mitochondrial inner membrane enzymes, most probably as result of Ca²⁺ overload of muscle fibers, is the reason for the bioenergetic deficits in dystrophin-deficient skeletal muscle.     

Phosphate starvation‐inducible pyrophosphate‐dependent phosphofructokinase occurs in plants whose roots do not form symbiotic associations with mycorrhizal fungi

Previously, we reported that phosphate (P_i) starvation of suspension cells or seedlings of Brassica nigra results in a large elevation in the activity of pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) (PFP). However, other researchers have found that P_i deprivation either causes a significant reduction or no change in extractable PFP activity of Catharanthus roseus suspension cells, or roots of Nicotiana tabacum and Phaseolis vulgaris seedlings. The present study was undertaken to examine the prevalence of P_i starvation-inducible PFP in seedlings, root cultures, or suspension cells of a variety of plant species differing in phylogenetic relatedness to B. nigra. In all species examined, fresh weights were decreased and acid phosphatase (EC 3.1.3.2) activities were increased by P_i limitation. Brassica napus suspension cells, Arabidopsis thaliana seedlings, and roots of B. napus, B. carinata, B. oleracea, Beta vulgaris, Fagopyrum esculentum, Sinapis alba, and S. arvensis seedlings grown with P_i-limited media contained 170–510% greater PFP activity than did nutrient-sufficient controls. In five of these species the induction of PFP activity by P_i limitation was based in part upon an increased susceptibility of the enzyme to its allosteric activator, fructose-2,6-bisphosphate. By contrast, the PFP activity in P_i-deprived Lycopersicon esculentum root cultures and Nicotiana silvestris suspension cells decreased by 45–65% relative to P_i-sufficient controls. Immunoblotting of extracts from A. thaliana seedlings, S. arvensis, F. esculentum and B. oleracea roots, and B. napus suspension cells probed with potato tuber PFP antibodies indicated that the upregulation of PFP activity by P_i stress in these species was not correlated with an alteration in the amount or subunit composition of PFP. Our findings suggest that induction of PFP during long-term P_i starvation may be characteristic of members of the Cruciferae, Chenopodiaceae and Polygonaceae families whose roots do not form symbiotic associations with mycorrhizal fungi.     

Light-mediated release of phytosiderophores in wheat and barley under iron or zinc deficiency

The effect of varied light intensity (50 – 600 mol m-2 s-1) on the rate of phytosiderophore release was studied under zinc (Zn) deficiency using a bread (Triticum aestivum L. cv. Aroona) and a durum wheat cultivar (Triticum durum Desf. cv. Durati) differing in zinc (Zn) efficiency and under iron (Fe) deficiency using a barley cultivar (Hordeum vulgare L. Europe). Plants were grown under controlled environmental conditions in nutrient solution for 15 days (wheat plants) or 11 days (barley plants). Phytosiderophore release was determined by measuring capacity of root exudates to mobilize copper (Cu) from a Cu-loaded resin.With increasing light intensity visual Zn deficiency symptoms such as whitish-brown lesions on leaf blade developed rapidly and severely in wheat, particularly in the durum cultivar Durati. In wheat plants supplied well with Zn, increases in light intensity from 100 to 600 mol m-2 s-1 did not clearly affect the rate of phytosiderophore release. However, under Zn deficiency increases in light intensity markedly enhanced release of phytosiderophores in Zn-deficient Aroona, but not in Zn-inefficient Durati. When Fe-deficient barley cultivar Europe was grown first at 220 mol m-2 s-1 and then exposed to 600 mol m-2 s-1 for 24 and 48 h, the rate of release of phytosiderophores was enhanced about 4-fold and 7-fold, respectively. Transfer of Fe-deficient plants from 600 to 50 mol m-2 s-1 for 48 h reduced the rate of release of phytosiderophores by a factor of 7. The effect of light on phytosiderophore release was similar regardless of whether the rate of phytosiderophore release was expressed per plant or per unit dry weight of roots.The results demonstrate a particular role of light intensity in phytosiderophore release from roots under both Zn and Fe deficiency. It is suggested that in the studies concerning the role of phytosiderophore release in expression of Zn or Fe efficiency among and within cereals, a special attention should be given to the light conditions.     

Phosphorus supply and the growth of frequently defoliated white clover (Trifolium repens L.) in dry soil

A previous study found that increased phosphorus (P) supply to frequently defoliated white clover plants, growing in a low-P, dry soil, alleviated water stress symptoms and increased plant recovery on rewatering. In this study we determined how these stresses influence white clover growth. Measurements were made of the leaf canopy, stolon infrastructure and root system of the white clover plants growing in a low-P soil. Treatments included the factorial combination of four levels of P supply, two defoliation frequencies and two soil water treatments. White clover growth declined markedly when P-deficient plants were exposed to frequent defoliation and dry soil conditions. Leaf area was more affected than other parameters, in that the combination of stresses reduced leaf area to 2% of maximum observed for infrequently defoliated plants growing in high-P soil, with adequate water. Increased P supply generally increased the growth of all plant parts. Frequently defoliated plants growing in dry soil produced similar or greater leaf mass and leaf area as plants from similar treatments growing in wet soil, when the P supply increased to 50 mg P kg-1 soil. Higher P rates were able to negate the effect of dry soil on these frequently defoliated plants, as a result of larger water and P uptake. Also, the frequently defoliated plants with restricted root growth did not respond to a small increase in P supply (17 mg P kg-1 soil) for the leaf growth, irrespective of whether they were growing in wet or dry soil. Infrequently defoliated plants with greater root growth, compared to frequently defoliated plants, more than doubled their leaf mass with this P treatment.     

Assessment of the Zn status of chickpea by plant analysis

Chickpea (Cicer arietinum L.) is extensively grown in areas where soils are deficient in zinc (Zn). To determine the response of chickpea to Zn nutrition and to diagnose Zn status in plant tissue, two glasshouse experiments were conducted using Zn-deficient siliceous sandy soil. In Experiment 1, two genotypes of desi chickpea (Dooen and Tyson) were grown at five Zn levels (0, 0.04, 0.2, 1.0 and 5.0 mg kg^-1 of soil). After 4 weeks, no difference in growth and no visible symptoms of Zn deficiency were detected. After 6–8 weeks of growth, chlorosis of younger leaves and stipules occured in the Zn₀ treatment, with shoot dry weight being only 24% of that recorded at the highest Zn level. Root growth increased from 0.52 g/plant when no Zn was applied to 1.04 g/plant in the treatment with 0.2 mg Zn kg^-1 of soil; no response to further increase of Zn fertilization occurred. Zinc concentration in the whole shoot increased significantly with increased in Zn application. The critical Zn concentration in the shoot tissue, associated with 90% of maximum growth, was 20 mg kg^-1 for both genotypes at flowering stage.In the second experiment, two genotypes of desi chickpea (Tyson and T-1587) were grown at three Zn levels (0, 0.5 and 2.5 mg kg^-1 of soil) under two moisture regimes (field capacity 12% w/w, and water stress 4% w/w). Shoot growth was influenced by both Zn supply and water stress. The effect of water stress was severe in the 0.5 and 2.5 mg Zn treatments where shoot dry matter was reduced 52 and 46%, respectively. T-1587 was less sensitive to Zn deficiency and produced higher shoot dry weight than Tyson in the Zn₀ treatment. Zinc concentration in shoots increased from 5 mg kg^-1 when no Zn was applied to 40 mg kg^-1 at the highest Zn level. The critical Zn concentration in shoots was 21 mg kg^-1.The results of the two experiments showed that the critical concentration for Zn did not differ amongst the three cultivars used and was not affected by soil moisture. Similar studies should be undertaken with a wider number of genotypes to discover if a critical concentration of 20–21 mg kg^-1 in the shoot can be used to diagose the Zn status of chickpea genotypes.     

Cardiac hypertrophy in copper-deficient rats is owing to increased mitochondria

Dietary copper depletion results in cardiac hypertrophy and ultrastructural alterations. The objective of this study was to determine the components that contribute to cardiac enlargement. Two groups (n = 4) of male, weaning, Sprague-Dawley rats were fed ad libitum with copper-adequate or copper-deficient diets for five weeks. Cross sectional transmission electron micrographs from both groups were evaluated using image analysis to quantify absolute area occupied by myocyte, mitochondria, myofibril, and other intracellular material. Copper-deficient rats had larger myocytes, increased area of mitochondria, and increased ratio of mitochondria :myofibril as well as mitochondria:myocyte. Copper deficiency did not change the absolute area occupied by myofibrils. These data suggested that increase in the absolute mitochondria area is the major contributory factor to the cardiac hypertrophy in copper deficiency. Under the conditions used, myofibril has minimal role toward contributing to the hypertrophic state. The pathology reported resembles human forms of genetic mitochondrial cardiomyopathies. The copper-deficient rat may be a useful model to investigate the underlying biochemical or molecular responses when peptides of enzymes are deleted.     

Nickel deficiency alters nickel flux in rat everted intestinal sacs

This study was conducted to determine nickel absorption in nickel-deficient rats. Jejunal segments obtained from dietary nickeldepleted (13 μg nickel/kg diet) and nickel-control (1 mg nickel/kg diet) adult rats from the first generation, and suckling pups from the second offspring were used. The nickel transfer across the intestinal epithelium and nickel uptake into the intestine were measured by use of everted jejunal sacs using a wide range of nickel concentrations administered on the luminal side (1.1 x 10^-8 M til 1.0 x 10^-4 M). Both the intestinal nickel transfer and nickel uptake were influenced by the dietary nickel supply in rat offspring, but not in the adult rats from the first generation. However, in nickel-deficient offspring, the nickel transfer across the small intestine was higher than in nickelcontrol offspring. This difference was greater using low intraluminal nickel concentrations than high nickel concentrations, and was significant at 1.1 x 10^-8 M, 6.1 x 10^-8 M, 5.1 x 10^-7 M, 1.0 x 10^-6 M, and 5.0 x 10^-6 M. Also, nickel uptake into the intestine was somewhat greater in nickel-deficient rat pups than in nickel-control pups, and significant using 1.1 x 10^-7 M and 1.0 x 10^-6 M nickel. A definite saturation type kinetic for the intestinal nickel absorption in relation to the intraluminal nickel concentration could not be observed.     

Iron reductase systems on the plant plasma membrane—A review

Higher plant roots, leaf mesophyll tissue, protoplasts as well as green algae are able to reduce extra-cellular ferricyanide and ferric chelates. In roots of dicotyledonous and nongraminaceous, monocotyledonous plants, the rate of ferric reduction is increased by iron deficiency. This reduction is an obligatory prerequisite for iron uptake and is mediated by redox systems localized on the plasma membrane. Plasma membrane-bound iron reductase systems catalyze the transmembrane electron transport from cytosolic reduced pyridine nucleotides to extracellular iron compounds. Natural and synthetic ferric complexes can act as electron acceptors.This paper gives an overview about the present knowledge on iron reductase systems at the plant plasma membrane with special emphasis on biochemical characteristics and localisation.