The Supply of Air to Leaves in Assimilation Chambers

Factors affecting the supply of air to leaves enclosed in assimilationchambers have been studied using models and leaves of appleand plum. The rate of CO₂ assimilation of a leaf does not increaselinearly with increasing rate of air flow. Explanations of thisand other observations are put forward in terms of the boundary-layerconcept and diffusion paths. Comparisons are made with free-airconditions, and it is concluded that air should be suppliedat the same rate (ml/min) for leaves of different size in thesame chamber. Three cup designs were also studied. A methodis described for making valid comparisons of assimilation ratesper unit area by extrapolation to an infinitely high rate ofair flow. Measurements of different sized leaves with chambersand cups can be compared in this way. Rates expressed per unitarea can be correctly compared only if the resistance to diffusionoutside the leaf is negligible, or under special conditions.Assimilation rates measured with rates of air flow giving CO₂availability equivalent to that occurring in static free airshould be expressed per unit perimeter. Methods of correctionof assimilation rates for the depletion of carbon dioxide fromthe air stream are considered.     

The Role of Leaves in the Perception of Vernalizing Temperatures in Sugar Beet

Annual and biennial sugar beet varieties require long days toinduce flowering but the biennial genotypes additionally requirevernalization. Previous research has suggested that the inabilityof non-vernalized biennial plants to flower can be explainedby a lack of competence of the leaves to respond to long days.In this study defoliation experiments were used to investigatewhich leaves could perceive long daylengths and, in particular,whether leaves initiated from a non-vernalized shoot apicalmeristem could perceive vernalizing temperatures and producea floral stimulus in long days. Annual and vernalized biennialplants flowered if young leaves (i.e. those formed during orafter vernalization) were kept on the plants, but they did notflower if only older expanded leaves (including those expandedprior to vernalization) were present. No evidence was obtainedto indicate that the older leaves contained inhibitors of floweringand it seems most likely that there is a decline in responsivenessto daylength with increasing leaf age. Exposure to vernalizingtemperatures accelerated flowering of the annual and was essentialfor flowering of the biennial. The presence of a single leafinitiated, but not expanded, prior to the transfer of biennialplants to vernalizing temperatures was sufficient to induceflowering. This indicates that expanding leaves do not needto be initiated from a vernalized apical meristem to becomecompetent to produce a floral stimulus in long days. Key words: Beta vulgaris L., sugar beet, vernalization, flowering     

Enzymes of Ammonia Assimilation, Photosynthesis, and Respiration in Alfalfa Leaves of Different Ages

The activities of enzymes involved in ammonia metabolism ferredoxin-dependent glutamate synthase (Fd-GOGAT), glutamine synthetase (GS) and glutamate dehydrogenase (GDH), the rates of photosynthetic oxygen evolution, dark respiration, and the activity of RuBP carboxylase (RuBPC) were determined in alfalfa (Medicago sativa L.) leaves taken from the apex (apical leaves), from the second to the fourth internode (mature leaves) and from the bottom of the canopy (basal leaves). Photosynthetic rate and the activities of RuBPC, GS and Fd-GOGAT showed their maximum in the mature leaves. The respiration rate together with amino acid and ammonium contents decreased with leaf age, whereas the opposite was true for GDH activity. Basal leaves still maintained substantial levels of chlorophylls, GS and Fd-GOGAT activities and oxygen evolution rate, thus suggesting that photosynthesis has some role in the reassimilation of the nitrogen liberated during protein degradation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO₂ assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO₂ assimilation and leaf dry matter accumulation. Chamber-grown `Amsoy 71' and `Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO₂ assimilation rates.

Rate of accumulation of leaf dry matter and rate of export both increased as CO₂ assimilation rate increased in each cultivar.

Starch concentrations were greater in Amsoy 71 than in Wells at all CO₂ assimilation rates. At low CO₂ assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH₂O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO₂ assimilation rate. The low leaf starch concentration in Wells at low CO₂ assimilation rates may have limited export by limiting carbon from starch remobilization.

Both cultivars exhibited positive correlations between CO₂ assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.

     

The Effect of Manganese on the Assimilation and Respiration Rate of Isolated Rooted Leaves

The effect of manganese on carbon assimilation, respiration,and translocation has been studied using isolated rooted potatoleaves and small potato plants. Methods are described for therooting and culture of the leaves and plants. It was found thatnormal potato leaves rooted readily when treated with -naphtha-leneaceticacid (2 p.p.m.), but that very few of the manganese-deficientleaves produced roots, the critical level being about 15 p.p.m.manganese on a dry weight basis. The growth of isolated deficientleaves was also much less than that of control leaves, but inno case did characteristic manganese-deficiency symptoms develop,although the manganese level had fallen below that of leaveswhich showed symptoms when attached to the plant. A marked differencein net assimilation rate was found between leaves which hada high or low manganese content at the time of rooting. Theaddition of manganese after rooting to low manganese leavesdid not, however, cause an increase in assimilation rate, althoughthe manganese content of the leaves had been raised to thatof the control leaves. Manganese was shown to have only a smalleffect on respiration, higher respiration rate being consistentlyassociated with a higher manganese content; the addition ofmanganese to ‘deficient’ leaves did not cause anyincrease in respiration. No effect of manganese on translocationwas detected.     

Leaf Temperatures in a Gas Exchange Chamber and in the Open Air

Leaf temperatures in a Koch fully climatized gas-exchange chamberas designed by Siemens and in a similarly equipped open-airreference were measured with horizontally and vertically insertedthermocouples on Nerium oleander L. On a sunny day with onlylittle air movement and an average air temperature of 20.4 °C,leaf over-temperatures in the gas-exchange chamber were loweron average by 2.2 K. The extent of reduction of over-temperaturein the chamber is determined by the reduced global radiationin the chamber and the differences of wind velocities in chamberand reference. Differences in the ventilation intensity in thechamber have no demonstrable influence on the leaf over-temperatures.The over-temperatures of the reference leaves, on the otherhand, depend to a large degree on air velocity. The changedradiation and air flow conditions in the chamber as comparedwith open-air conditions have consequences for the physiologicalreactions of the enclosed plant and must be taken into accountwhen comparing results from gas-exchange measurements with open-airconditions. For further improvements of gas-exchange measurementequipment, air flow conditions and radiation quantity and qualitymight be starting points     

Effects of Nitrate Withdrawal and Resupply on the Assimilation of Nitrate in Leaves of Tomato

Nitrate reduction in leaves of tomato occurred at the same ratein plants grown in 8.0 mol m^–3 nitrate as in plants grownin 2.0 mol m^–3 nitrate, but at a much slower rate in plantsgrown in 0.1 mol m^–3 nitrate. However, the plants grownin 8.0 mol m^–3 nitrate had a larger leaf system than theplants grown in 2.0 mol m^–3 nitrate, and so the totalcapacity to assimilate nitrate was greater in the plants grownin the higher concentration. It was shown that plants grownin 8.0 mol m^–3 nitrate were better buffered against nitratewithdrawal than plants grown in 2.0 mol m^–3 nitrate asthe rate of nitrate reduction declined more slowly when plantswere transferred to 0.1 mol m^–3 nitrate from the higherconcentration than from the lower concentration. Furthermore,leaf expansion continued in the plants transferred from thehigher concentration, whereas it ceased abruptly in the plantstransferred from the lower concentration. It was concluded thatboth continuing expansion and continuing nitrate reduction wereaccompanied, and possibly caused by, a release of nitrate fromstorage pools in the lower part of the stem or the roots. Duringwithdrawal of nitrate the leaves were shown to maintain potentialactivity of the enzyme nitrate reductase although there wasno nitrate to be reduced. When nitrate was resupplied it couldbe reduced very quickly and reduction in the leaves was seento increase within 5 h of resupply. By 3 d after resupply furtherenzyme activity had been induced. Key words: Lycopersicon esculentum Mill, nitrate assimilation, nitrate reductase activity, nitrate withdrawal     

14CO2 Assimilation and 14C-photosynthate Translocation in Different Leaves of Sunflower

Chlorophyll and nitrogen contents were highest in leaves of middle position, similarly as photosynthetic efficiency represented by ¹⁴C fixation (maxima in leaf 5 from the top). All the leaves lost ¹⁴C after 2 weeks of ¹⁴CO₂ exposure. However, the reduction in radioactivity was less in young upper leaves than in the mature lower leaves. Leaves exported ¹⁴C-photosynthates to stem both above and below the exposed leaf. Very little radioactivity was recovered from the seeds of plants in which only first or second leaves were exposed to ¹⁴CO₂ implying thereby that the carbon contribution of first two leaves to seed filling was negligible. The contribution of leaves to seed filling increased with the leaf position up to the sixth leaf from the top and after the seventh leaf their contribution to seed filling declined gradually.     

Effect of Chilling on Carbon Assimilation,Enzyme Activation,and Photosynthetic Electron Transport in the Absence of Photoinhibition in Maize Leaves

The relationships between electron transport and photosynthetic carbon metabolism were measured in maize (Zea mays L.) leaves following exposure to suboptimal temperatures. The quantum efficiency for electron transport in unchilled leaves was similar to that previously observed in C3 plants, although maize has two types of chloroplasts, mesophyll and bundle sheath, with PSII being largely absent from the latter. The index of noncyclic electron transport was proportional to the CO2 assimilation rate. Chilled leaves showed decreased rates of CO2 assimilation relative to unchilled leaves, but the integral relationships between the quantum efficiency for electron transport or the index of noncyclic electron transport and CO2 fixation were unchanged and there was no photoinhibition. The maximum catalytic activities of the Benson-Calvin cycle enzymes, fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase, were decreased following chilling, but activation was unaffected. Measurements of thiol-regulated enzymes, particularly NADP-malate dehydrogenase, indicated that chilling induced changes in the stromal redox state so that reducing equivalents were more plentiful. We conclude that chilling produces a decrease in photosynthetic capacity without changing the internal operational, regulatory or stoichiometric relationships between photosynthetic electron transport and carbon assimilation. The enzymes of carbon assimilation are particularly sensitive to chilling, but enhanced activation may compensate for decreases in maximal catalytic activity.     

5-硝基愈创木酚钠对韭菜叶片硝酸还原同化的影响

以3年生‘大金钩韭菜’为供试材料,采用10μmol·L-1的5-硝基愈创木酚钠(5-NGS)进行叶面喷施处理,分析对韭菜叶片硝酸盐含量、氮代谢关键酶活性、氨基酸组分、营养品质及叶绿素荧光动力学活性的影响,以探索5-NGS对促进NO3-还原同化的相关机制。结果表明:(1)5-NGS处理使韭菜叶片硝酸还原酶(NR)、谷氨酰胺合成酶(GS)、谷氨酸草酰乙酸转氨酶(GOT)和谷氨酸丙酮酸转氨酶(GPT)等氮代谢关键酶活性分别比对照显著提高58.7%、26.5%、179.4%、131.3%,硝酸盐含量显著降低26.1%。(2)5-NGS处理韭菜叶片的可溶性蛋白、维生素C分别显著提高49.5%、25.0%,氨基酸总量及其组分含量也显著增加,但可溶性糖含量显著降低。(3)5-NGS处理使韭菜叶片的PSⅡ电子传递速率显著增加11.3%,其生物量、叶面积、叶绿素含量等也显著增加。研究认为,韭菜叶面喷施5-NGS使得末端的蛋白质合成活性增强,拉动了游离氨基酸向蛋白质的转化,同时调动了转氨作用的积极协同配合,还可能调动碳同化产物的协同配合,促进了前端硝态氮转化为谷氨酸和衍生氨基酸,从而降低了韭菜叶片的NO3-累积,并改善营养品质,促进了植株生长。     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.     

Photosynthetic Units and Carbon Assimilation in Leaves of Grain Sorghum under Different Light Intensities

The Cyt f and P700 contents in leaves of three Sorghum, varietieswere measured, in relation to their carbon assimilation, underdifferent light intensities during growth. At the maximum irradiationused (1,800 µE m^–2 s^–1) the ratio of P700to Cyt f was close to unity, whereas under low irradiation (450µE m^–2 s^–1) the ratio of P700 to Cyt f rangedfrom two to three. A strikingly positive correlation existedbetween the P700 contents of the leaves and their rates of carbondioxide fixation, dry matter production and Cyt f contents,only when the plants were grown under high light intensities.The P700 content of the leaves in plants grown under low irradiationwas unrelated to the contents of Cyt f. Thus, at a high lightintensity there is a close relationship between the Cyt f andP700 levels, but at low intensities the amounts of electroncarriers and the reaction centre are independent. (Received March 7, 1983; Accepted August 24, 1983)     

Dependence of Net Assimilation Rate on Root Growth of Isolated Leaves

The phenomenon of inhibition of assimilation by carbohydrateaccumulation has been reinvestigated using single rooted leavesof dwarf bean. Such a system has the advantage that assimilatingarea remains constant and carbohydrate is translocated to asingle sink-the root system. The net assimilation rate of thesystem did not vary with season and was small compared withintact plants in the summer, suggesting that an internal factorcontrols assimilation rate. Evidence is given that this factoris the rate of translocation of carbohydrate from source tosink (from lamina to root) which in turn depends on growth-rateof the root system. The evidence came from experiments in whichroot growth was stimulated by increasing vessel size, by preliminarytreatment with IAA or by raising the root temperature, or wasretarded by kinetin treatment. In experiments at glasshouse temperature, lamina dry matterincreased by 0.75 mg. per cm.² per week. The maximum valuesattained depended on the time of the experiment. In August itwas greater than 7.0 mg. per cm.¹, but there was a seasonaltrend. In experiments with roots at 24?C, carbohydrate accumulatedin the lamina more slowly than at lower temperatures.     

The Effect of High Temperatures on the Photosynthetic Activity of Intact Barley Leaves at Low and High Irradiance

Light curves of CO₂ fixation by barley seedling leaves preliminarily heated at 30–43°C for 5 min were measured. The slope of the linear part of the light curve decreased after leaf heating at temperatures above 35°C; whereas, at a high light level, the photosynthesis rate decreased only at temperatures of 40°C and higher. The linear relationships between the photosynthetic CO₂-fixation rate and a photon flux density up to 1400 mol/(m² s) were found in leaves preheated at 42°C; this indicates the strong nonphotochemical dissipation of absorbed light quanta. The lowering of the oxygen concentration from 21 to 1% led to a CO₂ fixation maximum quantum yield and a photosynthesis-rate increase at the highest light intensity in leaves preheated at temperatures above 40°C as compared to the control leaves. Nevertheless, the linear relationship between the photosynthetic CO₂ fixation and the light intensity was found in leaves heated at 42°C at O₂ concentrations of both 21 and 1%. The latter fact suggests that the proton gradient of the thylakoid membrane, which causes an increase in the nonphotochemical dissipation of the quanta absorbed, could also be formed due to the cyclic electron transport over photosystem I.     

复硝酚钠对韭菜生长及硝酸盐还原同化效应研究

于收割后第12天对日光温室韭菜分别叶面喷施0.15、0.25、0.50mL·L-1复硝酚钠(CSN),并测定分析CSN处理不同时间韭菜生物量、叶片NO3-累积、氮代谢关键酶以及相关代谢产物含量的变化,探讨CSN对韭菜硝酸盐累积污染的减控效应及其相关生理机制。结果显示:(1)韭菜硝酸盐含量在叶面喷施CSN后呈先降低(3~6d)后增加(6~9d)的趋势,并以0.15 mL·L-1 CSN处理至第6天的韭菜硝酸盐含量最低,比对照降低了29.6%。(2)不同浓度CSN处理可显著提高韭菜叶片氮代谢关键酶的活性,其中0.15mL·L-1 CSN处理后第6天叶片硝酸还原酶(NR)、谷氨酰胺合成酶(GS)、谷氨酸草酰乙酸转氨酶(GOT)和谷氨酸丙酮酸转氨酶(GPT)活性分别较对照增加了34.4%、61.3%、208.8%、7.4%。(3)各浓度CSN处理可明显促进韭菜生长,提高韭菜营养品质,0.15mL·L-1 CSN处理后第6天叶片干、鲜重分别较对照增加10.7%、10.8%,叶片游离氨基酸含量、可溶性蛋白质含量、Vc含量分别较对照提高23.1%、23.3%、0.5%。研究表明,在韭菜氮代谢的还原同化途径中,CSN处理不仅能够显著提高氮还原动力泵(NR)和氮同化初级动力泵(GS)活性,而且能够同时调动氮同化次级动力泵(GOT和GPT)的转氨作用积极协同配合,还可能调动碳同化产物的积极协同配合(即可溶性糖含量降低),来促进硝态氮转化为游离氨基酸和可溶性蛋白以及次生代谢物质Vc的合成,减少了硝酸盐进入液泡贮积。     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Photosynthetic Rates, Gross Patterns of Carbon Dioxide Assimilation and Activities of Ribulose Diphosphate Carboxylase in Marine Algae Grown at Different Temperatures

Studies of the marine green flagellate Dunaliella tertiolecta have confirmed and extended previous observations of Steemann Nielsen and his colleagues. Algae, grown at 12°C, assimilated carbon dioxide under light-saturated conditions more rapidly than did those grown at 20°C; for both, the assimilation rate being higher at 20°C than at 12°C. Cells grown at the lower temperature contained higher concentrations of soluble protein, higher activities of ribulose diphosphate carboxylase and showed an enhanced relative rate of protein synthesis during the photosynthetic assimilation of carbon dioxide. This appears to represent true adaptation since it allowed the growth rate at 12°C to be almost the same as that at 20°C. Studies of the marine diatom Phaeodactylum tricornutum have not revealed the same picture of temperature adaptation. Cultures grown at 5°C had significantly higher rates of photosynthesis than did those grown at 10°C, but the same was not true when algae grown at 10°C were compared with those grown at 20°C. In this organism, growth at the lower temperatures reduced its ability to photosynthesize at 20°C. Cells grown at the lower temperatures contained more protein than did those grown at 20°C; this was particularly marked in cells growing at 5°C, a temperature which reduced the growth rate. The relative rate of protein synthesis was higher in Phaeodactylum grown at lower temperatures; but this difference was most marked when the measurements were made at 20°C.     

The Decreased Cold-Resistance of Chilling-Sensitive Plants Is Related to Suppressed CO2 Assimilation in Leaves and Sugar Accumulation in Roots

Tomato (Lycopersicon esculentum L., cv. Sibirskii skorospelyi) and cucumber (Cucumis sativus L., cv. Konkurent) plants were grown in a soil culture in a greenhouse at an average daily temperature of 20°C and ambient illumination until the development of five and eight true leaves, respectively. During the subsequent three days, some plants were kept in a climatic chamber at 6°C in the light, whereas other plants remained in a greenhouse (control). The cold-resistance of cucumber leaves and roots, as assayed from the electrolyte leakage, was reduced after cold exposure stronger than cold-resistance of tomato organs. The ratio photosynthesis/dark respiration was lower in cucumber than in tomato leaves at all measurement temperatures. The concentrations of sugars (sucrose + glucose + fructose) increased in chilled tomato roots but decreased in cucumber roots. Cold exposure changed the activities of various invertase forms (soluble and insoluble acidic and alkaline invertases). The total invertase activity and the ratio of mono- to disaccharides increased. The lower cucumber cold-resistance is related to the higher sensitivity of its photosynthetic apparatus to chilling and, as a consequence, insufficient root supply with sugars.