Growth and Biomass Allocation, with Varying Nitrogen Availability, of Near-isogenic Pea Lines with Differing Foliage Structure

The single-gene mutation afila in pea (Pisum sativum L.) resultsin the replacement of proximal leaflets with branched tendrils,thereby reducing leaf area. This study investigated whethertheafila line could adjust biomass partitioning when exposedto varying nutrient regimes, to compensate for reduced leafarea, compared with wild-type plants. Wild-type and afila near-isogeniclines were grown in solution culture with nitrate-N added toinitially N-starved seedlings at relative addition rates (R_N)of 0.06, 0.12, 0.15 and 0.50 d^-1. The relative growth rate (R_W)of the whole plants closely matched R_Nat 0.06 and 0.12 d^-1,but higher R_Nresulted in a slightly higher growth rate. At agiven R_N, the wild-type line had lower plant nitrogen statusthan the afila line. R_Wof the roots of the afila line was lessthan R_Wof the roots of the wild-type at the three higher ratesof N supply despite a greater accumulation of N in the rootsof the afila plants. Consequently, plant nitrogen productivity(growth rate per unit nitrogen) was lower for afila. Dry matterallocation was strongly influenced by nitrogen status, but nodifferences in shoot–root dry matter allocation were foundbetween wild-type and afila with the same plant N status. Theseresults imply that decreased leaf area as a result of the single-genemutation afila affects dry matter allocation, but only accordingto its effect on the nitrogen status. Copyright 2000 Annalsof Botany Company Pisum sativum, pea, nitrogen limitation, growth, shoot–root allocation, relative growth rate, nitrogen productivity, isolines     

Variation in the Photoperiodic Control of Flowering of Two Cultivars of Antirrhinum majus L.

The susceptibility to photoperiodic induction of an early (PinkIce) and a late (Orchid Rocket) flowering variety of Antirrhinummajus was investigated. At various times during developmentplants of both varieties were subjected to 1, 2, 3 or 4 long-daycycles. The early variety became increasingly more responsiveto long-days while the late variety showed no positive responsethroughout the course of the experiment. One hour light-breaksgiven midway through the 16-h dark period evoked a decreasein the leaf number of Pink Ice plants. The leaf areas of bothvarieties increased to levels equivalent to long-day grown plants.The net CO₂ uptake of plants grown in light-breaks increasedin proportion to the increase in leaf area. However the specificuptake of CO₂ per unit area was similar for light-break andshort-day grown plants but was substantially lower than thelong-day plants. Suggestions are made to explain the differencesin photoperiodic response of the two varieties in terms of variationin assimilation rate.     

Dependence of Yields of Wheat Varieties on their Leaf Area Durations

In a field experiment three wheat varieties: autumn-sown CappelleDesprez, Prestige, both autumn-sown and spring-sown, and spring-sownJufy I, each supplied with 0.5 or 1.0 cwt nitrogen/acre (6₃or 126 kg/ha), had grain yields nearly proportional to theirLeaf Area Durations (D) during grain development. Squarehead'sMaster had a smaller grain yield relative to its total D, butwith 0.5 cwt N/acre the ratio of its grain yield to D of partsabove the flag leaf node (D_F) was similar to the other varieties.The difference in this ratio between varieties was less whenD_Fwas calculated from anthesis than from ear emergence to ripening.Squarehead’s Master with 1.0 cwt N/acre had a smallerratio of grain yield to D_F than other treatments, implying lessgrain per ear relative to D_Y per shoot, perhaps because of lodging,or because factors limiting size of ears restricted their abilityto accept all the assimilate the shoots could produce with thisamount of N.     

Photosynthetic Responses of Different Varieties of Wheat to High Temperature: I. EFFECT OF GROWTH TEMPERATURE ON DEVELOPMENT AND PHOTOSYNTHETIC PERFORMANCE

Both fast and slow chlorophyll fluorescence kinetics were usedto assess the differential heat sensitivity of ten wheat (Triticumaestivum L.) varieties commonly grown in northern, temperateor (sub-) tropical climate zones. No consistent differencesbetween the groups were found. Studies on the role of growthtemperature in modulating the response of photosynthesis toheat stress were carried out on two of the varieties, APU (Finnish)and K65 (Indian), the former being more sensitive to heat stress.Growth and development of these varieties were similar underboth cool (13 °C day, 10 °C night) and warm (30 °Cday, 25 °C night), regimes. The cool-grown plants exhibitedhigher rates of net CO₂ exchange than the warm-grown plantswhen expressed on a chlorophyll basis and, in both regimes,photosynthesis declined with age prior to reduction in chlorophyllcontent. Net CO₂ exchange in leaves of K65 showed greater sensitivityto short-term heat stress than APU when plants were grown underthe cool regime. Plants grown under the warm regime exhibitedan upward shift in the optimum temperature for net CO₂ exchangein both varieties, with K65 showing somewhat superior performanceat high temperature compared with APU. Stomatal aperture wasessentially unaffected by assay temperature during these measurements. Key words: CO₂ exchange, growth temperature, Triticum aestivum     

Limitations to net photosynthesis as affected by nitrogen status in jack pine (Pinus banksiana Lamb.) seedlings

Relative limitations of nitrogen (N) status on the processescontributing to photosynthetic rate (A) were investigated. Jackpine {Pinus banksiana Lamb.) seedlings from seeds grown in sandculture were supplied with four different N treatments for 6weeks, which resulted in a needle N content ranging from 50–85mmol m^–2 (14–32 mg g^–1 dry weight). Leaf gasexchange at varying CO₂ levels was measured and limitationson A₃₅₀ (A at ambient CO₂ level) caused by finite, limitingcarboxylation efficiency (c.e.), maximum A (A_max)and stomatalconductance were estimated from an analysis of the responseof A to internal CO₂ concentration. Although c.e. and A_max decreasedlinearly with the decline in needle N, the magnitudes of theirchanges relative to A₃₅₀ differed. A_max varied with A₃₅₀ andalways exceeded A₃₅₀ by 37–38% c.e., however, declinedfaster than A₃₅₀, as needle N level decreased. Consequently,relative limitation on A₃₅₀ caused by inefficient A_max remainedconstant, but limitations caused by c.e. increased by 10–15%at low N levels. In contrast, the limitation by stomatal conductancedeclined initially, but remained stable when N content droppedbelow 75 mmol m^–2. The results suggest: (1) a decreasein biochemical capacity, but not stomatal conductance, contributedto the reduction of A₃₅₀ induced by N-deficiency in jack pineseedlings; and (2) the capacity of carboxylation appeared tobe impaired more than that of electron transport and/or photophosphorylationand its reduction may be the major reason for the reductionin A₃₅₀. Key words: A–C_i analysis, carboxylation efficiency, electron transport, nitrogen deficiency, stomatal conductance     

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.     

Effectiveness of Lotus Root Nodules: III. EFFECT OF COMBINED NITROGEN ON NODULE EFFECTIVENESS AND FLAVOLAN SYNTHESIS IN PLANT ROOTS

When grown in a nutrient solution containing combined nitrogen(NH₄NO₃), Lotus pedunculatus and L. tenuis seedlings inoculatedwith a fast-growing strain of Rhizoblum (NZP2037) did neitherdevelop root nodules nor develop flavolans in their roots. Incontrast, the roots of nodulated seedlings growing in a nitrogen-freenutrient solution contained flavolans. Flavolan synthesis coincidedwith root nodule development on these plants. When added as a single dose, high concentrations of NH₄NO₃ (5and 10 mg N per plant) stimulated the growth of L. pedunculatusplants but suppressed nodulation and nitrogen fixation. In contrastthe continued supply of a low concentration of NH₄NO₃ (1?0 mgN d^–1 per plant) stimulated nitrogen fixation by up to500%. This large increase in nitrogen fixation was associatedwith a large increase in nodule fresh weight per plant, a doublingof nodule nitrogenase activity, and a lowering of the flavolancontent of the plant roots. The close relationship between nitrogendeficiency, nodule development, and flavolan synthesis in L.pedunculatus meant that it was not possible (by nitrogen pretreatmentof plants) to alter the ineffective nodule response of a Rhizobiumstrain (NZP2213) sensitive to the flavolan present in the rootsof this plant.     

Above- and Below-ground Production of Young Scots Pine (Pinus sylvestris L.) Trees after Three Years of Growth in the Field under Elevated CO2

Scots pine (Pinus sylvestris L.) seedlings were grown for 3years in the ground in open top chambers and exposed to twoconcentrations of atmospheric CO₂(ambient or ambient + 400 µmol mol^-1) without addition of nutrients and water. Biomassproduction (above-ground and below-ground) and allocation, aswell as canopy structure and tissue nitrogen concentrationsand contents, were examined by destructive harvest after 3 years.Elevated CO₂increased total biomass production by 55%, reducedneedle area and needle mass as indicated, respectively, by lowerleaf area ratio and leaf mass ratio. A relatively smaller totalneedle area was produced in relation to fine roots under elevatedCO₂. The proportion of dry matter in roots was increased byelevated CO₂, as indicated by increased root-to-shoot ratioand root mass ratio. Within the root system, there was a significantshift in the allocation towards fine roots. Root litter constituteda much higher fraction of fine roots in trees grown in the elevatedCO₂than in those grown in ambient CO₂. Growth at elevated CO₂causeda significant decline in nitrogen concentration only in theneedles, while nitrogen content significantly increased in branchesand fine roots (with diameter less than 1 mm). There were nochanges in crown structure (branch number and needle area distribution).Based upon measurements of growth made throughout the 3 years,the greatest increase in biomass under elevated CO₂took placemainly at the beginning of the experiment, when trees grownin elevated CO₂had higher relative growth rates than those grownunder ambient CO₂; these differences disappeared with time.Symptoms of acclimation of trees to growth in the elevated CO₂treatmentwere observed and are discussed. Copyright 2000 Annals of BotanyCompany Elevated CO₂, Pinus sylvestris, biomass production, allocation, fine roots, root litter, crown structure, nitrogen, C/N ratio     

Leaf Properties Related to the Photosynthetic Response to Drought in Upland and Lowland Rice Varieties

Four upland and two lowland varieties were grown on floodedand dry soil in pots in a glasshouse. Photosynthetic rate (P),transpiration rate (T), and water content (W) of the secondexpanded leaf from the top of the main stem were measured undercontrolled aeration and illumination in a leaf chamber in thelaboratory, together with leaf area (La), dry matter content(DM), nitrogen content (N), stomatal frequency (Sf), and totalvessel cross-sectional area at the base of the leaf blade (Va).P/La was positively related to T/La and Sf/La among six varietieswhen they were grown on flooded soil. IR 8, a semidwarf indicalowland variety, showed the highest P/La with the highest Sf/Laand T/La. When grown on dry soil P/La was positively correlatedwith W/DM, the latter being negatively related to T/Va. Twoupland varieties, African Moroberekan and Brazilian IAC 1246,showed the highest P/La on dry soil, keeping a higher W/DM witha lower T/La and a lower T/Va. Daytime leaf diffusive conductance(l/r_L) and leaf water potential (_L) measured on the same orthe same stage leaf in the glasshouse were positively correlatedwith the W/DM measured in the laboratory among varieties grownon dry soil. Simultaneous observation of P, T and W in the laboratoryindicated nonstomatal reduction in P/La due to leaf water deficitin sensitive varieties, although these varieties also showeda markedly lower daytime l/r_L in the glasshouse as comparedwith resistant varieties. Oryza sativa L., rice, drought resistance, photosynthesis, transpiration, water deficit, stomatal frequency, vessel size     

Growth Analysis of Solution Culture-grown Winter Rye, Wheat and Triticale at Different Relative Rates of Nitrogen Supply

At low nitrogen (N) supply, it is well known that rye has ahigher biomass production than wheat. This study investigateswhether these species differences can be explained by differencesin dry matter and nitrogen partitioning, specific leaf area,specific root length and net assimilation rate, which determineboth N acquisition and carbon assimilation during vegetativegrowth. Winter rye (Secale cereale L.), wheat (Triticum aestivumL.) and triticale (X Triticosecale) were grown in solution cultureat relative addition rates (R_N) of nitrate-N supply rangingfrom 0.03–0.18 d^-1and at non-limiting N supply under controlledconditions. The relative growth rate (R_W) was closely equalto R_Nin the range 0.03–0.15 d^-1. The maximalR_W at non-limitingnitrate nutrition was approx. 0.18 d^-1. The biomass allocationto the roots showed a considerable plasticity but did not differbetween species. There were no interspecific differences ineither net assimilation rate or specific leaf area. Higher accumulationof N in the plant, despite the same relative growth rate atnon-limiting N supplies, suggests that rye has a greater abilityto accumulate reserves of nitrogen. Rye had a higher specificroot length over a wide range of sub-optimal N rates than wheat,especially at extreme N deficiency (R_N=0.03–0.06 d^-1).Triticale had a similar specific root length as that of wheatbut had the ability to accumulate N to the same amount as ryeunder conditions of free N access. It is concluded that thebetter adaptation of rye to low N availability compared to wheatis related to higher specific root length in rye. Additionally,the greater ability to accumulate nitrogen under conditionsof free N access for rye and triticale compared to wheat maybe useful for subsequent N utilization during plant growth.In general, species differences are explained by growth componentsresponsible for nitrogen acquisition rather than carbon assimilation.Copyright 1999 Annals of Botany Company Growth analysis, nitrogen, nitrogen productivity, partitioning, specific root length, Secale cereale L.,Triticum aestivum L., X Triticosecale, winter rye, winter wheat, winter triticale.     

Effects of elevated carbon dioxide on three grass species from montane pasture II. Nutrient uptake, allocation and efficiency of use

Agrostis capillaris L.⁴ Festuca vivipara L. and Poa alpinaL.were grown in outdoor open-top chambers at either ambient (340µmol mol^–1) or elevated (680 µmol^–1)CO² for periods from 79 to 189 d. Under these conditions thereis increased growth of A. caplllarls and P. alpina, but reducedgrowth of F. vivipara. Nutrient use efficiency, nutrient productivity(total plant dry weight gain per unit of nutrient) and nutrientallocation of all three grass species were measured in an attemptto understand their individual growth responses further andto determine whether altered nutrient-use efficiencies and productivitiesenable plants exposed to an elevated atmospheric CO₂ environmentto overcome potential limitations to growth imposed by soilfertility. Total uptake of nutrients was, in general, greater in plantsof A. capillaris and P. alpina (with the exception of N andK in the latter) when grown at 680 µmol mol^–1 CO₂.In F. vivipara, however, uptake was considerably reduced inplants grown at the higher CO₂ concentration. Overall, a doubling of atmospheric CO₂ concentration had littleeffect on the nutrient use efficiency or productivity of A.capillaris. Reductions in tissue nutrient content resulted fromincreased plant growth and not altered nutrient use efficiency.In P. alpina, potassium, magnesium and calcium productivitieswere significantly reduced and photosynthetic nitrogen and phosphorususe efficiencies were doubled at elevated CO₂ with respect toplants grown at ambient CO₂ F. vivipara grown for 189 d showedthe most marked changes in nutrient use efficiency and nutrientproductivity (on an extracted dry weight basis) when grown atelevated CO₂, F. vivipara grown at elevated CO₂ however, showedlarge increases in the ratio of non-structural carbohydrateto nitrogen content of leaves and reproductive tissues, indicatinga substantial imbalance between the production and utilizationof assimilate. Key words: Nutrient, allocation, nutrient use efficiency, grasses, nutrient productivity, elevated CO₂, cliniate change     

Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes

The influence of elevated CO₂ concentration (670 ppm) on thestructure, distribution, and patterning of stomata in Tradescantialeaves was studied by making comparisons with plants grown atambient CO₂. Extra subsidiary cells, beyond the normal complementof four per stoma, were associated with nearly half the stomatalcomplexes on leaves grown in elevated CO₂. The extra cells sharedcharacteristics, such as pigmentation and expansion, with thetypical subsidiary cells. The position and shape of the extrasubsidiary cells in face view differed in the green and purplevarieties of Tradescantia. Substomatal cavities of complexeswith extra subsidiary cells appeared larger than those foundin control leaves. Stomatal frequency expressed on the basisof leaf area did not differ from the control. Stomatal frequencybased on cell counts (stomatal index) was greater in leavesgrown in CO₂-enriched air when all subsidiary cells were countedas part of the stomatal complex. This difference was eliminatedwhen subsidiary cells were included in the count of epidermalcells, thereby evaluating the frequency of guard cell pairs.The extra subsidiary cells were, therefore, recruited from theepidermal cell population during development. Stomatal frequencyin plants grown at elevated temperature (29 C) was not significantlydifferent from that of the control (24 C). The linear aggregationsof stomata were similar in plants grown in ambient and elevatedCO₂. Since enriched CO₂ had no effect on the structure or patterningof guard cells, but resulted in the formation of additionalsubsidiary cells, it is likely that separate and independentevents pattern the two cell types. Plants grown at enrichedCO₂ levels had significantly greater internode lengths, butleaf area and the time interval between the appearance of successiveleaves were similar to that of control plants. Porometric measurementsrevealed that stomatal conductance of plants grown under elevatedCO₂ was lower than that of control leaves and those grown atelevated temperature. Tradescantia was capable of regulatingstomatal conductance in response to elevated CO₂ without changingthe relative number of stomata present on the leaf. Key words: Elevated CO₂, stomata, subsidiary cells, patterning     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Effect of Low Relative Humidity on {delta}13C Value in Two C3 Grasses and in Panicum milioides, a C3-C4 Intermediate Species

When grown under conditions of low relative humidity, the C₃–C₄intermediate Panicum milioides, as well as the C₃ grasses Triticumaestivum and Poa pratense, exhibited ¹³C values which were upto 2–7%o less negative than the ¹³C values of the correspondingplants grown at high relative humidity. At both humidity levels,there was no evidence of a substantial contribution of phosphoenolpyruvatecarboxylase to carbon gain in Panicum milioides     

Fractionation of Nitrogen Isotopes during the Uptake and Assimilation of Ammonia by Plants

Two varieties (Nihonbare and Koshihikari) of rice plants (Oryzasativa L.) were grown hydro-ponically with two levels (20 and100 mg N liter ^–1) of ammonia. Variations in levels ofnatural abundance of ¹⁵N (¹⁵N) were analyzed in the ammoniaand organic nitrogen of shoots and roots, as well as in theammonia in the culture solution. There was substantial fractionationof nitrogen isotopes during the uptake of ammonia. When plantsabsorbed a large proportion of ammonia from a solution witha low concentration, less negative ¹⁵N values in plants andhigh positive ¹⁵N values in the ammonia remaining in solutionwere observed. The reverse was found when a smaller fractionof ammonia was absorbed from a solution with a higher concentrationof ammonia. The ^l5N values of ammonia in shoots and roots werehigher than in the respective constituent organic nitrogen,suggesting the fractionation of nitrogen isotopes during theassimilation of ammonia. Wild-type and mutant cells of the cyanobacterium(blue-green alga) Synechococcus PCC 7942 were grown in nitrate-or ammonia-containing medium as the source of nitrogen. Fractionationof nitrogen isotopes during the uptake of nitrate was limited,whereas that during the uptake of ammonia was considerable. ¹ In this report, the term ammonia refers indiscriminately toboth NH₃ or NH₄⁺. (Received June 13, 1991; Accepted September 12, 1991)     

Compensatory Roles of Nitrogen Uptake and Photosynthetic N-use Efficiency in Determining Plant Growth Response to Elevated CO2: Evaluation Using a Functional Balance Model

We used a modified functional balance (FB) model to predictgrowth response of Helianthus annuus L. to elevated CO₂. Modelpredictions were evaluated against measurements obtained twiceduring the experiment. There was a good agreement between modelpredictions of relative growth rate (RGR) responses to elevatedCO₂and observations, particularly at the second harvest. Themodel was then used to compare the relative effects of biomassallocation to roots, nitrogen (N) uptake and photosyntheticN-use efficiency (PNUE) in determining plant growth responseto elevated CO₂. The model predicted that a rather substantialincrease in biomass allocation to root growth had little effecton whole plant growth response to elevated CO₂, suggesting thatplasticity in root allocation is relatively unimportant in determininggrowth response. Average N uptake rate at elevated comparedto ambient CO₂was decreased by 21–29%. In contrast, elevatedCO₂increased PNUE by approx. 50% due to a corresponding risein the CO₂-saturation factor for carboxylation at elevated CO₂.The model predicted that the decreased N uptake rate at elevatedCO₂lowered RGR modestly, but this effect was counterbalancedby an increase in PNUE resulting in a positive CO₂effect ongrowth. Increased PNUE may also explain why in many experimentselevated CO₂enhances biomass accumulation despite a significantdrop in tissue nitrogen concentration. The formulation of theFB model as presented here successfully predicted plant growthresponses to elevated CO₂. It also proved effective in resolvingwhich plant properties had the greatest leverage on such responses.Copyright 2000 Annals of Botany Company Elevated CO₂, functional balance model, Helianthus annuus L., N uptake, photosynthetic nitrogen use efficiency, root:shoot ratio     

Mitochondrial Complementation and Grain Yield in Hybrid Wheat

ADP/O ratios, cytochrome c oxidase and adenosine triphosphatasehave been measured in mitochondria and mixtures of mitochondriaisolated from two day-old shoots of wheat of known F₁ hybridgrain yield performance. Mixtures of mitochondria from two varieties,Peko and Cappelle-Desprez, which have considerable F₁ hybridyield heterosis, showed a significantly increased ADP/O ratioover the mean value for mitochondria from the varieties assayedindividually, i.e. these varieties showed ‘mitochondrialcomplementation’. No mitochondrial complementation wasdetected for cytochrome c oxidase or adenosine triphosphatase.In other mitochondrial mixtures no complementation in ADP/Oratios were found even when the varieties showed F₁ hybrid yieldheterosis. Mitochondrial ADP/O ratios were studied in six varietiesindividually and in mixtures. In only one mixture was any significantcomplementation detected. However, when all the results wereconsidered together, mitochondrial complementation was significantlycorrelated with F₁ hybrid grain yield heterosis when the plantswere grown at a low seed density but not at a high seed density.New hypotheses are offered to account for mitochondrial complementationand its statistical relationship with yield heterosis.     

A comparison between methods used to control nutrient supply

Experimental methods to supply nutrients to culture solutionsin order quantitatively to control plant nutrition are compared.In experiments with tomato and birch plants, for which the dataare available in databases (Ingestad et al., 1994a, b), thenutrients were supplied at constant relative addition rates(R_A over sufficiently long periods of time to achieve acclimatedplants and reliable measurements of plant responses. The plantswere maintained under steady-state conditions, i.e. the internalnutrient concentrations (c₁) remained constant, as a resultof a numerical equality between the relative uptake rate (R_U)and the relative growth rate (R_G). These results are comparedto experiments with pea plants (Macduff et al., 1993). In oneseries (a), R_A was applied, but without strict control of internalsteady-state, and in the other series (b), the external concentration(c_e) was maintained constant. With limiting nitrogen, in bothseries, there was a substantial deviation from equality betweenR_U and R_G. In (a), c_I changed during the experimental periodand the purpose of the R_A approach was lost. In (b), a constantc_e had little effect on nitrogen uptake and plant growth. Atthe three highest concentrations, steady-states were obtainedat non-limiting uptake rates. At the lowest concentration, theuptake rate of nitrogen was about the same, but there was adecrease of R_a, which apparently was not caused by reduced uptake.Clear-cut relationships can not therefore be established betweentreatment variables and plant responses and the conclusionsreached by Macduff et al. (1993) have little support in theirexperimental results. This indicates an urgent need to updateboth theories and experimental methods together: in particular,it is important to identify the system under investigation andto distinguish between control of the medium and control ofthe plant. Key words: Experimental control, external nutrient concentration, non-limiting and limiting nutrient supply, relative addition rate, relative uptake rate, relative growth rate, steady-state     

Influence of nitrogen on growth and photosynthesis of a C3 cereal, Oryza sativa, infected with the root hemiparasite Striga hermonthica

Striga hermonthica is a root hemiparasitic angiosperm nativeto the African semi-arid tropics. It is a major weed of C₄ cerealsbut locally it is also an important weed of the C₃ plant, rice[Oryza sativa). Infected rice plants produced 17% and 42% ofthe total biomass of uninfected plants when grown at two differentammonium nitrate concentrations, 1 and 3 mol m^–3, respectively.S. hermonthica prevented grain production at both concentrationsof nitrogen. At the lower concentration no heads were produced.At the higher concentration head weight was only 6% of uninfectedcontrols. S. hermonthica also altered the partitioning of drymatter between plant parts, such that shoot growth was reducedto a greater extent than root growth. As a consequence the root-to-shootratio of infected plants was approximately five times greaterthan that of uninfected control plants. Light saturated ratesof photosynthesis In infected plants were 56% and 70% of thoseof uninfected controls, at low and high nitrogen, respectively.Infection also led to lower values of stomatal conductance althoughthe substom-atal CO₂ concentration was unaffected. Analysisof the response of photosynthesis to substomatal CO₂ concentration(A/C_I curves) demonstrated that lower rates of photosynthesiscould not be solely attributed to lower stomatal conductances.Lower initial slopes and asymptotic rates suggest that bothcarboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussedwith respect to the influence of S. hermonthica on the growthand photosynthesis of C₄ hosts, where in contrast to the situationwith rice, nitrogen feeding results in a marked alleviationof the effects of the parasite on the host. Key words: Rice, Striga, growth, photosynthesis, nitrogen     

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.