The Effects of Copper Supply and Shading on Retranslocation of Copper from Mature Wheat Leaves

Plants of Gamenya wheat (Triticum aestivum L.) were grown inpots of a Cu-deficient sand at two levels of Cu (deficient andsufficient), and harvested on days 13, 22, 28 and 38. In 50per cent of the pots in each Cu treatment, the oldest leaf andleaf 2 of the main stem were shaded when they reached full expansion. The Cu content of the oldest leaf of Cu-sufficient, unshadedplants was high at day 13 and declined rapidly to day 38. Thatof Cu-deficient, unshaded plants was initially relatively lowand declined much more slowly, so that at day 38 it resembledthat of Cu-sufficient plants. Shading the oldest leaf acceleratedthe loss of its Cu in both Cu-deficient and Cu-sufficient plants.The effects of shading and of Cu supply on the loss of Cu fromthe oldest leaf paralleled their effects on the loss of N andchlorophyll. The results suggest that most of the Cu in theoldest leaf does not move out until the leaf senesces. In Cu-deficient plants retention of Cu by old green leaves accentuatedCu deficiency. The release of Cu, resulting from shading theold leaves of Cu-deficient plants, stimulated the growth ofnew leaves. In Cu-sufficient plants, shading depressed growth. copper, shading, retranslocation, wheat, Triticum aestivum L.     

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     

Copper supply and the leaf emergence rate of spring wheat

Copper deficiency can delay flowering and plant maturity. However, the effect of copper deficiency on the rate of leaf emergence has not been quantified. We tested the hypothesis that low copper supply decreases the rate of leaf emergence of wheat (Triticum aestivum L. cv Gamenya). Copper foliar sprays are commonly applied to wheat. We examined the response of the rate of leaf emergence to a foliar application of copper sulphate.Wheat was grown in root cooling tanks (20°C) in the glasshouse. Soil copper treatments were applied as solutions of CuSO₄.5H₂O at three rates: Cu₀=no added Cu, Cu₄₀₀=400 g Cu per 3 kg pot, and Cu₁₆₀₀=1600 g Cu per pot. An additional treatment of a foliar spray of CuSO₄.5H₂O (0.4 mg Cu per plant) was applied to Cu₀ and Cu₄₀₀ plants 45 days after sowing (5.5 leaves on the main stem). Leaves on the main stem were counted and the rate of leaf emergence was estimated from the regression of number of emerged leaves against thermal time (base 0°C). The phyllochron was calculated as 1/rate of emergence.Leaves on Cu₀ and Cu₄₀₀ plants took longer to emerge than on Cu₁₆₀₀ plants, with the phyllochron of Cu₁₆₀₀ plants being 130 compared to 137 for the Cu₄₀₀ plants and 158 for the Cu₀ plants. The foliar application of CuSO₄ at the 5–6 leaf stage resulted in a decrease in the phyllochron of the Cu₀ plants to 127, but no change in that of the Cu₄₀₀ plants.     

An Analysis of the Effects of Gibberellic Acid on Tomato Leaf Growth

The effects of four consecutive daily sprays of gibberellicacid (GA₃) on the growth of leaves of young tomato plants cv.Potentate were studied. Total leaf weight and area were increasedby GA₃. The percentage changes were larger in the younger leavesthan in the older but the absolute increases of the middlleleaves accounted for most of the total response. Chlorophyllcontent, both total and per unit weight, was reduced by GA inthe older leaves and increased in the younger; on an area basisit was reduced in all but the youngest leaves. Palisade cell length and palisade cell number per unit sectionlength were reduced by GA₃ in the oldest leaves and increasedin the youngest. There were larger intercellular spaces in bothmesophyll layers and a larger transectional area of the mid-ribsof the oldest and two youngest leaves in GA₃ plants. The ‘surfaceareas’ of epidermal cells were also increased by GA₃ treatment.Leaf fresh weight per unit area was only a true index of laminathickness in the two oldest leaves.     

Oxidative Damage Caused by an Excess of Copper in Oat Leaves

The relationship between the toxicity of Cu²⁺ ions and oxidativereactions in plant cells was studied. Segments of leaves from6- and 9-day-old oat seedlings were incubated in solutions thatcontained Cu²⁺ ions at various concentrations for 24 h in thelight. High concentrations of Cu²⁺ ions caused the breakdownof chlorophyll and carotenoid, as well as an increase in membranepermeability and rates of lipid peroxidation. These effectswere more pronounced in older leaves than in younger ones. Scavengersof free radicals, such as Tiron, sodium benzoate and mannitol,prevented the increases in these parameters of senescence. WhileTiron was more effective in this regard in younger leaves, sodiumbenzoate was more effective in older ones. The treatment withCu²⁺ ions enhanced the activity of superoxide dismutase, especiallyin the younger leaves. By contrast, Cu²⁺ ions decreased theactivities of catalase and ascorbate peroxidase in both oldand young leaves. Scavengers of free radicals protected theseenzymes against inactivation. These results indicate that an excess of Cu²⁺ ions causes rapidsenescence in plant leaves via oxidative reactions in the light. The reactions involve formation of ‘O^–₂, H₂O₂ andHO’ and a subsequent decrease in antioxidant defenceswhich in turn enhances the efectiveness of toxic species ofoxygen. (Received March 17, 1993; Accepted September 28, 1993)     

Effects of Nitrogen Fertilizer on Growth and Yield of Spring Wheat

Nine amounts of nitrogen fertilizer, ranging from 0 to 200 kgN ha^–1, were applied to spring wheat cv. Kleiber in the3 years 1972-1974. In 1972 grain dry weight with 125 kg N ha^–1or more was 100 g m^–2 (23 per cent) greater than withoutnitrogen. Grain yield was unaffected by nitrogen in the otheryears. Leaf area at and after anthesis was increased throughoutthe range of nitrogen tested, most in 1972 and least in 1973.Consequently, the addition of 200 kg N ha^–1 decreasedthe amount of grain produced per unit of leaf area by approximately25 per cent in all years. The dry weight of leaves and stems at anthesis and maturitywas increased by nitrogen in all years, similarly to leaf area.However, the change in stem dry weight between anthesis andmaturity was not affected by nitrogen; stems increased in dryweight for about 20 days after anthesis and then decreased tovalues similar to those at anthesis. The uptake of CO₂ per unit area of flag leaf or second leaf(leaf below the flag leaf) was slightly decreased by nitrogenwhen the increase in leaf area caused by nitrogen appreciablydecreased the light intensity at the surface of these leaves.In spite of such decreases the CO₂ absorbed by flag and secondleaves per unit area of land was always increased by nitrogen,and relatively more than was grain yield. It is suggested that increases in respiratory loss of CO₂ withincreasing nitrogen fertilizer may explain why nitrogen increasedvegetative growth and leaf area relatively more than grain yield.     

Water relations of cotton plants under nitrogen deficiency

Nitrogen deficiency in cotton plants (Gossypium hirsutum L.) increased the threshold water potentials for both stomatal closure and leaf senescence (defined as loss of chlorophyll and protein) during drought. These studies attempted to answer two questions: (1) What is the basis for the N/water interaction on senescence? (2) Is there a direct relationship between stomatal closure and senescence? Young and old leaves from N-deficient and N-sufficient plants maintained their relative sensitivities to water stress when excised leaf discs were floated on solutions of polyethylene glycol in dim light. In this leaf disc system, both leaf aging and N deficiency increased the threshold water potential for senescence. Leaf aging and N deficiency also decreased the concentration of exogenous abscisic acid necessary to initiate senescence in discs. A role for cytokinins in controlling senescence could not be clearly shown. In young leaves of both N-deficient and N-sufficient plants, stomata closed at water potentials much higher than those causing senescence. During leaf aging, the water potentials causing senescence increased more than those causing stomatal closure. The two processes thus occurred at about the same potentials in the oldest leaves. These data argue against a general cause-and-effect relationship between stomatal closure and senescence. Rather, each process apparently responded independently to absicsic acid accumulated during drought.     

Effects of elevated carbon dioxide and ozone on the growth and yield of spring wheat (Triticum aestivum L.)

Spring wheat cv. Minaret was grown under three carbon dioxide(CO₂) and two ozone (O₃) concentrations from seedling emergenceto maturity in open-top chambers. Under elevated CO₂ concentrations,the green leaf area index of the main shoot was increased, largelydue to an increase in green leaf area duration. Biomass increasedlinearly in response to increasing CO₂ (ambient, 550 and 680ppm). At anthesis, stem and ear dry weights and plant heightwere increased by up to 174%, 5% and 9 cm, respectively, andbiomass at maturity was 23% greater in the 680 ppm treatmentas compared to the ambient control. Grain numbers per spikeletand per ear were increased by 0.2 and 5 grains, respectively,and this, coupled with a higher number of ears bearing tillers,increased grain yield by up to 33%. Exposure to a 7 h daily mean O₃ concentration of 60 ppb inducedpremature leaf senescence during early vegetative growth (leaves1–7) under ambient CO₂ concentrations. Damage to the mainshoot and possible seedling mortality during the first 3 weeksof exposure altered canopy structure and increased the proportionof tillers 1 and 2 which survived to produce ears at maturitywas increased; as a result, grain yield was not significantlyaffected. In contrast to the older leaves, the flag leaf (leaf8) sustained no visible O₃ damage, and mean grain yield perear was not affected. Interactions between elevated CO₂ andO₃ influenced the severity of visible leaf damage (leaves 1–7),with elevated CO₂ apparently protecting against O₃-induced prematuresenescence during early vegetative growth. The data suggestthat the flag leaf of Minaret, a major source of assimilateduring grain fill, may be relatively insensitive to O₃ exposure.Possible mechanisms involved in damage and/or recovery are discussed. Key words: Carbon dioxide, ozone, spring wheat (cv. Minaret), leaf damage, tiller, yield     

The Growth Rate of Successive Leaves of Wheat Plants in Relation to Sugar and Protein Concentrations in the Extension Zone

The growing part of a wheat leaf (the extension zone) is loocatedat the leaf base and following from this it was proposed thatthe absolute leaf extension rate (L_er) can be partitioned intotwo components: the length of the extension zone (L_ez) and therelative extension rate of that extension zone (R_ez). R_ez isan appropriate measure of the efficiency of leaf growth forcomparing different leaves. This model of a wheat leaf was thenused to investigate whether differences in growth rates betweensuccessive leaves on wheat plants were due to differences inhexose sugar or protein concentrations within the extensionzone. Measurements were done in an irrigated field crop suppliedwith 0, 3, 10, or 30 kg N ha^–1 per week. The mean values of L_er at 15 °C increased with leaf numberand with nitrogen supply as did values for L_ez. In contrastR_ez at 15 °C declined from 0.9 d^–1 for the first leavesto 0.3 d^–1 for the flag leaf. Nitrogen supply had littleeffect on R_ez. A separate measure of the efficiency of leafgrowth, the responsiveness of L_er to temperature (measured asthe slope of the temperature response curve), also decreasedwith leaf number by the same order as R_ez and was similarlyunaffected by nitrogen supply. The protein concentrations in the extension zones of the firstleaves were around 40 mg g^–1 fr. wt. and this declinedto approximately 20 mg g^–1 fr. wt for leaves emergingafter tillers emerged and remained low thereafter. Protein concentrationswere not correlated with the external supply of nitrogen. Hexosesugar concentrations followed a reverse pattern of increasingin the later order leaves and these also increased as nitrogensupply decreased. Both R_ez and the responsiveness of L_er to temperature were positivelycorrelated with the protein concentration, the relationshipin each case being described by a rectangular hyperbola equation(P < 0.01), and negatively correlated with hexose concentrations.It was concluded that internal competition between growing pointsfor reduced nitrogen caused the observed effects. However, differencesin protein concentrations may not simply reflect differencesin enzyme concentrations; rather these differences may indicatechanges in some other character such as cell numbers.     

Photosynthesis of Ears and Flag Leaves of Wheat and Barley

Immediately after anthesis ears of spring wheat absorbed lessthan 0.5 mg CO₂, per hour in daylight and later evolved CO₂,in the light and in the dark. The rate of apparent photosynthesisof the combined flag-leaf lamina and sheath and peduncle (collectivelycalled flag leaf) of two spring wheat varieties, Atle and JufyI, was 3–4 mg per hour; the rates of the flag leaf andthe ear of two spring barleys, Plumage Archer and Proctor, wereeach about 1 mg per hour. The gas exchange of ears and flag leaves between ear emergenceand maturity accounted for most of the final grain dry weight.The CO₂, fixed by the wheat ear was equivalent to between 17and 30 per cent of the grain weight, but more than this waslost by respiration, so assimilation in the flag leaf was equivalentto 110–20 per cent of the final grain weight. In barley,photosynthesis in the flag leaf and the net CO₂ uptake by theear each provided about half of the carbohydrate in the grain. Barley ears photosynthesized more than wheat ears because oftheir greater surface, and flag leaves of wheat photosynthesizedmore than those of barley because they had more surface anda slightly greater rate of photosynthesis per dm².     

The Effect of Copper Supply on the Senescence and the Retranslocation of Nutrients of the Oldest Leaf of Wheat

The contents of Cu, N, P, K, Zn, Mn and Ca were followed duringthe life of the oldest leaf of wheat plants (Triticum aestivumL. cv. Gamenya) grown at deficient and sufficient supplies ofCu. At both levels of Cu, the Cu content of the oldest leafbehaved in a similar way to the contents of N and Zn, whichdeclined markedly during leaf senescence. By contrast to Cu the P and K contents declined markedly, priorto leaf senescence, whereas the Ca and Mn contents increasedthroughout the life of the leaf and did not decline during leafsenescence. Interactions among Cu supply, the supply of other nutrients(e.g. N), and leaf senescence account for the variable mobilityof Cu in wheat. Similar interactions between nutrient supplyand senescence may explain contradictory reports on the redistributionof other nutrients which are variably mobile in plants.     

Dark Respiration of Several Varieties of Winter Wheat Given Different Amounts of Nitrogen Fertilizer

Carbon dioxide production in the dark by ears and by the restof the shoot of winter wheat grown in the field was measuredin 2 years during grain growth. The respiration rate per g d.wt of the ears was increased by nitrogen fertilizer. Ears ofthe semi-dwarf varieties Maris Fundin and Hobbit respired moreslowly than ears of Maris Huntsman and Cappelle-Desprez. Respirationrates of the rest of the shoot were unaffected by nitrogen orvariety. The amount of carbohydrate required to provide the CO₂ respiredduring the whole period of grain growth varied from 163 to 443g m^–2, or 42 to 76 per cent of the dry weight of the grain.More than half the CO₂ lost was respired by the ear. The additionof 180 kg N ha^–1, which increased grain yield by 78 percent in 1975, almost trebled the amount of CO₂ lost by the ears.The semi-dwarf varieties lost less CO₂ from ears and shootsthan did the taller ones, and had larger yields of grain. Respiration was also estimated from the difference between the¹⁴C contents of shoots sampled immediately after a 30 s exposureto ¹⁴CO₂ and at maturity. When 14C was supplied 10 days afteranthesis, the loss by maturity amounted to 16–28 per centof that initially absorbed by flag leaves and 40 per cent ofthat absorbed by the leaf below the flag leaf. Most of the lossoccurred in the first day. The loss of 14C by maturity was significantlyincreased by nitrogen fertilizer in 1975. Triticum aestivum L., wheat, respiration, nitrogen supply, fertilizer treatment     

Profiles of Leaf Senescence During Reproductive Growth of Sunflower and Maize

We investigated the effect of reproductive growth on the profilesof leaf senescence in maize (Zea mays L.) and sunflower (Helianthusannuus L.). Leaf senescence after flowering was assessed usingboth structural (leaf chlorophyll, nitrogen and dry matter)and functional (photosynthesis) variables in undisturbed plants(+G) and in plants in which grain set was prevented (-G). Twoweeks after flowering, lack of grain accelerated senescencein maize and delayed senescence in sunflower as indicated byleaf chlorophyll; leaf nitrogen and dry matter were less sensitiveresponse variables. Lack of interaction between reproductivetreatment and leaf position indicates that the senescence signal,whatever its nature, was equally effective throughout the plantin both species. In both species, feedback inhibition of photosynthesiswas first detected 30–35 d after flowering; excess carbohydratein the leaves was therefore an unlikely trigger of acceleratedsenescence in maize. As reproductive development progressed,differences between +G and -G plants were more marked in sunflower,and tended to disappear or reverse in maize. In sunflower, interactionsbetween leaf position and reproductive treatment—attributableto the local effect of grain—were detected around 20–27d after flowering. Copyright 2000 Annals of Botany Company Helianthus annuus, Zea mays, chlorophyll, light, nitrogen, photosynthesis, reproductive growth, senescence, source-sink, SPAD.     

NaCl-induced Senescence in Leaves of Rice (Oryza sativa L.) Cultivars Differing in Salinity Resistance

The influence of NaCl on senescence-related parameters (proteinand chlorophyll concentrations, membrane permeability and chlorophyllfluorescence) was investigated in young and old leaves of fiverice cultivars differing in salt resistance. NaCl hastened thenaturally-occurring senescence of rice leaves which normallyappears during leaf ontogeny: it decreased chlorophyll and proteinconcentrations and increased membrane permeability and malondialdehydesynthesis. Such an acceleration of deteriorative processes affectedall leaves in salt-sensitive cultivars while it was more markedin oldest than in youngest leaves of salt-resistant genotypes.NaCl-induced senescence also involved specific modifications,such as an increase in basal non-variable chlorophyll fluorescence(F ₀) recorded in all cultivars or a transient increase in solubleprotein concentration recorded in salt-resistant genotypes only.Alteration of membrane permeability appeared as one of the firstsymptoms of senescence in rice leaves and allowed discriminationamong cultivars after only 7 d of stress. In contrast, F _v/F _mratio (variable fluorescence/maximal fluorescence) was thesame for all cultivars during the first 18 d of stress and thuscould not be used for identifying salt-resistant rice exposedto normal light conditions. Relationships between parametersinvolved in leaf senescence are discussed in relation to salinityresistance of rice cultivars. Chlorophyll concentration; chlorophyll fluorescence; electrolyte leakage; magnesium; malondialdehyde; membrane permeability; NaCl; Oryza sativa L.; protein; rice; salinity resistance; senescence; UV absorbing substances     

Effect of Nitrogen Fertilizer on Photosynthesis of Several Varieties of Winter Wheat

The rates of gross photosynthesis of the flag leaf and the nextleaf below (second leaf) in crops of winter wheat were estimatedfrom the ¹⁴C uptake of the leaves after exposure to short pulsesof ¹⁴CO₂. The photosynthetic rates of both leaves during thegrain-filling period decreased with increase in nitrogen fertilizerbecause the intensity of photosynthetically active radiationwas less at the surface of the leaves in the dense crops withadditional nitrogen. In addition, the rate of photosynthesisat saturating light intensity was slightly decreased by nitrogen.The effects of nitrogen, in decreasing the rate of photosynthesisper unit area of leaf and in increasing the leaf-area indexof the top two leaves, were such that the photosynthetic productivityper unit area of land of the flag leaf was increased by nitrogenbut the productivity of the second leaf was unaffected. Applying180 kg N ha^–1 increased the productivity of the top twoleaves by a factor of 2.3 but increased grain yield by only1.8. The photosynthetic productivity of the second leaf duringthe grain-filling period was about half that of the flag leaf. There was no difference in photosynthetic rate per unit areaof leaves of Cappelle-Desprez and Maris Huntsman which couldaccount for the larger yield of the latter cultivar. There wasa slight indication that the leaves of the semi-dwarf cultivarsMaris Fundin and Hobbit photosynthesized faster than those ofMaris Huntsman. Triticum aestivum L., winter wheat, photosynthesis, nitrogen fertilizer     

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Single, seed-grown plants of ryegrass (Lolium perenne L. cv.Melle) were grown for 49 d from the early seedling stage ingrowth cabinets at a day/night temperature of 20/15 C, witha 12 h photoperiod, and a CO₂ concentration of either 340 or680µI 1^–1 CO₂. Following complete acclimation tothe environmental regimes, leaf and whole plant CO₂ effluxesand influxes were measured using infra-red gas analysis techniques.Elevated CO₂ increased rates of photosynthesis of young, fullyexpanded leaves by 35–46% and of whole plants by morethan 50%. For both leaves and whole plants acclimation to 680µI^–1 CO₂ reduced rates of photosynthesis in bothCO₂ regimes, compared with plants acclimated to 340µll^–1. There was no significant effect of CO₂ regime onrespiration rates of either leaves or whole plants, althoughleaves developed in elevated CO₂ exhibited generally lower ratesthan those developed in 340µI I^–1 CO₂. Initially the seedling plants in elevated CO₂ grew faster thantheir counterparts in 340µI I^–1 CO₂, but this effectquickly petered out and final plant weights differed by onlyc. 10%. Since the total area of expanded and unexpanded laminaewas unaffected by CO₂ regime, specific leaf area was persistently13–40% lower in elevated CO₂ while, similarly, root/shootratio was also reduced throughout the experiment. Elevated CO₂reduced tissue nitrogen contents of expanded leaves, but hadno effect on the nitrogen contents of unexpanded leaves, sheathsor roots. The lack of a pronounced effect of elevated CO₂ on plant growthwas primarily due to the fact that CO₂ concentration did notinfluence tiller (branch) numbers. In the absence of an effecton tiller numbers, any possible weight increment was restrictedto the c. 2.5 leaves of each tiller. The reason for the lackof an effect on tillering is not known. Key words: Lolium perenne, ryegrass, elevated CO₂, photosynthesis, respiration, growth, development     

Changing Responses of Stomata to Abscisic Acid and CO2 as Leaves and Plants Age

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changingresponses of stomata to abscisic acid and CO₂ as leaves andplants age.—J. exp. Bot. 39: 401–410. Stomatal conductances were measured in ageing leaves of Commelinacommunis L. as plants developed; stomatal responses to CO₂ andabscisic acid (ABA) in epidermal strips of C. communis takenfrom ageing leaves of developing plants and in epidermal stripsfrom the same-aged leaves (the first fully-expanded leaf) ofdeveloping plants were also monitored. Stomatal behaviour wascorrelated with parallel measurements of photosynthesis andleaf ABA concentrations. Stomatal conductance in intact leavesdecreased from a maximum of 0-9 cm s^{– 1} at full leaf expansionto zero about 30 d later when leaves were very senescent. Conductancesdeclined more slowly with age in unshaded leaves. Photosynthesisof leaf slices also declined with age from a maximum at fullleaf expansion until about 30 d later when no O₂ exchange wasdetectable. Exogenously applied ABA (0.1 mol m^{– 3}) didnot affect respiration or photosynthesis. In epidermal stripstaken from ageing leaves the widest stomatal apertures occurredabout 10 d after full leaf expansion (just before floweringbegan) and then decreased with age; this decrease was less dramaticin unshaded leaves. The inhibitory effects of ABA on stomatalopening in epidermal strips decreased as leaves aged and wasgreater in the presence of CO₂ than in its absence. When leaveswere almost fully-senescent stomata were still able to open.At this stage, guard cells remained healthy-looking with greenchloroplasts while mesophyll cells were senescing and theirchloroplasts were yellow. Similar data were obtained for stomatain epidermal strips taken from the same-aged leaves of ageingplants. The inhibitory effects of ABA on stomatal opening alsodecreased with plant age. In ageing leaves both free and conjugated ABA concentrationsremained low before increasing dramatically about 30 d afterfull leaf expansion when senescence was well advanced. Concentrationsof free and conjugated ABA remained similar to each other atall times. It is concluded that the restriction of stomatal movements inintact leaves as the leaves and plants age is due mainly toa fall in photosynthetic capacity of the leaves which affectsintracellular CO₂ levels rather than to an inherent inabilityof the stomata to function normally. Since stomatal aperturein epidermal strips declines with plant and leaf age and stomatabecome less responsive to ABA (while endogenous leaf ABA levelsremain fairly constant until leaf senescence) it is suggestedthat some signal, other than ABA, is transmitted from the leafor other parts of the plant to the stomata and influences theirbehaviour. Key words: Abscisic acid, CO₂, Commelina, leaf age, senescence, stomatal sensitivity     

Relationship between Leaf Structure and Gas Exchange in Wheat Leaves at Different Insertion Levels

Net photosynthesis rate (P_n), stomatal conductance to CO₂ andresidual conductance to CO₂ were measured in the last six leaves(the sixth or flag leaf and the preceding five leaves) of Triticumaestivum L. cv. Kolibri plants grown in Mediterranean conditions.Recently fully expanded leaves of well-watered plants were alwaysused. Measurements were made at saturating photosynthetic photonflux density, and at ambient CO₂ and O₂ levels. The specificleaf area, total organic nitrogen content, some anatomical characteristics,and other parameters, were measured on the same leaves usedfor gas exchange experiments. A progressive xeromorphic adaptation in the leaf structure wasobserved with increasing leaf insertion levels. Furthermore,mesophyll cell volume per unit leaf area (V_mes/A) decreasedby 52·6% from the first leaf to the flag leaf. Mesophyllcell area per unit leaf area also decreased, but only by 24·5%.However, nitrogen content per unit mesophyll cell volume increasedby 50·6% from the first leaf to the flag leaf. This increasecould be associated to an observed higher number of chloroplastcross-sections per mm² of mesophyll cell cross-sectional areain the flag leaf: values of 23000 in the first leaf and 48000in the flag leaf were obtained. P_n per unit leaf area remainedfairly constant at the different insertion levels: values of33·83±0·93 mg dm^–2 h^–1 and32·32±1·61 mg dm^–2 h^–1 wereobtained for the first leaf and the flag leaf, respectively.Residual conductance, however, decreased by 18·2% fromthe first leaf to the flag leaf. Stomatal conductance increasedby 41·7%. The steadiness in P_n per unit leaf area across the leaf insertionlevels could be mainly accounted for by an opposing effect betweena decrease in V_mes/A and a more closely packed arrangement ofphotosynthetic apparatus. Adaptative significance of structuralchanges with increasing leaf insertion levels and the steadinessin P_n per unit leaf area was studied. Key words: Photosynthesis, structure, wheat     

Nitrogen Deficiency Slows Leaf Development and Delays Flowering in Narrow-leafed Lupin

Effects of nitrogen (N) supply on leaf and flower developmentinLupinus angustifolius L. cv Merrit were examined in a temperature-controlledglasshouse. Low N supply (0.05 or 0.4 mM N) had little effecton leaf initiation but slowed leaf emergence on the main stemcompared with plants receiving high N supply (6.0 or 6.4 mMN), or with symbiotic N₂-fixation. Plants experiencing transientN deficiency had slower leaf emergence than plants with a continuoussupply of 6.4 mM N. Nitrogen supply did not affect the timeof floral initiation, which occurred within 4 weeks of sowing,by which time nine to ten leaves had emerged. However, the floweringof low-N plants was delayed by 68 to 220 °C d (i.e. 4–14d) even though they had fewer leaves. The effect of N deficiencyon flowering time was largely a result of slower leaf emergence. Lupinus angustifolius L.; nitrogen; leaf; flower initiation; thermal time; plastochron; phyllochron     

Species- and age-dependent sensitivity to ozone in young plants of pea,wheat and spinach: Effects on acyl lipid and pigment content and metabolism

Acyl lipids and pigments were analyzed in young plants of garden pea, spring wheat and spinach exposed to < 5 or 65 nl l^?1 ozone 12 h per day for 6 days. In one set of experiments, the plants were exposed to ¹⁴CO₂ for 2 h 3 days prior to ozone exposure. The plants responded differently to the moderately enhanced level of ozone used Spinach was not at all sensitive while in both pea and wheat, leaves of different ages differed in ozone sensitivity. In pea, ozone sensitivity increased with leaf age. In the second and third oldest leaves, the amounts of galactolipids per leaf area and the proportions of 18:3 of the total lipid extract and of phosphatidylglycerol decreased. In the second oldest leaf, ozone also caused a decreased proportion of 18:3 of monogalactosyldiacylglycerol. In the fourth oldest leaf, lipid composition and galactolipid unsaturation was unaffected, but ozone caused decreased leaf expansion resulting in increased acyl lipid content per leaf area. In both the first and second leaves of wheat, ozone fumigation caused a marked decrease in the content of monogalactosyldiacylglycerol and in the first leaf, the contents of phosphatidylcholine and phosphatidylethanolamine increased. The proportion of 18:3 in phosphatidylcholine was larger in ozone-fumigated than in control plants, while the reverse applied for phosphatidylglycerol. In the oldest sampled leaves of pea and wheat, ozone caused an increase in the radioactivity associated with β-carotene, indicating increased turnover. Thus, while spinach was unaffected, in both pea and wheat ozone caused a decrease in the proportion of chloroplast membrane lipids to non-chloroplast membrane lipids in older leaves while younger leaves were less sensitive.