The influence of the parasitic angiosperm Striga gesnerioides on the growth and photosynthesis of its host, Vigna unguiculata

Cowpea plants (Vigna unguiculata) infected with the root hemiparasiticangiosperm Striga gesnerioides accumulated less biomass thanuninfected plants over a growth period of 60 d. The allometricrelationship between shoot and root dry weight was similar inparasitized plants relative to control plants, as was the proportionof dry matter partitioned into leaf, stem and root tissue. However,infected plants failed to make any significant investment ofdry matter in pods. The rate of photosynthesis of the youngestfully expanded leaf of parasitized plants was significantlylower than for control plants. The lower rates of photosynthesiswere not attributable to stomatal limitation, a loss of chlorophyllor to an accumulation of carbohydrate. The depression of photosynthesisin the young leaves was transient. As control leaves aged, photosynthesisdeclined. This also occurred in Striga infected plants, butto a lesser extent resulting in higher rates of photosynthesisin mature leaves when compared to those of uninfected plants.The foliar nitrogen content of parasitized plants was higherthan control plants consistent with the slower rate of photosyntheticdecline of older leaves. The data are discussed with respectto the influence of parasitic weeds on host growth and photosynthesis. Key words: Cowpea, hemiparasite, allometry, nitrogen     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

Triadimefon Protects Bean Plants from Water Stress through its Effects on Abscisic Acid

Triadimefon is a fungicide that has plant growth regulatingproperties. In beans (Phaseolus vulgaris L.) it significantlyreduced shoot weight, shoot length and leaf area, and rootsappeared whiter and thicker in the treated plants. Chlorophylland carotenoid levels were increased in the leaves, but triadimefondid not affect protein levels in either leaves or roots. Triadimefonreduced transpiration and protected the plants from drought.It increased leaf diffusive resistance indicating partial closureof the stomates, and treated plants maintained their water potentialswhile those of the controls declined. Osmotic potentials ofboth treated and control leaves fell, but values in the controlswere significantly lower than those from the treated plants.Three days after treatment with triadimefon in both water stressedand non-stressed plants the abscisic acid levels in the leavesof the treated plants were more than twice the levels of thecontrols. It appears therefore that the protection conveyedby triadimefon during water stress is mediated at least partially,via its effects on ABA levels in treated tissue. (Received October 12, 1985; Accepted January 8, 1986)     

Photosynthetic Determinants of Growth in Maize Plants: Effects of Nitrogen Nutrition on Growth, Carbon Fixation and Photochemical Features

Maize(Zea mays L.) plants were grown in a greenhouse with differentlevels of nitrate-N (2 to 20 millimolar). Nitrogen nutritionhad dramatic effects on plant growth and photosynthetic characteristicsof mature leaves. Increasing nitrogen resulted in greater biomassproduction, shoot/root ratios, and rates of leaf expansion duringthe day. The elongating zone of high-N plants had higher activities(per gram fresh weight) of sucrose synthase and neutral invertasethan low-N plants, suggesting that increased leaf growth wasrelated to a greater biochemical capacity for sucrose metabolism. Mature leaves of high-N plants had higher rates of photosynthesisand assimilate export (sucrose formation), and partitioned morecarbon into sucrose relative to starch. Increased photosyntheticrates (leaf area basis) were associated with higher levels ofribulose-l,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylaseand pyruvate, phosphate dikinase (determined immunochemically).In addition, N-nutrition affected the functional organizationof chlorophyll in the leaves. Large increases in the numberof PS I reaction centers were observed which fully accountedfor increases in leaf chlorophyll content with increasing nitratesupply. Collectively, the results suggest that increased growth of maizeplants at high light and optimal nitrogen nutrition is relatedto greater capacity for photosynthesis and translocation inmature leaves, and possibly increased capacity for sucrose metabolismin expanding leaves. (Received May 22, 1989; Accepted August 28, 1989)     

The Effect of Light Intensity during Growth on the Subsequent Rate of Photosynthesis of Leaves of Tall Fescue (Festuca arundinacea Schreb.)

Leaves of tall fescue (Festuca arundinacea Schreb.) plants grownin bright light had higher rates of apparent photosynthesisper unit leaf area in bright light, and slightly lower ratesin dim light than did those of plants grown in dim light. Darkrespiration rates were higher in plants grown in bright lightthan in plants grown in dim light and the decline of photosynthesiswith increasing leaf age was faster. The rate of apparent photosynthesis in bright light of the firstleaf to become fully expanded after plants were transferredfrom bright to dim light was lower than that of plants remainingin bright light. The decline in the rate of photosynthesis ofa leaf already fully expanded at the time of transfer was notaffected. Transferring from dim to bright light increased therate of photosynthesis of the next expanded leaf; it also increasedthe rate of an already fully expanded leaf during the firstweek in bright light. After this, photosynthesis fell at a ratesimilar to that of plants remaining in dim light.     

RNA Synthesis in Phaseolus Chloroplasts: II. RIBONUCLEIC ACID SYNTHESIS IN CHLOROPLASTS FROM DEVELOPING AND SENESCING LEAVES

The rate of RNA synthesis in chloroplasts from the primary leavesof Phaseolus vulgaris L. cv. Canadian Wonder was measured invitro as plant age increased. The rate per leaf began to fallbefore the leaf was 70% expanded. At full expansion, activityhad fallen by 70%. Chloroplast RNA synthesis per unit chlorophyllwas falling before the leaf was 25% expanded. When all parts of the plant above the mature primary leaveswere removed (detopping) chloroplast RNA synthesis in theseleaves rose within 36 h. The rate increased to a maximum 3–4d after detopping, when it was 5–10 times control values;thereafter it fell again. The chlorophyll content began to increaseabout 4 d after detopping, eventually rising by 100%. Detoppingcaused a 3-fold increase in the Triton X-100-soluble DNA contentof chloroplast preparations, measured after 3.5 d. At that timethe rate of RNA synthesis per unit Triton-soluble DNA was thesame in chloroplasts from the primary leaves of intact and detoppedplants. Detopping also resulted in an increase in the depthof the leaf palisade layer. The effects of detopping on chloroplasts were prevented by darknessand reduced by shading. Increased chloroplast RNA polymerase activity was also inducedin the primary leaves by placing a polythene bag over intactplants, enclosing everything above these leaves. Removal ofthe roots from detopped plants prevented the rise in the rateof chloroplast RNA synthesis.     

Photosynthesis by Flag Leaves of Wheat in Relation to Protein, Ribulose Bisphosphate Carboxylase Activity and Nitrogen Supply

The effects of nitrate supply on the composition (cell numbers,protein and chlorophyll contents) of flag leaves of winter wheatgrown with two amounts of N fertilizer and of spring wheat grownin the glasshouse under controlled nitrate supply are describedand related to photosynthesis. Nitrogen deficiency decreasedthe size of leaves, mainly by reducing cell number and, to asmaller extent, by decreasing cell volume. Protein content perunit leaf area, per cell and per unit cell volume was largerwith abundant N. Total soluble protein, ribulose bisphosphatecarboxylase-oxygenase (RuBPc-o) protein and chlorophyll changedin proportion irrespective of nitrogen supply and leaf age.Photosynthesis per unit area of flag leaf and carboxylationefficiency in both winter and spring wheat were proportionalto the amount of total soluble protein up to 7.0 g m^–2and to the amount of RuBPc-o protein up to 4.0 g m^–2.However, photosynthesis did not increase in proportion to theamount of total soluble or RuBPc-o protein above these amounts.In young leaves with a high protein content the measured ratesof photosynthesis were lower than expected from the amount andactivity of RuBPc-o. Carboxylation per unit of RuBPc-o protein,measured in vitro, was slightly greater in N-deficient leavesof winter wheat but not of spring wheat. RuBPc-o activity perunit of RuBPc-o protein was similar in winter and spring wheatleaves and remained approximately constant with age, but increasedin leaves showing advanced senescence. RuBPc-o protein fromN-deficient leaves migrated faster on polyacrylamide gels thanprotein from leaves with high N content. Regulation of the rateof photosynthesis in leaves and chloroplasts with a high proteincontent is discussed. The conductance of the cell to the fluxof CO₂ from intercellular spaces to RuBPc-o active sites iscalculated, from cell surface areas and CO₂ fluxes, to decreasethe CO₂ partial pressure at the active site by less than 0.8Pa at an internal CO₂ partial pressure of 34 Pa. Thus the decreasein partial pressure of CO₂ is insufficient to account for theinefficiency of RuBPc-o in vivo at high protein contents. Otherlimitations to the rate of photosynthesis are considered. Key words: Wheat, photosynthesis, nitrogen, ribulose, bisphosphate carboxylase     

Striga hermonthica decreases photosynthesis in Zea mays through effects on leaf cell structure

Maize seedlings were grown in pots either with or without preconditionedseeds of the parasitic weed, Striga hermonthica. After between4 and 8 weeks, net photosynthesis in the leaves of maize plantsinfected with Striga decreased compared to leaves of uninfectedcontrol plants. The activities of four enzymes of photosyntheticmetabolism were, however, little affected by infection. A pulse-chaseexperiment using ¹⁴CO₂ showed that C₄ acids were the main earlyproducts of assimilation even when the rate of photosynthesiswas much decreased by infection, but more radio-activity appearedin glycine and serine than in leaves of healthy maize plants.Leaves of infected maize required longer to reach a steady rateof photosynthesis upon enclosure in a leaf chamber than leavesof uninfected plants after similar treatment. Electron microscopy of transverse sections of the leaves ofinfected maize indicated that the cell walls in the bundle sheathand vascular tissue were less robust than in leaves of healthyplants. The results suggest that infection with Striga causesan increase in the permeability of cell walls in the bundlesheath, leakage of CO₂ from the bundle sheath cells and decreasedeffectiveness of C₄ photosynthesis in host leaves. Key words: Zea mays, Striga hermonthica, photosynthesis, photorespiration, enzyme activity     

Regulation of Photosynthetic Activity in the Primary Leaves of Bean (Phaseolus vulgaris L.) by Materials Moving in the Water-conducting System

Endogenous factors which determine the photosynthetic capacity of the leaf were studied in the fully expanded, primary leaves of young seedings of bean (cv. Bulgarian). Following removal of the shoot above the primary leaf node and excision of all axillary buds, the primary leaves increased in area and thickness, in chlorophyll content, in levels of soluble protein, and in the specific activity of ribulose-1,5-bisphosphate carboxylase. Plants in which phloem continuity was disrupted by heat-girdling of the stem, between the shoot above the primary leaf node and the organs below, did not exhibit similar increases, whereas the shoot above the girdle continued to grow for several days. Plants in which all developing trifoliate leaves were excised as soon as they became macroscopic exhibited an increase in their photosynthetic activity, area, and thickness, while their main stem and (leafless) branches made considerable growth. Transpiration from the primary leaves was the same in decapitated plants as in the heat-girdled ones, although in the latter it accounted for only about 30% of total transpiration.     

Leaf Nucleic Acids:: II. METABOLISM DURING SENESCENCE AND THE EFFECT OF KINETIN

The nucleic acids in the green and in the senescent leaves ofthree types of plant have been studied. High and low molecularweight RNA of the chloroplast is not present in senescent leavesof Xanthium pensylvanicum, but both cytoplasmic and chloroplasticfractions are found in yellow leaves of Vicia faba and Nicotianatabacum. RNA is more rapidly degraded than DNA in the leavesof these plants when they are detached, and kinetin treatmenttemporarily arrests the loss of chlorophyll and nucleic acid.Once X. pensylvanicum leaves are yellow and senescent they cannotbe re-greened, whereas those of Nicotiana spp., and to someextent those of V. faba, can be rejuvenated. We suggest thatthe retention of chloroplast RNA in yellow leaves may be a majorfactor determining their ability to re-green and that the patternof organelle senescence prior to the first stages of leaf autolysisand dehydration is species-specific.     

Leaf Growth in Dactylis glomerate following Defoliation

Defoliation to a height of 2.5 cm considerably reduced the increasein leaf area in young Dactylis glomerata (cocksfoot) plantscompared with that of intact plants, the reduction in the rateof appearance of new leaves being relatively greater than thereduction in expansion of existing leaves. The growth of thoseexpanding leaves which were cut during defoliation accountedfor 94 per cent of the total increase in leaf area during thefirst four days after defoliation. In such a leaf, expandingcells were confined to a basal section which was well belowthe ligule of the enclosing fully expanded leaf. There was a positive relationship between rate of leaf expansionand total soluble carbohydrate content of the stubble when thecarbohydrate content was varied by placing the plants in thedark, but not when it was varied by defoliation and subsequentgrowth. These and other results suggest that the concentrationof soluble carbohydrate in the bases of expanding leaves wasa factor controlling leaf expansion following defoliation, andthat the concentration in any one leaf depended on the photosyntheticcontribution from its exposed portion. When the external nutrient supply was high, removal of the laminaeof fully expanded leaves, which comprised about two-thirds ofthe total leaf area, did not reduce leaf expansion. When thenutrient status was low, these leaves were of primary importance,presumably because of their role as a source of labile nutrients.     

Leaf Primordium Initiation and Expanded Leaf Production are Co-ordinated through Similar Response to Air Temperature in Pea (Pisum sativumL.)

Accurate prediction of the timing of leaf area development isessential to analyse and predict the responses of crops to theenvironment. In this paper, we analyse the two processes determiningthe chronology of leaf development—initiation of leafprimordia by the shoot meristem and production of expanded leavesout of the shoot tip—in several pea (Pisum sativumL.)cultivars in response to air temperature and plant growth rate.Contrasting levels of air temperature and plant growth rateduring leaf development were induced by a wide range of sowingdates and plant densities in glasshouse or field experiments.Full leaf expansion was found to occur one phyllochron afterfull leaf unfolding, whatever the leaf nodal position. Primordiuminitiation and expanded leaf production rates presented similarquantitative responses to air temperature (linear response andcommonx-intercept), whatever the plant growth rate, cultivaror period of cycle. As a consequence, they were co-ordinatedand the numbers of initiated primordia or expanded leaves wereeasily deduced from simple visual observation of leaf unfolding.The change, over time, of the numbers of initiated leaf primordiaand fully expanded leaves correlated with cumulated degree-days,with stable relationships in a wide range of environmental conditions.Two phases, with different production rates, had to be considered.These results allowed us to predict accurately the beginningand the end of individual leaf development from daily mean airtemperatures. The relationships obtained here provide an effectiveway of analysing and predicting leaf development responses tothe environment. Pisum sativumL.; pea; number of leaf primordia; number of leaves; temperature; modelling     

Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance in Kleinia articulata

Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associatedchanges during long-day-induced leaf senescence and the natureof the graft-transmissible senescence substance in Kleinia articulata.— J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaveswas characterized by a loss in fresh and dry weight, in therate of leaf expansion and progressive loss of chlorophyll inthe detached rooted leaves. Ultrastructural examination of mesophyllcells of leaves from plants grown in continuous light showedthat osmiophilic globules accumulating in the chloroplasts werethe first visible sign of senescence in the organdies. Thesefirst signs of senescence could be detected in very young leavesof plants in continuous light, even before the leaves had expanded.Attempts were made to study the cause of this photoperiodicsenescence which, from previous work, appeared to involve agraft-transmissible substance. Leaves in continuous light showed reduced stomatal opening andextracts from them had very much higher activity in the Commelinastomatal closure assay (ABA-like activity ?) compared with non-senescingleaves grown in short days (8 h). However, even if all the activitywere due to ABA, this on its own does not appear to be the senescencesubstance because a much longer exposure to continuous lightwas required to induce irreversible senescence than to reachmaximum stomatal closure promoting activity in the bioassay.Moreover, severe water stress (high ABA?) did not lead to senescenceunless combined with continuous light or ethylene treatment.It is postulated that while ABA may play an important role inKleinia leaf senescence its lethal effect may not be realizedunless ethylene-induced membrane changes may synergisticallyassist. Key words: Leaf senescence, ABA, Daylength, stomatal movement, Kleinia     

Nitrate Assimilation in Higher Plants with Special Reference to the Cocklebur (Xanthium pennsylvanicum Wallr.)

A soluble NADH-dependent nitrate reductase is described forthe shoot system of Xanthium. Young leaves and immature stemtissues contain high levels of the enzyme. They are relativelyrich in free amino acids and amides but store little free nitrate.The specific activity of the enzyme is lower in fully expandedleaves, although these leaves exhibit higher rates of fixationof carbon in photosynthesis than do younger leaves. Neithernitrate nor free amino acids accumulate in the mesophyll ofthe leaf. Older parts of the stem axis accumulate large amountsof soluble nitrogen, almost entirely as free nitrate. Reservesof nitrate in the shoot and root are rapidly depleted if nitrateis removed from the external medium. Nitrate reductase is apparently absent from roots of Xanthium.This finding is supported by analyses of bleeding sap from nitrate-fedplants which show that 95 per cent of the nitrogen exportedfrom roots is present as free nitrate. However, roots are capableof synthesizing and exporting large amounts of amino nitrogenif supplied with reduced nitrogen such as urea or ammonium. A scheme is presented summarizing the main features of the metabolismof nitrate in Xanthium and this is compared with the situationin nitrate-fed plants of the field pea (Pisum arvense L.), aspecies previously shown to be capable of reducing nitrate inits root system.     

Developmental Physiology of Sugar-beet: IV. EFFECTS OF GROWTH SUBSTANCES AND DIFFERENTIAL ROOT AND SHOOT TEMPERATURES ON SUBSEQUENT GROWTH

The effects of three growth substances, viz. indol-3yl-aceticacid (IAA), gibberellic acid (GA₃), and kinetin (KIN), and differentialshoot and root temperatures on growth of sugar-beet (Beta vulgarisL.) plants have been studied. IAA, GA₃, and KIN were applied in aqueous lanolin at differentconcentrations (50 ppm to 5000 ppm) to decapitated sugar-beetplants at the eight-leaf stage, one group also having alternateleaves removed. The growth substances significantly increasedthe dry weights of the plants when all the leaves were present,which was mainly explained by the large increase in roots. Thegrowth substances probably stimulated cambial activity and hencethe mobilization of substrates resulting in a bigger root whena relatively large leaf area existed. The failure of the plantsto respond to treatments following the removal of alternateleaves suggests that under such conditions the growth substanceshave hardly any major effect on the production of substrates;rather they influence growth by regulating the movement of substratesby altering the ‘sink strength’ if the supply ofsubstrates is not limiting. It could also be that the rootsproduce sufficient growth substances to maintain half the leavesat maximum expansion and maximum photosynthesis. Treatment withgrowth substances would therefore have little effect. When allthe leaves were present, they are limited by insufficient growthsubstances. All combinations of root and shoot temperatures of 17 and 25°C were imposed on plants decapitated at the eight-leafstage, one group also having each alternate leaf removed. Leaf8 expanded most at shoot and root temperature of 25 °C whereasother leaves had the largest areas at shoot and root temperatureof 17 °. When all the leaves were present root growth wasmaximal at shoot temperature of 17°C and root temperatureof 25 °C, but when alternate leaves were removed maximumroot growth occurred at shoot and root temperatures of 25 °C.Generally, a higher concentration of soluble carbohydrates wasfound in the roots and leaves when either the shoot or rootor both were kept at 17 °C. Concentrations of nitrogen,phosphorus, and potassium in different organs were less at 17°C than at higher shoot or root temperatures and decreasedwith age.     

Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. I. Photosynthesis, Respiration, Dry Matter Production and Partition

Small communities of S24 ryegrass were grown under supplementarylights in a glasshouse at 20°C, and abundantly suppliedwith a complete nutrient solution containing 300 p.p.m. of nitrogen,until they had a leaf area index of 5 and fully interceptedthe light. Half were then given a solution containing only 3p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m.(HN). The LN plants had a rate of single leaf photosynthesis lowerthan that of the HN plants at all but the lowest light intensities(33 per cent lower at the saturating irradiance of 170 W m^–2).Similarly, the LN communities had rates of canopy gross photosynthesis(P_sc) markedly lower than those of the HN communities. A comparisonof the observed rates of P_sc with those predicted by a mathematicalmodel of canopy photosynthesis indicated that it was the effectof nitrogen on single leaf photosynthesis, rather than differencesbetween the communities in leaf area, which led to the observeddifferences in P_sc. The superiority of the HN communities in terms of P_sc was partlyoffset by a higher rate of respiration so that they only exceededthe LN communities in terms of canopy net photosynthesis atirradiances in excess of 180 W m_–2, and produced only15 per cent more total dry matter. Nevertheless, the HN plantsdirected less of that dry matter into root and more into topsso that they came to possess twice the weight of live laminae,and the HN communities twice the leaf area, of their nitrogendeficient counterparts. Lolium perenne, S24 ryegrass, photosynthesis, respiration, dry matter production and partition, nitrogen dekieacy     

Cultivar-specific impairment of strawberry growth,photosynthesis, carbohydrate and nitrogen accumulation by ozone

In a 2-year study, fruiting plants of strawberry (Fragaria × ananassa Duch.) cv. ‘Korona’ and ‘Elsanta’ were exposed for 2 months to 78 ppb ozone on average or filtered air without ozone in controlled environment chambers. Plant growth, photosynthesis, carbohydrate accumulation, and macronutrient concentrations were investigated in order to demonstrate cultivar-specific differences in the ozone sensitivity of ‘Korona’ and ‘Elsanta’ on the whole plant level. Moreover, the hypothesis was tested whether properties of the root system in strawberry were involved in ozone tolerance, for example, the roots’ ability to store or make available carbohydrates and their capacity to secure plants’ supply with nitrogen during a stress situation. In strawberry, ozone reduced leaf area by reducing leaf number. Moreover, specific leaf area (SLA) and relative leaf water content were reduced. Net photosynthesis was only slightly impaired, but activity of Rubisco and chlorophyll content in older leaves of cv. ‘Elsanta’ were significantly reduced. The most important, indirect impairment of photosynthesis was the reduction of plants’ total leaf area, which resulted in a decrease in plant biomass. The reduction of root biomass, the root/shoot ratio, and also the distribution of carbohydrates indicated a partitioning priority of the shoot at expense of the root system. Cultivar ‘Elsanta’ was characterized by significantly lower carbohydrate levels in ozone-exposed leaves, whereas levels remained fairly stable in ‘Korona’ leaves. In addition, nitrogen concentrations in leaves and roots decreased significantly in ‘Elsanta’, not in ‘Korona’. The reduced nitrogen concentration in leaves may be related with the more distinct reduction in Rubisco activity and chlorophyll content in older leaves of ‘Elsanta’.     

Differences in Nitrogen Metabolism During Growth of Plants from Aged Potato (Solanum tuberosum L.) Meristems

The effect of advanced meristem age on growth and accumulationof plant nitrogen (N) in potato (Solanum tuberosum L.) was studied.Etiolated plantlets, excised from sprouted, single-eye-containingcores from 7 and 19-month-old seed-tubers, were transplantedinto aerated nutrient culture. Rates of shoot and root dry matterand shoot soluble-N (which included nitrate-N) accumulationwere similar for plants from both meristem ages over a 30 dinterval of log-linear growth. The rate at which nitrate-N accumulatedwas consistently 17 per cent higher in shoots from 19-month-oldcompared to those from 7-month-old meristems. However, accumulationof free amino-N and soluble protein-N were 21 and 15 per centlower, respectively in shoots from 19-month-old meristems. Abuild-up of shoot nitrate, along with lower rates of accumulationof amino-N and soluble protein-N, suggests a lower capacityfor nitrate reduction during early growth of plants from oldermeristems. Furthermore, these effects can be attributed to age-inducedchanges in the meristem or bud tissue as the plants were separatedfrom the tuber tissue initially in the study. Long-term ageingof seed-potatoes apparently affects changes within meristemsthat translate into a lower capacity to accumulate reduced formsof nitrogen during early plant growth. Potatoes (Solanum tuberosum L.), meristem age, nitrogen metabolism, plant growth potential     

Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

Fiskeby V soya bean was grown from seed germination to seedmaturation with two contrasting patterns of nitrogen metabolism:either wholly dependent on dinitrogen fixation, or with an abundantsupply of nitrate nitrogen, but lacking root nodules. The carbonand nitrogen economies of the plants were assessed at frequentintervals by measurements of photosynthesis, shoot and rootrespiration, and organic and inorganic nitrogen contents. Plantsfixing atmospheric nitrogen assimilated only 25–30 percent as much nitrogen as equivalent plants given nitrate nitrogen:c. 40 per cent of the nitrogen of ‘nitrate’ plantswas assimilated after dinitrogen fixation had ceased in ‘nodulated’plants. The rates of photosynthesis and respiration of the shootsof soya bean were not markedly affected by source of nitrogen;in contrast, the roots of ‘nodulated’ plants respiredtwice as rapidly during intense dinitrogen fixation as thoseof ‘nitrate’ plants. The magnitude of this respiratoryburden was calculated to increase the daily whole-plant respiratory loss of assimilate by 10–15 per cent over thatof plants receiving abundant nitrate. It is concluded that ‘nodulated’plants grew more slowly than ‘nitrate’ plants inthese experiments for at least two reasons: firstly, the symbioticassociation fixed insufficient nitrogen for optimum growth and,secondly, the assimila tion of the nitrogen which was fixedin the root nodules was more energy-demanding in terms of assimilatethan that of plants which assimilated nitrogen by reducing nitratein their leaves.     

Photosynthesis by White Clover Leaves in Mixed Clover/Ryegrass Swards

Measurements of rates of net photosynthesis were made on singleBlanca white clover leaves on plants taken from a field-grown,mixed clover/perennial ryegrass sward during two regrowth periods. Net photosynthesis fell by 20 per cent in the first measurementperiod as leaf area index increased and the grass componentof the crop flowered, but did not change significantly in thesecond measurement period during which the grass remained vegetative. Leaves which had been artificially protected from shading inthe sward did not have significantly different photosyntheticcapacities from leaves in the undisturbed sward, even in thefirst measurement period. As leaf area index and sward height increased, successive cloverpetioles were longer, keeping the newly expanded leaves nearthe top of the sward where they received full light. It is suggestedthat it is this which allows successive clover leaves, unlikethose of vegetative grasses, to attain a high photosyntheticcapacity throughout a growth period. Trifolium repens, Lolium perenne, Photosynthetic capacity, shading, growth