Studies on the Expansion of the Leaf Surface: IV. THE CARBON AND PHOSPHORUS ECONOMY OF A LEAF

The fixation, utilization, and translocation of carbon and thenet import and export of phosphorus by three leaves of Cucumissativus over the course of their lives were measured in a controlledenvironment. The rate of photosynthesis of a leaf followed a regular dailypattern, rising to a maximum during the first 2 hrs. of thelight period and subsequently falling. Dark respiration wasusually highest at the beginning of the dark period and fellthroughout it. The daily rate of photosynthesis per unit areaof a leaf fell during its later life partly as a result of shadingby upper leaves and also because of an independent age factor.The rate of dark respiration per unit area was high in veryyoung leaves but fell rapidly with age. The amount of phosphorus in each leaf reached a maximum beforethe leaf had reached its maximum dry weight. There was thensubstantial net loss of phosphorus from the leaf. The changing function of each leaf as a sink or source of carbohydrateand mineral nutrients was determined. Four stages were recognized:(1) early development from inception until some time after unfolding,when the leaf was dependent upon imported carbohydate; (2) aperiod of rapid expansion, associated with a high rate of uptakeor mineral nutrients, during which translocation of assimilatedcarbon from the leaf was most rapid; (3) a time of decliningrates of growth, photosynthesis and export of carbon, associatedwith substantial loss of phosphorus; (4) finally, a short sensescentphase with net loss of CO₂, terminating in the death of theleaf.     

Studies on the Expansion of the Leaf Surface: VI. SENESCENCE AND THE USEFULNESS OF OLD LEAVES

An investigation was made of the usefulness of old leaves andof some effects of light and nutrition upon their senescence.Three experiments were carried out on plants of Cucumis sativusgrown in growth rooms and in a glasshouse. In the first, theeffects of removing and shading old leaves suggested that theyserve a useful function, not as photosynthetic organs, but assources of substantial quantities of mobile elements which canbe exploited to greater advantage by leaves in more favourablyilluminated positions. In the second experiment, plants weregrown horizontally so that individual leaves could be shadedindependently of their neighbours. Shading hastened senescence.In the third, the effects of light intensity, defoliation, andnutrient deficiency upon the senescence of lower leaves weremeasured. The effects of light seemed to dominate those of nutritionin influencing the speed of senescence. The results are discussed in relation to the concept of theparasitism of lower leaves, the importance of redistributionof minerals, and the control of the course of senescence.     

Studies on the Expansion of the Leaf Surface: II. THE INFLUENCE OF LIGHT INTENSITY AND DAYLENGTH

The parts played by constant amounts of visible radiation perday and its two components—daylength and intensity—ininfluencing the growth of Cucumis sativus have been investigated.The amount of radiation per day had a far greater influencethan either of its components per se. Nevertheless, small significanteffects of photoperiod were found, leaf expansion and dry weightincrease being greatest at daylengths between 10 and 15 hr.rather than with longer days which, with similar daily totals,would be expected to give the greatest amounts of assimilation. Rates of leaf production and appearance were greatest with thehighest amounts of radiation, but the rates of expansion ofindividual leaves and their maximum areas were greatest withintermediate amounts of radiation. This response resulted inan optimum curve relating the leaf surface and the dry weightattained after a given period to radiation. The amount of radiationgiving the maximum leaf surface and dry weight decreased withage and with external nutrient supply, but at any one age washigher for increase in dry weight than for increase in leafsurface; stem and root tissues responded more to high radiationthan did the leaf surface. The net assimilation rate was a linearfunction of visible radiation over the range of 15–120cal. cm.^-2 day^-1 explored, the highest value of radiation usedrepresenting the intensity at which photosynthesis would beexpected to be maximal over a 12–15 hr. day. The inhibitory effect of high radiation on leaf expansion andthe resultant influence on the growth of the plant are explainedin terms of the number and intensity of ‘sinks’for carbohydrate and mineral nutrients within the plant.     

LEAF ONTOGENY AS A FACTOR IN SUSCEPTIBILITY TO OZONE: AMINO ACID AND CARBOHYDRATE CHANGES DURING EXPANSION

During development (expansion), the cotton leaf passes through a stage in which it is highly susceptible to ozone. This period of susceptibility occurs after the maximum expansion rate but before complete expansion occurs. The period of maximum susceptibility corresponds to a minimum concentration of soluble sugars and free pool amino acids. Specific amino acids such as aspartate, serine, threonine, glutamate, asparagine, alanine, glycine, valine, isoleucine, leucine, histidine, and tryptophan tended to reach a minimum concentration at about the same time leaves were susceptible to ozone. Ozone exposure during the susceptible period is manifested by visible leaf flecking of the upper surface and a dramatic (up to 2 fold) increase in total free pool amino acids. Most individual amino acids tended to increase except for phenylalanine, alanine, phosphoserine, phosphoethanolamine, and ethanolamine. Soluble protein is decreased but not to the same extent that the free pool amino acids increase. Because there is ample evidence that ozone enters the leaf during nonsusceptible periods as well as susceptible periods, it is postulated that ozone damage results because of the depletion of soluble reserves (carbohydrates and amino acids). Perhaps repair of damage cannot occur.     

STUDIES ON CELL METABOLISM AND CELL DIVISION : V. CYTOCHROME OXIDASE ACTIVITY IN THE EGGS OF ARBACIA PUNCTULATA

1. An enzyme capable of oxidizing reduced cytochrome c (i.e. a cytochrome oxidase) has been obtained from Arbacia eggs. In 0.02 M hydroquinone, the cytochrome oxidase was half activated at a cytochrome c concentration of approximately 4 x 10^–6 M. The concentration of the cytochrome oxidase was found to be nearly the same in unfertilized and fertilized eggs, the amount of the enzyme—as measured by means of its activity toward cytochrome c as a representative substrate—being more than sufficient to account for the highest rate of oxygen utilization yet observed in the intact, living, fertilized eggs, and of the same order as that in certain rat tissues. 2. The Arbacia cytochrome oxidase was strongly inhibited by carbon monoxide in the dark, the inhibition being almost completely reversed by light. The inhibition constant was not greatly altered by variation in the concentration of cytochrome c or the concentration of hydroquinone used as reductant for the cytochrome c, having a value of 3 to 5 under the conditions used. The inhibition constant was about 2 with p-phenylenediamine as reductant for the cytochrome c, but apparently had the surprisingly low value of about 0.5 with 0.02 M cysteine as reductant. 3. The cytochrome oxidase was completely inhibited by sufficiently high concentrations of sodium cyanide, sodium azide, and sodium sulfide. It was also completely inhibited in 0.6 M sodium chloride. It was not inhibited by two inhibitors of copper containing enzymes, 8-hydroxyquinoline and sodium diethyldithiocarbamate. It was also not significantly inhibited by 2,4-dinitrothymol, 2,4-dinitro-o-cyclohexylphenol, phenylurethane, 5-isoamyl-5-ethylbarbituric acid, or iodoacetic acid. 4. Quantitative examination of the fertilized eggs showed that cytochrome c, if present at all, occurred in a concentration of less than 2 micrograms per gram of wet fertilized Arbacia eggs. On the basis of these data and those of Fig. 2, above, it seems safe to conclude that cytochrome c cannot carry a significant fraction of the oxygen consumption of fertilized Arbacia eggs. It was also found that, in contrast to similar preparations from certain other animal tissues, the Arbacia cytochrome oxidase preparation displayed no succinic dehydrogenase activity when tested manometrically in the presence of excess cytochrome c. 5. Extending previously reported (3) experiments with other inhibitors, the effects of sodium azide and sodium sulfide on the respiration and cell division of fertilized Arbacia eggs were determined, the eggs being initially exposed to the reagents 30 minutes after fertilization at 20°C. With either reagent cleavage was completely blocked by a concentration of reagent which reduced the respiration to approximately 50 per cent of the normal level. 6. On the basis of certain theoretical considerations regarding the possible mechanism of action of cyanide and other respiratory inhibitors it is suggested that a fraction of the respiration apparently concerned with supplying energy for division processes in the fertilized Arbacia egg may be keyed into the respiratory cycle through a carrier having a somewhat higher potential than those which carry the larger portion of the egg respiration. The theory is also employed in an effort to resolve a number of hitherto apparently paradoxical observations regarding the effects of cyanide, azide, and carbon monoxide on cell respiration.     

Field Studies of Cereal Leaf Growth: II. THE RELATION BETWEEN AUXANOMETER AND DISSECTION MEASUREMENTS OF LEAF EXTENSION AND THEIR RELATION TO CROP LEAF AREA EXPANSION

Destructive measurements of laminar, sheath, and internode lengthmade on spring barley and winter wheat enabled the contributionof these phytomer components to apparent leaf extension rate(R_E), as measured by auxanometer or rule, to be determined.R_E for early formed leaves consisted entirely of laminar andsheath extension. However, during stem extension, internodeextension also contributed to measured R_E. For the last twoleaves of winter wheat, the extension of internode below thephytomer being measured contributed to R_E. For these leaves,R_E should therefore be interpreted cautiously. Calculating mainstem and tiller leaf areas, and allowing for changes of leafwidth with leaf number, showed that leaf extension was closelyrelated to crop leaf area expansion.     

Field Studies of Cereal Leaf Growth: I. INITIATION AND EXPANSION IN RELATION TO TEMPERATURE AND ONTOGENY

Measurements of leaf initiation, appearance, and expansion arepresented for winter wheat and spring barley crops. For winterwheat, these processes occurred during periods of several weekswhen fluctuating temperatures influenced process rates. Analysisof these measurements was facilitated by plotting variablesagainst the time integral of temperature above an appropriatebase temperature (O °C), here called thermal time with unitsof °C d. Leaf primordial number and appearance stage increasedlinearly with thermal time for both winter wheat and springbarley which initiated 12 and 9 leaves respectively. When plottedagainst thermal time 90% of laminar and leaf length growth and80% of laminar width growth was satisfactorily described bya straight line for both species. This enabled an average extensionrate and duration of linear growth to be defined for each leaf.When expressed in thermal time, wheat leaves had a similar durationof linear growth (210 °C d; s.d. 30 °C d) with insolationexerting a negligible influence. The first seven barley leaveshad a shorter duration of linear growth (151 °C d; s.d.8 °C d). For wheat, final leaf length and laminar widthincreased with leaf number and were not apparently associatedwith changes in apical development stage. Changes of barleyleaf dimensions with leaf number were more complex.     

Studies on the Expansion of the Leaf Surface: I. THE INFLUENCE OF TEMPERATURE

An investigation was made of the expansion of the leaf surfaceof cucumber at temperatures of 12°, 18°, 24°, and30°C. with two levels of visible radiation (40 and 80 cal.cm.^–2 day^–1). The relative rate of expansion ofthe leaf surface increased with temperature up to 24° butwas lower at 30° than at 24°. It was slightly greaterwith the higher than the lower level of radiation at the lowertemperatures only. This rate was the resultant of the rate ofunfolding of new leaves and the rate of expansion of the componentleaves. The rate of leaf production increased with increasingtemperature up to 24° and was constant there-after, butleaves unfolded from the terminal bud more rapidly with increasein temperature over the entire range. The rate of expansionof individual leaves was greatest at 24°, being less atboth lower and higher temperatures. Differences in this ratebetween temperatures increased in the order: cotyledon, leaf1, leaf 2. Leaf production and unfolding was greater with thehigher level of radiation but the expansion of individual leaveswas not influenced. These results suggested the following interpretation of theexpansion the leaf surface. Its potential rate is set by therate of unfolding of leaves from the terminal bud, which dependsmainly on the temperature and the rate of assimilation by theupper leaves and the terminal bud, the demand for assimilateexceeding the supply in this region. The demand for mineralsubstrates by the terminal bud is low and not influenced bya wide variation in potential supply. After unfolding from theterminal bud, the leaf provides most of its own supply of carbohydrateby assimilation and this can be met at a low level of radiation.Surplus assimilate is diverted to the roots and stems whichrespond much more to increased radiation than does the leafsurface. The demand for mineral substrates by expanding leaves,however, is high—the greater the number expanding at anytime the more likely is the demand by any one leaf to exceedthe supply. This leads to a reduction in the number of celldivision and, consequently, a reduced rate of expansion anda smaller leaf. The optimum level of any environmental factoris that at which the most effective compromise between theseconflicting processes is reached.     

Field Studies of Cereal Leaf Growth: IV. WINTER WHEAT LEAF EXTENSION IN RELATION TO TEMPERATURE AND LEAF WATER STATUS

Throughout winter and early spring, rule and auxanometer measurementsshowed that leaf extension rate (R_E) was directly related totemperature and stopped at about 0°C. During this period,both night and day time R_E responded similarly to temperature.Bright sunshine in late April and May caused fast transpirationwhich was associated with low leaf water potential () and slowR_E. When bright sun was obscured by cloud, R_E increased butthis did not compensate for previous slow R_E. Leaf turgor potential,calculated as the difference between and leaf osmotic potential,was large (0.6–1.8 MPa) and bore little relation to R_E.Low was associated with slower R_E than would have been expectedwithout water stress, but the relation was not unique. On abright day in May, adaptation to low occurred and during theafternoon R_E was faster than at similar values of and meristemtemperatures before noon. The response of R_E and duration ofleaf extension to temperature suggested that for any particularleaf grown under field conditions, variation in mean growingtemperature would affect final leaf length only slightly. Becausesevere water stress slows R_E without affecting the durationof leaf extension markedly, it decreases final leaf size.     

Field Studies of Cereal Leaf Growth: III. BARLEY LEAF EXTENSION IN RELATION TO TEMPERATURE, IRRADIANCE, AND WATER POTENTIAL

A mechanical auxanometer, suitable for measuring hourly ratesof leaf extension of grass and cereal crops, is described. Severalof these instruments were used to monitor leaf extension rates(R_E) of a spring barley crop. R_E of main stem and first leaftillers responded similarly to environmental factors. DuringMay, when soil water deficits were less than 50 mm, and on dulldays later in the season, R_E was directly related to meristemtemperature with night and day measurements responding similarly.During the central 10 h of bright days in late May and June,R_E was unrelated to temperature but slowed during bright sunshineand accelerated at the start of cloudy periods. Pressure chambermeasurements of total leaf water potential () showed that brightsun caused to decrease rapidly and that this was associatedwith slow R_E. Analysis of 2 h mean values of and R_E indicatedthat, at any given temperature, R_E slowed in direct proportionto decrease of .     

Light Actions in the Germination of Cocklebur Seeds: I. DIFFERENCES IN THE LIGHT RESPONSES OF THE UPPER AND LOWER SEEDS

Germination responses to light were studied in the upper andlower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Thelower seed was dark-germinating and negatively photoblastic;the upper one had a red-light (R) requirement and was positivelyphotoblastic. Germination of the lower seeds was inhibited bya prolonged single irradiation with R, blue (B) or far-red (FR)light applied during imbibition. The maximal inhibitory effectof a single irradiation occurred 9 h and 13 h after the startof soaking at 33 °C and 23 °C, respectively. However,the inhibitory effect of R differed from that of B and FR, byonly delaying germination. A single exposure to B or FR lightcould be replaced by intermittent B or FR irradiation, and theireffects were repeatedly reversible by the following R irradiation.If the upper seeds were not exposed to R during imbibition,they failed to germinate even at 33 °C which was optimalfor germination, and the promotive effect of R increased withdelay of its application time. The photoperceptive locus incocklebur seeds was the axial tissue for all B, R and FR. Lightreceived by the cotyledonary tissue had little effect. Germinationdimorphism in response to light is discussed with respect tothe phytochrome content and the ageing of axial tissues. Key words: Blue light, Dimorphism, Far red light, Germination, Red light, Xanthium seed     

Factors controlling Flowering in the Chrysanthemum: V. DE-VERNALIZATION IN RELATION TO HIGH TEMPERATURE AND LOW LIGHT INTENSITY TREATMENTS

Experiments are described which indicate that the annual vernalizationrequirement of the basal shoots of the Chrysanthemum is dueto annual devernalization of these shoots as the main axis growsup and flowers. Plants sprayed with varying concentrations ofmaleic hydrazide were arrested in their growth for considerableperiods, but this enforced ‘dormancy’ did not affecttheir vernalization status. This makes it appear unlikely thatmere suppression of growth through apical dominance of the mainshoot is the cause of this de vernalization of basal shoots.Fully or partly vernalized plants heated to 40° C. for upto 30 hours did not become dc-vernalized. Heat treatment at35° C. for as long as 30 days also failed to achieve completedc-vernalization, but here flowering was delayed by periodsequivalent to the time spent at high temperature. However, atthe end of the heat treatment progress towards flowering wasresumed at the normal rate. Complete dc-vernalization can bebrought about by prolonged exposure to low intensity illumination.This treatment appears to be effective right up to the stagewhen the first morphological changes leading to inflorescenceformation take place. These results are discussed in relationto similar experiments on the de-vernalization of rye and Hyoscyamusniger.     

Leaf Extension in Zea mays: II. LEAF EXTENSION IN RESPONSE TO INDEPENDENT VARIATION OF THE TEMPERATURE OF THE APICAL MERISTEM, OF THE AIR AROUND THE LEAVES, AND OF THE ROOT-ZONE

The temperatures of the roots, the apical meristem, and theshoots of Zea mays plants were varied independently of eachother and the rates of leaf extension were measured. When thetemperature of the apical meristem and region of cell expansionat the base of the leaf was kept at 25 °C, changes of leafextension in response to changes of root and shoot temperatureswere less pronounced. When the temperature of the meristematicregion was changed by increments of 5 or 10 °C from 0 to40 °C, and the root and shoot temperatures were kept at25 °C, rapid changes in leaf extension occurred. It was concluded that the rates of leaf extension were controlledat root-zone temperatures of 5 to 35 °C by heating or coolingof the meristematic region. Changes in rates of leaf extensionin response to changes in air temperature were attributed todirect effects on the temperature of the meristematic regionand on the physiology of the leaf.     

Genome Expression during Normal Leaf Development : I. CELLULAR AND CHLOROPLAST NUMBERS AND DNA, RNA, AND PROTEIN LEVELS IN TISSUES OF DIFFERENT AGES WITHIN A SEVEN-DAY-OLD WHEAT LEAF

Changes in genome expression during normal cellular and plastid development in the first leaf of young (7-day-old) wheat (Triticum aestivum var. Maris Dove) were investigated by examining homogeneous populations of leaf cells and plastids of several developmental ages present in the same leaf. The cells were characterized over a period immediately following the last cell division. All of the leaf cells had cytoplasmic contents and nuclei, and between 44% (young tissue) and 54% (older tissue) of the leaf cells were mesophyll cells. Chloroplast development was complete 36 hours after the chloroplasts had ceased dividing. Extremely large changes occurred in cellular constituents over a very short period of leaf development. Maximum rates of accumulation of ribulose bisphosphate carboxylase per mesophyll cell (80 picograms/hour), chlorophyll per mesophyll cell (9 picograms/hour), and 70S ribosomes per mesophyll cell (19 × 10⁵/hour) were recorded.     

Studies in the Respiratory and Carbohydrate Metabolism of Plant Tissues1: XVI. FURTHER STUDIES OF THE INFLUENCE OF IODOACETATE AS A STIMULANT OF RESPIRATION IN STRAWBERRY LEAVES

Low concentrations of iodoacetate produced large increases inCO₂ output of strawberry leaves which were not completely accountedfor by the observed losses of sugars and starch. No significantchange was observed in RQ in iodoacetate but the values, eitherin water or in iodoacetate, were generally below unity. Thus,in addition to other carbohydrates not estimated in our work,there may also be respiration of protein or organic acids. The increased rate of CO₂, output in iodoacetate was associatedwith rises in pyruvate and oxaloacetate but citrate and -ketoglutarateremained unchanged. The former changes suggested a faster trafficinto the tricarboxylic acid cycle; the content of oxaloacetateappeared to be a close index of the rate of this traffic. Atentative interpretation is given of the anomalous behaviourof citrate and -ketoglutarate in iodoacetate.     

The Effect of Wind on Grasses: V. LEAF EXTENSION DIFFUSIVE CONDUCTANCE PHOTOSYNTHESIS IN THE WIND TUNNEL

Festuca arundicacea and Lolium perenne were grown in a controlled-environmentwind tunnel at high (7.4 m s^–1) or low (1.0 m s^–1)windspeed. The rate of leaf extension was markedly reduced atthe high windspeed. This effect could not be attributed to waterstress, for, although the leaf conductance increased with exposureto high wind, no effect on leaf water potential was detected.Nor was the rate of photosynthesis affected when the windspeedwas changed. Moreover, concentrations of ethylene in the windtunnel were too low to explain the observations. It is suggestedthat mechanical stimulus itself may have caused the reductionin leaf growth rate.     

Studies on the Growth in Culture of Plant Cells: V. LARGE-SCALE CULTURE OF ACER PSEUDOPLATANUS L. CELL SUSPENSIONS

A technique is described for the large-scale culture of sycamorecell suspensions. Two 10-1 wide-mouthed flat-bottomed culturebottles are spun continuously at 120 rpm inclined at 45°from the vertical. Each bottle contains a 4.5 1 culture which,after 21 d at 25 °C yields a suspension of 10⁹ cells, 1.41 of packed-cell volume and 40 g dry weight. The growth characteristicsof the cultures are described and it is demonstrated that largevolumes of suspension can be withdrawn during incubation withoutaltering the general growth pattern or the yield of cells perunit volume of the final culture.