Cold responses of Arabidopsis mutants impaired in freezing tolerance

Mutants of Arabidopsis thaliana L. (Heynh), characterized asdeficient in their freezing tolerance after cold acclimation,were surveyed for some of the normal responses to cold exposure.In foliar tissue, the coldinducibility of three proteins, thelevels of sucrose and glucose, the fatty acyl composition oflipids, and the accumulation of anthocyanin was examined. Fourmutations (sfr3, sfr4, sfr6, and sfr7) reduced or eliminatedthe accumulation of anthocyanin during cold acclimation. Onemutation (sfr4) prevented the normally cold-induced elevationof sucrose and glucose levels; both sfr4 and another mutation(sfr7) affected fatty acid composition after (and only after)cold acclimation. On the other hand mutations sfr1, sfr2 andsfr5 did not differ significantly from the wild type in anyof the parameters tested, suggesting that they have other, perhapshighly specific, effects on lowtemperature responses. Key words: Arabidopsis thaliana, cold acclimation, freezing tolerance, mutation     

An analysis of seed development in Pisum sativum XIX. Effect of mutant alleles at the r and rb loci on starch grain size and on the content and composition of starch in developing pea seeds

The effects of mutant alleles at the r and rb loci on starchgrain size and the levels of starch and amylose in developingpea (Pisum sativum L.) seeds have been examined. Four lines,near-isogenic except for genes at these loci, have been usedto show that both mutations reduce levels of starch throughoutembryo development and reduce levels still further when combinedin the ‘double mutant’. The reduction in starchcontent was due, at least in part, to a reduction in starchgranule size. Although the proportion of starch in mature embryoswas similar in the rrRbRb and RRrbrb lines, the starch contentdiffered between these two lines during development, as a percentageof embryo dry weight. This difference was due to a reductionin the absolute growth of the embryo caused by the rb mutation.Lines homozygous for the mutant r allele with either wild-type(RbRb) or mutant (rbrb) alleles at the rb locus contained increasedproportions of amylose in their starch throughout development,due to a reduced production of amylopectin. The presence ofthe rb mutation, however, also reduced the amount of amylosein relation to the reduction in total starch levels. Mutantalleles at both loci also reduced starch levels in the testaduring development, the reduction due to rb being more extreme.Reciprocal crosses showed a maternal effect of the rb mutationon final seed size and on the absolute amount of starch in theembryo. Key words: Pisum sativum L., seed, starch, development, mutant     

Pre-flowering Growth and Development of the Inflorescences of Maize: II. ACCUMULATION AND PARTITIONING OF DRY MATTER AND NITROGEN BY INFLORESCENCES

The accumulation and partitioning of dry matter and nitrogenwere examined in the developing tassel and two uppermost earshoots of field-grown maize under varying levels of appliednitrogen and times of sowing. Accumulation of dry matter and nitrogen within an axillary branchalways favoured the ear over the husk and shank. Dry matterand nitrogen accumulated faster in the first ear than in thetassel or second ear and the partitioning between inflorescencesof dry matter and nitrogen was not affected by treatment. Therelative rate of growth, RGR(dry matter), of the first and secondear shoots increased by up to 42% at high levels of appliednitrogen and with early sowing. In contrast, the relative rateof accumulation of nitrogen (RNAR) was not sensitive to N supply,although it was reduced, on average, by 22% at the late timeof sowing. We conclude that accumulation, but not partitioning, of drymatter and nitrogen between developing inflorescences of maizeare altered by nitrogen application, time of sowing, and positionof the inflorescence on the stem. Key words: Maize, N-application, partitioning, inflorescence, sowing time     

Effects of Constant and Variable Nitrogen Supply on Sunflower (Helianthus annuus L.) Leaf Cell Number and Size

The effects of nitrogen (N) availability on cell number andcell size, and the contribution of these determinants to thefinal area of fully expanded leaves of sunflower (Helianthusannuus L.) were investigated in glasshouse experiments. Plantswere given a high (N =315 ppm) or low (N=21 ppm) N supply andwere transferred between N levels at different developmentalstages (5 to 60% of final size) of target leaves. The dynamicsof cell number in unemerged (< 0.01 m in length) leaves ofplants growing at high and low levels of N supply were alsofollowed. Maximum leaf area (LA_max) was strongly (up to two-fold)and significantly modified by N availability and the timingof transfer between N supplies, through effects on leaf expansionrate. Rate of cell production was significantly (P<0.05)reduced in unemerged target leaves under N stress, but therewas no evidence of a change in primordium size or in the durationof the leaf differentiation–emergence phase. In fullyexpanded leaves, number of cells per leaf (N_cell), leaf areaper cell (LA_cell) and cell area (A_cell) were significantly reducedby N stress. WhileLA_cell and A_cellresponded to changeover treatmentsirrespective of leaf size, significant (P<0.05) changes inN_cellonly occurred when the changeover occurred before the leafreached approx. 10% of LA_max. There were no differential effectsof N on numbers of epidermal vs. mesophyll cells. The resultsshow that the effects of N on leaf size are largely due to effectson cell production in the unemerged leaf and on both cell productionand expansion during the first phase of expansion of the emergedleaf. During the rest of the expansion period N mainly affectsthe expansion of existing cells. Cell area plasticity permitteda response to changes in N supply even at advanced stages ofleaf expansion. Increased cell expansion can compensate forlow N_cellif N stress is relieved early in the expansion of emergedleaves, but in later phases N_cellsets a limit to this response.Copyright 1999 Annals of Botany Company Helianthus annuus, leaf expansion, leaf cell number, leaf cell size, nitrogen, leaf growth, sunflower.     

Sulphydryl and Disulphide Levels in Protein Fractions from Hydrated and Dry Leaves of Resurrection Plants

Significant changes in sulphydryl (‘SH’) and disulphide(‘SS’) levels during air-drying in leaves of ‘resurrection’plants (whose protoplasm survives dehydration) stemmed mainlyfrom protein turnover effects. No significant changes were foundin the SH, SS levels in leaves of the desiccation sensitivespecies Sporobolus pyramidalis following air-drying. The three tolerant species studied differed in the directionof change. Some data were consistent with Levitt's SH, SS hypothesis:increases in protein-SS levels in Sporobolus stapfianus (desiccationtolerant) were consistent with a stabilization of new proteinby SS bonds; lower reactivity of protein-SH in the tolerantspecies Talbotia elegans (which on the other hand has decreasedprotein-SS) is consistent with a second mechanism of decreasingprotein denaturation proposed in Levitt's hypothesis. Evidence of some conversion of SH to SS in the soluble proteinsof Xerophyta viscosa (a tolerant species) would on Levitt'shypothesis indicate an injurious process. Some degree of proteindenaturation might be indicated by partial inactivation of thesoluble enzyme ribulose bisphosphate carboxylase in this species,and loss of some soluble isoenzymes (peroxidase and alkalinephosphatase). An apparent lack of SH conversion to SS in thesensitive species Sporobolus pyramidalis was not consistentwith the SH, SS hypothesis. Resurrection plants, Sporobolus pyramidalis, Sporobolus stapfianus, Talbotia elegans, Xerophyta viscosa, drought resistance, desiccation tolerance, protein turnover, sulphydryl groups     

Gene and Enhancer Trap Transposable Elements Reveal Oxygen Deprivation-regulated Genes and their Complex Patterns of Expression in Arabidopsis

Transposon tagging with modified maize Ds–GUS constructswas used to isolate genes induced by oxygen deprivation in Arabidopsisthaliana. Seedlings of 800 gene-trap (DsG) and 600 enhancer-trap(DsE) lines were grown on vertically positioned plates for 1 week,oxygen deprived for up to 24 h and stained for GUS activity.Oxygen deprivation induced intricate patterns of gene expressionin seedlings of 65 lines. The insertion site and phenotypesof 15 lines were examined. Surprisingly, none of the insertionswere into genes that encode known anaerobic polypeptides. Insertionswere identified within or adjacent to genes encoding proteinsof regulatory, enzymatic, mitochondrial protein import and unknownfunction, as well as adjacent to genes encoding a putative receptor-likekinase and putative sensor-histidine kinase. Four lines hadsignificantly lower ADH activity after 24 h of oxygen deprivationand three of these showed reduced stress tolerance. Two lineswith wild-type levels of ADH were low-oxygen intolerant. Paradoxically,several lines had significantly higher ADH activity after 12 hof oxygen deprivation but reduced stress tolerance. Caffeinetreatment, which increased ADH specific activity in wild-typeseedlings under aerobic conditions, was sufficient to increaseGUS staining in seven of the 15 lines, providing evidence thatthese genes may be regulated by cytosolic calcium levels. Theseresults demonstrate the effectiveness of the Ds–GUS taggingsystem in the identification of genes that are regulated inresponse to oxygen deprivation and a calcium second messenger.     

Mutations that affect flightin expression in Drosophila alter the viscoelastic properties of flight muscle fibers

Striated muscles across phyla share a highly conserved sarcomere design yet exhibit broad diversity in contractile velocity, force, power output, and efficiency. Insect asynchronous flight muscles are characterized by high-frequency contraction, endurance, and high-power output. These muscles have evolved an enhanced delayed force response to stretch that is largely responsible for their enhanced oscillatory work and power production. In this study we investigated the contribution of flightin to oscillatory work using sinusoidal analysis of fibers from three flightless mutants affecting flightin expression: 1) fln⁰, a flightin null mutant, 2) Mhc¹³, a myosin rod point mutant with reduced levels of flightin, and 3) Mhc⁶, a second myosin rod point mutant with reduced levels of phosphorylated flightin. Fibers from the three mutants show deficits in their passive and dynamic viscoelastic properties that are commensurate with their effect on flightin expression and result in a significant loss of oscillatory work and power. Passive tension and passive stiffness were significantly reduced in fln⁰ and Mhc¹³ but not in Mhc⁶. The dynamic viscous modulus was significantly reduced in the three mutants, whereas the dynamic elastic modulus was reduced in fln⁰ and Mhc¹³ but not in Mhc⁶. Tension generation under isometric conditions was not impaired in fln⁰. However, when subjected to sinusoidal length perturbations, work-absorbing processes dominated over work-producing processes, resulting in no net positive work output. We propose that flightin is a major contributor to myofilament stiffness and a key determinant of the enhanced delayed force response to stretch in Drosophila flight muscles. flight muscles; muscle mutants; myosin     

Effects of acetylcholine and nitric oxide on forearm blood flow at rest and after a single muscle contraction

We tested thehypothesis that ACh or nitric oxide (NO) might be involved in thevasodilation that accompanies a single contraction of the forearm.Eight adults (3 women and 5 men) completed single 1-s-durationcontractions of the forearm to raise and lower a weight equivalent to~20% maximal voluntary contraction through a distance of 5 cm. In asecond protocol, each subject had a cuff, placed completely about theforearm, inflated to 120 mmHg for a 1-s period, then released as asimulation of the mechanical effect of muscle contraction. Threeconditions were studied, always in this order:1) control, with intra-arterialinfusion of saline; 2) after muscarinic blockade withatropine; and 3) after NO synthase inhibitionwith N^G-monomethyl-L-arginine(L-NMMA) plus atropine. Forearm blood flow (FBF),measured by combined pulsed and echo Doppler ultrasound, was reduced atrest with L-NMMA-atropinecompared with the other two conditions. After the single contraction,there were no effects of atropine, butL-NMMA reduced the peak FBF andthe total postcontraction hyperemia. After the single cuff inflation,atropine had no effects, whereasL-NMMA caused changes similar tothose seen after contraction, reducing the peak FBF and the totalhyperemia. The observation thatL-NMMA reduced FBF in responseto both cuff inflation and a brief contraction indicates that NO fromthe vascular endothelium might modulate the basal level of vasculartone and the mechanical component of the hyperemia with exercise. It isunlikely that ACh and NO from the endothelium are involved in thedilator response to a single muscle contraction.

     

Mechanism and regulation of folate uptake by human pancreatic epithelial MIA PaCa-2 cells

After the liver, the pancreas contains the second highest level of folate among human tissues, and folate deficiency adversely affects its physiological function. Despite that, nothing is currently known about the cellular mechanisms involved in folate uptake by cells of this important exocrine organ or about folate uptake regulation. We have begun to address these issues, and in this report we present the results of our findings on the mechanism of folate uptake by the human-derived pancreatic MIA PaCa-2 cells. Our results show folic acid uptake to be 1) temperature and energy dependent; 2) pH dependent, with a markedly higher uptake at acidic pH compared with neutral or alkaline pH; 3) Na⁺ independent; 4) saturable as a function of substrate concentration (apparent K_m = 0.762 ± 0.10 µM); 5) inhibited (with similar affinity) by reduced, substituted, and oxidized folate derivatives; and 6) sensitive to the inhibitory effect of anion transport inhibitors. RT-PCR and Western blot analysis showed expression of the human reduced folate carrier (hRFC) at the RNA and protein levels, respectively. The functional contribution of hRFC in carrier-mediated folate uptake was confirmed by gene silencing using gene-specific small interfering RNA. Evidence also was found suggesting that the folate uptake process by MIA PaCa-2 cells is regulated by cAMP- and protein tyrosine kinase (PTK)-mediated pathways. These studies demonstrate for the first time the involvement of a specialized, acidic pH-dependent, carrier-mediated mechanism for folate uptake by human pancreatic MIA PaCa-2 cells. The results also show the involvement of hRFC in the uptake process and suggest the possible involvement of intracellular cAMP- and PTK-mediated pathways in the regulation of folate uptake. human reduced folate carrier; small interfering RNA; transport regulation     

Effects of Atmospheric NO2 on Azolla-Anabaena Symbiosis

Cultures of the water fern Azolla pinnata R, Br. exposed for1 week to atmospheric NO₂ (50, 100 or 200 nl l^-1) induced additionallevels of nitrate reductase (NaR) protein and nitrite reductase(NiR) activity. At low concentrations of NO₂ (50 nl l^-1), nitratederived from NO₂ provides an alternative N source for Azollabut does not affect rates of acetylene reduction. However, thesymbiotic relationship between Azolla and its endosymbiont,Anabaena azollae is only affected adversely by high concentrations(100 and 200 nl l^-1) of atmospheric NO₂. The resultant decreasesin rate of growth, nitrogen fixation, heterocyst formation,and overall nitrogen cycling are probably due to the additionalaccumulation of N products derived from higher levels of atmosphericNO₂. Parallel increases in levels of polyamines suggest thatAzolla partially alleviates these harmful effects by incorporatingsome of the extra NO₂-induced N into polyamines.Copyright 1994,1999 Academic Press Azolla-Anabaena symbiosis, nitrogen dioxide pollution, nitrogen metabolism, polyamines     

Changes of action potentials and force at lowered [Na+]o in mouse skeletal muscle: implications for fatigue

We examined 1) whether the effects of lowered trans-sarcolemmal Na⁺ gradient on force differed between nonfatigued fast- and slow-twitch muscles of mice and 2) whether effects on action potentials could explain the decrease of force. The Na⁺ gradient was reduced by lowering the extracellular [Na⁺] ([Na⁺]_o). The peak force-[Na⁺]_o relationships for the twitch and tetanus were the same in nonfatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na⁺]_o and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lowered [Na⁺]_o that had little effect in nonfatigued soleus muscle. This finding suggests that substantial differences exist in the Na⁺ effect on force between nonfatigued and fatigued muscle. The reduced contractility in nonfatigued muscles at lowered [Na⁺]_o was largely due to 1) an increased number of inexcitable fibers and threshold for action potentials, 2) a reduction of action potential amplitude, and 3) a reduced capacity to generate action potentials throughout trains. sodium gradient; muscle contraction; action potential train; extensor digitorum longus; soleus     

Photoinduction of Spore Germination in a Fern, Mohria caffrorum

Irradiation of spores of the fern Mohria caffrorum Sw. witheither red light (67.4 µW cm^–2) or far-red light(63.2 µW cm^–2) for a period of 24 h induced maximumlevels of germination. Brief irradiations with blue light (127.6µW cm^–2) administered before or after photoinductioncompletely nullified the effects of red or far-red light; however,with prolonged exposure to blue light, germination levels roseto near maximum. The similar effects of red and far-red lightin promoting spore germination makes the involvement of phytochromein this process questionable. Based on energy requirements,the promotive and inhibitory phases of blue light appear toinvolve independent modes of action. Mohria caffrorum, ferns, spore germination, photoinduction, phytochrome     

An Analysis of Growth of Oil Palm Seedlings in Full Daylight and in Shade

Growth of seedling oil palms (Elaeis guineensis), in full daylightand under three levels of shade, was studied using growth analysistechniques. In full day-light, net assimilation rates (E_A) betweeno.15 and 0.32 g./dm.²/week were obtained associated with lowleaf-area ratios (F) giving relative growth-rates (R_W) rangingfromI I.8 to 3.2 per cent, per day. There were no indicationsof seasonal differences within the small range of values found. The plants take about 90 days to adapt to shade conditions becausethe mean plastochron is 24 days, and shading effects are beststudied on plants grown since germination under the shade treatments.Very different pictures of response to shade were obtained usingplants grown initially in full light followed by 90 days' shadebefore sampling compared with plants grown under shade sincegermination. In the latter, except at the lowest light levelused, I I.I per cent, of full day-light, there was very littleeffect of light on E_A or R_w, although the F values decreasedas light increased. Extrapolation of the F values to the extinctionpoint gave values similar to those obtained in another experimenton the effect of a number of shade levels on F. The physiological and ecological implications of these findings,particularly the low growth-rates and shade tolerance, are discussed.     

Glucosamine protects neonatal cardiomyocytes from ischemia-reperfusion injury via increased protein-associated O-GlcNAc

Increased levels of protein O-linked N-acetylglucosamine (O-GlcNAc) have been shown to increase cell survival following stress. Therefore, the goal of this study was to determine whether in isolated neonatal rat ventricular myocytes (NRVMs) an increase in protein O-GlcNAcylation resulted in improved survival and viability following ischemia-reperfusion (I/R). NRVMs were exposed to 4 h of ischemia and 16 h of reperfusion, and cell viability, necrosis, apoptosis, and O-GlcNAc levels were assessed. Treatment of cells with glucosamine, hyperglycemia, or O-(2-acetamido-2-deoxy-D-glucopyranosylidene)-amino-N-phenylcarbamate(PUGNAc), an inhibitor of O-GlcNAcase, significantly increased O-GlcNAc levels and improved cell viability, as well as reducing both necrosis and apoptosis compared with untreated cells following I/R. Alloxan, an inhibitor of O-GlcNAc transferase, markedly reduced O-GlcNAc levels and exacerbated I/R injury. The improved survival with hyperglycemia was attenuated by azaserine, which inhibits glucose metabolism via the hexosamine biosynthesis pathway. Reperfusion in the absence of glucose reduced O-GlcNAc levels on reperfusion compared with normal glucose conditions and decreased cell viability. O-GlcNAc levels significantly correlated with cell viability during reperfusion. The effects of glucosamine and PUGNAc on cellular viability were associated with reduced calcineurin activation as measured by translocation of nuclear factor of activated T cells, suggesting that increased O-GlcNAc levels may attenuate I/R induced increase in cytosolic Ca²⁺. These data support the concept that activation of metabolic pathways leading to an increase in O-GlcNAc levels is an endogenous stress-activated response and that augmentation of this response improves cell survival. Thus strategies designed to activate these pathways may represent novel interventions for inducing cardioprotection. hexosamine biosynthesis; calcium; protein O-glycosylation     

The Influence of Foliar Applications of Urea on the Nodulation Pattern of Certain Leguminous Species

In greenhouse experiments with seven species of legumes, thespraying of urea on to the leaves was shown to affect nodulationadversely, without impairing the growth of the plants. In Phaseolusvulgaris, Vicia sativa, and Pisum sativum three-times-weeklysprays of 1 per cent. aqueous urea either prevented or markedlyreduced nodule development during the 8-week experimental period.In Medicago sativa and Trifolium pratense the urea treatmentresulted in a delay in nodulation so that numbers of nodulesat the first sampling (4 weeks) were reduced, while numbersat later samplings were higher since nodulation had been delayeduntil the root system was larger and provided a greater numberof potential nodule sites. In Trifolium hybridum and T. repensthe urea-treated plants showed reduced nodulation throughoutthe 6-week experimental period. In these experiments the advance effects on nodulation cannotbe due to high concentration of combined nitrogen in the rootingmedium, but it is suggested they derive from a high level ofnitrogen within the plant.     

Identification and utilization of genetic determinants of trait measurement errors in image-based,high-throughput phenotyping

The accuracy of trait measurements greatly affects the quality of genetic analyses. During automated phenotyping, trait measurement errors, i.e. differences between automatically extracted trait values and ground truth, are often treated as random effects that can be controlled by increasing population sizes and/or replication number. In contrast, there is some evidence that trait measurement errors may be partially under genetic control. Consistent with this hypothesis, we observed substantial nonrandom, genetic contributions to trait measurement errors for five maize (Zea mays) tassel traits collected using an image-based phenotyping platform. The phenotyping accuracy varied according to whether a tassel exhibited “open” versus. “closed” branching architecture, which is itself under genetic control. Trait-associated SNPs (TASs) identified via genome-wide association studies (GWASs) conducted on five tassel traits that had been phenotyped both manually (i.e. ground truth) and via feature extraction from images exhibit little overlap. Furthermore, identification of TASs from GWASs conducted on the differences between the two values indicated that a fraction of measurement error is under genetic control. Similar results were obtained in a sorghum (Sorghum bicolor) plant height dataset, demonstrating that trait measurement error is genetically determined in multiple species and traits. Trait measurement bias cannot be controlled by increasing population size and/or replication number.

The accuracy of high-throughput phenotyping can be affected by genetically determined measurement biases, which can alter the results of genetic analyses.     

Sodium-dependent inactivation of sodium/calcium exchange in transfected Chinese hamster ovary cells

High concentrations of cytosolic Na⁺ ions induce the time-dependent formation of an inactive state of the Na⁺/Ca²⁺ exchanger (NCX), a process known as Na⁺-dependent inactivation. NCX activity was measured as Ca²⁺ uptake in fura 2-loaded Chinese hamster ovary (CHO) cells expressing the wild-type (WT) NCX or mutants that are hypersensitive (F223E) or resistant (K229Q) to Na⁺-dependent inactivation. As expected, 1) Na⁺-dependent inactivation was promoted by high cytosolic Na⁺ concentration, 2) the F223E mutant was more susceptible than the WT exchanger to inactivation, whereas the K229Q mutant was resistant, and 3) inactivation was enhanced by cytosolic acidification. However, in contrast to expectations from excised patch studies, 1) the WT exchanger was resistant to Na⁺-dependent inactivation unless cytosolic pH was reduced, 2) reducing cellular phosphatidylinositol-4,5-bisphosphate levels did not induce Na⁺-dependent inactivation in the WT exchanger, 3) Na⁺-dependent inactivation did not increase the half-maximal cytosolic Ca²⁺ concentration for allosteric Ca²⁺ activation, 4) Na⁺-dependent inactivation was not reversed by high cytosolic Ca²⁺ concentrations, and 5) Na⁺-dependent inactivation was partially, but transiently, reversed by an increase in extracellular Ca²⁺ concentration. Thus Na⁺-dependent inactivation of NCX expressed in CHO cells differs in several respects from the inactivation process measured in excised patches. The refractoriness of the WT exchanger to Na⁺-dependent inactivation suggests that this type of inactivation is unlikely to be a strong regulator of exchange activity under physiological conditions but would probably act to inhibit NCX-mediated Ca²⁺ influx during ischemia. ischemia; cytosolic calcium concentration; cytosolic sodium concentration; cellular phosphatidylinositol-4,5-bisphosphate     

Contrasting CO2 and temperature effects on leaf growth of perennial ryegrass in spring and summer

The effects of increased atmospheric carbon dioxide (CO₂) of700 µmol mol^–1 and increased air temperature of+ 4C were examined in Lolium perenne L. cv. Vigor, growingin semi-controlled greenhouses. Leaf growth, segmental elongationrates (SER), water relations, cell wall (tensiometric) extensibility(%P) and epidermal cell lengths (ECL) were measured in expandingleaves in spring and summer. In elevated CO₂, shoot dry weight (SDW) increased in mid-summer.In both seasons, SDW decreased in elevated air temperatureswith this reduction being greater in summer as compared to spring.Specific leaf area (SLA) decreased in elevated CO₂ and in CO₂ temperature in both seasons. In spring, increased leaf extensionand SER in elevated CO₂ were linked with increased ECL, %P andfinal leaf size whilst in summer all were reduced. In high temperature,leaf extension, SER, %P and final leaf size were reduced inboth seasons. In elevated CO₂ temperature, leaf extension,SER, %P, and ECL increased in spring, but final leaf size remainedunaltered, whilst in summer all decreased. Mid-morning waterpotential did not differ with CO₂ or temperature treatments.Leaf turgor pressure increased in elevated CO₂ in spring andremained similar to the control in summer whilst solute potentialdecreased in spring and increased in summer. Contrasting seasonalgrowth responses of L. perenne in response to elevated CO₂ andtemperature suggests pasture management may change in the future.The grazing season may be prolonged, but whole season productivitymay become more variable than today. Key words: Lolium perenne, ryegrass, CO₂ and temperature, leaf extension, cell wall rheology     

Water Deficit and Inflorescence Development in Zea mays L.

A water deficit imposed during the period of terminal male inflorescenceinitiation and early development reduced both the growth rateand the mature size of that organ in Zea mays (cv. Iochief).Growth and development of the axillary shoots, the potentialfemale inflorescences, was inhibited during the episode of waterdeficit but promoted thereafter. As a result, plants which hadbeen subjected to a water deficit at that period produced 2–3mature cobs and relatively large axillary shoots at the lowernodes, whereas plants supplied with water throughout produceda single mature cob and relatively small axillary shoots. A water deficit imposed during other growth phases did not producethis response and, moreover, a further period of deficit imposedlater in development, following a deficit at the sensitive stage,inhibited the enlargement of the axillary shoots invoked bythe earlier deficit. It did not, however, inhibit the enhancedfloral development of those axillary shoots nor reverse theinhibition of tassel growth. The data are discussed in relation to correlative inhibitionin Zea mays.     

Effects of UV radiation on grazing by two marine heterotrophic nanoflagellates on autotrophic picoplankton

Phytoplankton <4-3 µm in diameter, or autotrophic picoplankton,can constitute the majority of the biomass and productivityof photosynthetic organisms in marine and freshwater systems.Indirect evidence has indicated that mortality of autotrophicpicoplankton occurs principally at night in the open ocean,but continuously in coastal water. The predominant view of thefate of autotrophic picoplankton production in the ocean isthat they are consumed by heterotrophic nanoflagellates. A possiblemechanism to explain these observations is that grazing of heterotrophicnanoflagellates on autotrophic picoplankton is inhibited byultraviolet radiation (W), at least in clear open-ocean environments.A series of laboratory experiments was conducted to examinethe effects of UV radiation on the grazing impact of two heterotrophicnanoflagellates on Synechococcus spp., a commonly occurringgenus of autotrophic picoplankton. The two nanoflagellates usedwere Paraphysomonas bandaieensis and Paraphysomonas imperforata.For both nanoflagellates, there was an inverse relationshipbetween the grazing mortality of Synechococcus and UV irradiance.The grazing mortality of Synechococcus was reduced less withP.imperforata than with P.bandaiensis. In some experiments,the effect of UV on the grazing impact of the nanoflagellatepopulations was caused in part by UV-related reductions in nanoflagellatesurvival. However, UV reduced the grazing impact of nanoflagellatesprimarily by reducing the rates of consumption of Synechococcusby individual nanoflagellates, to a degree directly relatedto UV irradiance. The results suggat that UV radiation may bean important selection factor in clear open-ocean water, andthat in order to predict the effect of increasing UV radiationon marine microbial plankton communities, we must consider interactionsbetween trophic levels as well as effects on single trophiclevels.