Pre-flowering Growth and Development of the Inflorescences of Maize: II. ACCUMULATION AND PARTITIONING OF DRY MATTER AND NITROGEN BY INFLORESCENCES

The accumulation and partitioning of dry matter and nitrogenwere examined in the developing tassel and two uppermost earshoots of field-grown maize under varying levels of appliednitrogen and times of sowing. Accumulation of dry matter and nitrogen within an axillary branchalways favoured the ear over the husk and shank. Dry matterand nitrogen accumulated faster in the first ear than in thetassel or second ear and the partitioning between inflorescencesof dry matter and nitrogen was not affected by treatment. Therelative rate of growth, RGR(dry matter), of the first and secondear shoots increased by up to 42% at high levels of appliednitrogen and with early sowing. In contrast, the relative rateof accumulation of nitrogen (RNAR) was not sensitive to N supply,although it was reduced, on average, by 22% at the late timeof sowing. We conclude that accumulation, but not partitioning, of drymatter and nitrogen between developing inflorescences of maizeare altered by nitrogen application, time of sowing, and positionof the inflorescence on the stem. Key words: Maize, N-application, partitioning, inflorescence, sowing time     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. I. ANATOMY,APICAL DOME DIAMETER,AND PLASTOCHRON DURATION

Developing embryos in immature Picea abies seeds already have well-delineated shoot apical meristems with clearly evident cytohistological zonation. During early seedling development the zonation characteristic of gymnospermous apical meristems is attained. Seedling development is also accompanied by an approximately threefold increase in apical dome diameter. The latter approaches a steady state about 140 days after germination. Seedlings display a spiral phyllotaxis consisting of a contact parastichy system, usually of the primary Fibonacci series. As the seedlings age and apical domes enlarge, higher Fibonacci number-pairs characterize their phyllotaxis. Mathematical analysis of the relation between cumulative leaf number and age revealed that the length of the plastochronic time interval declines from about 18.5 hr to 5.7 hr as seedling age increases from 20 to 140 days.     

Growth and Development in the Young Tomato: I. THE EFFECT OF TEMPERATURE AND LIGHT INTENSITY ON GROWTH OF THE SHOOT APEX AND LEAF PRIMORDIA

Tomato seedlings were grown at constant temperatures of 25°and 15° C. in a 12-hour day at light intensities of 1,600,800, and 400 f.c. The rate of increase in size of the shootapex and the rates of formation and growth of leaf primordiaduring the vegetative phase were followed by dissecting samplesfrom the time of cotyledon emergence onwards. The rate of enlargement of the shoot apex increased with lightintensity, but apical enlargement was delayed at the highertemperature, the delay being longer the lower the light intensity.The rates of leaf formation and leaf growth increased with bothtemperature and light intensity. Temperature had a larger effecton leaf growth than on leaf formation. More leaves were formedbefore flowering at 25° C. than at 15° C., the increasein leaf number being greater the lower the light intensity. It is suggested that the delay in the enlargement of the apexat high temperature can be explained in terms of competitionfor assimilate, the competitive potential of the expanding leafprimordia exceeding that of the apex at higher temperatures.     

Tissue Culture of Maize I. Callus Induction and Growth

Callus induction and subculture was successful with mature embryos and stem sections of seedlings of Zea mays L. on Linsmaier and Skoog's medium modified to contain 4 mg/I of 2,4-D and 1 g/I of casamino acids. — 2,4-D was superior to NAA and IAA for both callus induction and growth. Callus subcultured on NAA formed abundant roots on agar-solidified media and numerous root-like primordia in liquid cultures. — Kinetin had no effect on callus induction in the presence of 2,4-D and neither kinetin nor gibberellic acid stimulated callus growth during subculture. — Callus grew equally well on the medium of Linsmaier and Skoog, that of Schenk and Hildebrandt, and the B-5 medium of Gamborg and Eveleigh containing 2% sucrose, 4 mg/I of 2,4-D and 1 g/I of casamino acids. — The callus grew more rapidly at 25°C than at 30°C or 35°C. Little difference was noted at any temperature in callus growth in alternating light (16 h) and dark (8 h) or continuous dark. — Sucrose was superior to glucose and maltose in both liquid and agar-solidified cultures. Lactose and galactose failed to support callus growth.     

THE PICEA ABIES SHOOT APICAL MERISTEM IN CULTURE. I. AGAR AND AUTOCLAVING EFFECTS

Shoot apical meristems of Picea abies seedlings can be cultured on a relatively simple, defined, basal medium. Dome-like explants initially about 200 μ tall, without externally obvious primordia, and having dry weights of about 3 μg, usually initiate 5–10 new primordia within a week. They typically show 10- to 30-fold dry weight increases in three weeks. None of the 5,000 meristems cultured has produced any basal callus. Growth is strongly influenced by both the type and concentration of agar used to gel the medium. Dry weight yield increases as agar concentration decreases. This is probably partly due to increased diffusion rates of enzymes or other large molecules through more dilute agar gels but possibly also partly ascribable to unknown agarborne inhibitors. About half of the agar concentration effect can be eliminated by substituting glucose and fructose for sucrose in the medium. This suggests that diffusion of invertase through the agar gel in this medium may be a growth limiting factor. Growth of cultures is also promoted by autoclaving sucrose in the presence of the agar. The basis of this effect is not yet understood.     

The Role of the Coleoptile Apex in Controlling Organ Elongation: I. THE EFFECTS OF DECAPITATION AND APICAL INCISIONS

It has been widely accepted for over 50 years that the elongationrate of a coleoptile is dependent on the supply of auxin fromthe apex. The original coleoptile decapitation experiments whichprovided support for this view have been repeated but the measurementsof coleoptile elongation were made with greater temporal andspatial precision. The experiments confirm that Avena and Zeacoleoptile elongation is retarded by decapitation but the locationand timing of the growth rate changes are not consistent withthe hypothesis that decapitation reduces growth rate solelyby removing the major supply of auxin. Evidence is presentedthat wounding is the prime cause of the effects of decapitation.Data are also presented showing that the recovery of growthrate of coleoptiles after decapitation or wounding is not dearlyassociated with any events near the cut surface and hence thetraditional explanation of this phenomenon (‘regenerationof the physiological tip’) is misleading. Key words: Coleoptile, decapitation, apex     

The Role of Boron in Plant Growth: I. THE EFFECT ON GENERAL GROWTH, SEED PRODUCTION AND CYTOLOGICAL BEHAVIOUR

The relation of boron to vegetative growth and seed productionof Trifolium pratense L. and Vicia faba L. has been studiedusing sand and water culture techniques. It was found that theaddition of a very small quantity of boron was sufficient torestore deficient plants to normal growth and it would appearthat boron fertilizers will only be beneficial where deficiencysymptoms have been shown. The rapid cessation of cell division under conditions of borondeficiency was demonstrated and was responsible for a decreasein both vegetative and seed yields through reduced branchingand flower differentiation. No special association of boronwith the functioning of pollen or with chromosome behaviourwas found. The cessation of cell division is responsible forthe abnormalities associated with boron deficiency but the cellularreactions involved remain unknown. Previous suggestions of the role of boron in plant metabolismare discussed in the light of the conclusion that boron is essentialfor the maintenance of meristems.     

EUGLENA GRACILIS IN SYNCHRONOUS DIVISION I. DRY MASS AND VOLUME CHARACTERISTICS

Volume distributions and dry mass have been measured in synchronouspopulations of Euglena gracilis, grown in a salt medium at aconstant temperature. In keeping with the approximate doublingof cell number observed in each division burst, the averagedry mass and volume are doubled in each inter-division period. ¹ Present address: Department of Biology, Tokyo MetropolitanUniversity, Setagaya.ku, Tokyo. (Received February 17, 1961; )     

INTERCELLULAR COMMUNICATION AND TISSUE GROWTH : I. Cancerous Growth

Intercellular communication was examined with intracellular electrical techniques in primary and transplanted rat liver cancers. Normal liver cells communicate rather freely with each other through permeable junctional membranes. Cancer liver cells show no communication at all; their surface membrane is a strong barrier to diffusion all around the cell. Cancer cells induce alterations in membrane permeability in normal liver cells; communication among the latter is markedly reduced when cancer cells grow near them.     

Changes in Enzyme Regulation during Growth of Maize: I. Progressive Desensitization of Homoserine Dehydrogenase during Seedling Growth

The sensitivity of homoserine dehydrogenase (EC 1.1.1.3) to inhibition by the feed-back modifier, l-threonine, was examined in preparations derived from etiolated shoots, roots, and lightgrown tissues of Zea mays L. var. earliking. A progressive decrease in enzyme sensitivity was observed during seedling growth. Enzyme derived from internode tissue retained a greater sensitivity to the effector than enzyme derived from apical portions of etiolated shoots, whereas enzyme from root tips was characteristically more sensitive than that prepared from mature cells of the root. Enzyme desensitization occurred rapidly during culture of excised shoots and the activities of both homoserine dehydrogenase and aspartokinase (EC 2.7.2.4) declined during shoot culture under a variety of conditions. The initial enzyme levels and the characteristic sensitivity of homoserine dehydrogenase were preserved during culture at 5 to 7 C, but desensitization was not prevented by inclusion of cycloheximide in the culture medium.Results of control experiments provide evidence that desensitization occurs in vivo. No alteration of the enzyme properties was detected during extraction or concentration of sensitive or insensitive enzyme or during coextraction of enzyme from mixed populations of different age shoots; nor was a differential distribution of inhibitors or activators indicated during assay of mixed preparations. The change in enzyme sensitivity was apparent under a variety of assay conditions and was not accompanied by changes in the apparent affinity of the enzyme for the substrate, homoserine. It is suggested that systematic changes in the regulatory characteristics of certain enzymes could be an important level of metabolic regulation during cellular differentiation.Three forms of maize homoserine dehydrogenaase were detected after acrylamide gel electrophoresis of samples derived from 72-hr shoots. Similar analysis of samples from older shoots revealed a broad asymmetric band of enzyme activity, suggesting that changes in the relative distribution of specific forms of the enzyme could be related to the growth-dependent changes in the sensitivity of maize homoserine dehydrogenase.     

The Growth of Shoot Apices and Inflorescences of Carex flacca Schreb. in Aseptic Culture

The vegetative apex and young inflorescence of Carex flaccahave been grown in aseptic culture for several weeks on a definedmedium. Explants comprised the apical dome and the three youngestleaf primordia, and the young inflorescence complete with bracts,excised shortly after the initiation of the lateral spikes.Some growth occurred on the basal medium which contained inorganicsalts, sucrose, and vitamins, but growth was increased and thelife span extended by the addition of certain other compounds.The most effective additive was gibberellic acid, which, however,resulted in precocious differentiation of the meristematic tissuesand the differentiation of abnormal xylem. These deleteriouseffects of gibberellic acid were counteracted by the additionof kinetin. This substance did not otherwise affect vegetativeapices but it resulted in a further increase in growth and lifespan of inflorescences.     

Growth and Cellular Differentiation in the Wheat Coleoptile (Triticum vulgare L.): I. ESTIMATION OF CELL NUMBER, CELL VOLUME, AND CERTAIN NITROGENOUS CONSTITUENTS

The numbers and volumes of cells were determined for consecutivestages in the growth and development of the wheat coleoptile(var. ‘King II’) when grown at 25° C. in darknessfrom soon after germination to senescence. Cell expansion occurredthroughout growth and development up to 96 hours, and was accompaniedby cell division between 18–60 hours. Evidence is presentedthat suggests there are two phases of cell expansion concernedin coleoptile growth. Determinations were made at daily intervals from 24 to 120 hoursafter sowing of protein nitrogen, trichloroacetic acid solublenitrogen (TCA-sol. N), ribonucleic acid (RNA), and deoxyribonucleicacid (DNA) in the coleoptile, and the results expressed on aper-coleoptile and per-cell basis. The maximum rate of net proteinsynthesis took place during or after the cell-multiplicationphase of growth, depending on whether the results were expressedper coleoptile or per cell respectively. The ratio of proteinnitrogen to TCA-sol. N changed considerably during growth, from4·7 for young cells to 0·68 for mature cells. The fluctuations in the values for RNA and protein are consistentwith the template theory of protein synthesis and the DNA dataare discussed in relation to polyploidy in differentiated cells.No significant difference was found in the nucleotide compositionof RNA during the growth and development of the coleoptile.     

Allometric Growth Studies of the Carrot Crop: I. Effects of Plant Development and Cultivar

The allometric equation Y = aW^k was fitted to data from a fieldexperiment in which carrots were harvested at 14 weekly intervalsfrom a succession of 14 weekly sowings. Use of the allometricequation enabled more than 95 per cent of the variation in theweight of tap roots (Y), to be accounted for by reference tothe weight of leaves (W). The residual mean square error ofthe predicted root weight was less than the within-harvest meanerror. The exponent k, representing the ratio of relative growthrates of roots to that of leaves was 1.268 ± 0.019, similarto values calculated from field data from California and centralRussia. Large differences in the relative growth rate ratiowere found in the germination and flowering phases of growth,when values of k were 0.814 and -0.095 respectively. These differences,together with those found between cultivars, were associatedwith differing root shapes.     

Growth and Development of the Banana Plant: I. The Growing Regions of the Vegetative Shoot

This is a study of the vegetative growth of the banana plant,with special reference to the structure of the shoot apex, theorigin of the leaf primordia and buds, and the growth of theleaf base into the pseudostem. The various regions in whichintercalary growth contributes to the vegetative plant bodyare described. The anatomical structures observed are illustratedby photomicrographs. Binucleate cells are conspicuous in theleaf bases and in cells produced by intercalary men-stems. Theformation of the air chambers which are characteristic of themature leaf and of the septa, which are formed as persistentsheets of cells which bound these chambers, is described. Thecell divisions which build the septa, and also those which causethe eccentric growth of the midrib are noted, and their proximityto adjacent vascular strands is stressed. Other marginal meristemsbuild the lamina of the leaf. The function of the central apicalmeristem of the shoot is not to create a massive axis whichgrows in length, for this vegetative function is taken overby the lateral organs, the growth of which greatly overshadowsthat in the main axis. However, as the vegetative shoot growsolder, its central mass of meristem does become progressivelylarger. Cell divisions in this central area are sparse, thoughsufficient to increase its bulk slowly, while the main organ-buildingand cell-multiplying functions are delegated to the lateralorgans. This condition changes on flowering when a massive,true, erect stem forms. Axillary buds do not occur in the vegetativeshoot, but adventitious buds appear in an anomalous situation.The vegetative shoot behaves as though there is an extremelystrong apical dominance, which suppresses all buds and growthin the axis itself. But an elusive question is the mechanismwhich stimulates, or controls, the behaviour of so many dividingcells, distributed so widely, through so many discrete areasof cell division or intercalary meristematic activity. The frequentproximity of vascular strands, as probable sources of both nutrientsand stimuli to cell division, is suggestive here.     

Phosphate Deficiency in Maize. I. Leaf Phosphate Status, Growth, Photosynthesis and Carbon Partitioning

Maize plants (Zea mays L.) were cultured with nutrient solutioncontaining 0.001 or 0.5 mM orthophosphate (Pi). Effects of lowphosphate (low-P) nutrition on growth, leaf phosphate status,photosynthesis, and carbon partitioning were investigated. Withlow-P treatment, the fresh weight of aerial parts decreasedby about 40% by 24 days after planting. Detailed studies ofthe effects of low-P treatment on the other characteristicsof maize leaves-were done using the middle part of the thirdleaf, counting from the base. Low-P treatment had almost noeffect on specific leaf weight or soluble protein content measured13 to 21 days after planting. Low-P treatment decreased Chicontent slightly (by 15% 19 days after planting). Twenty onedays after planting, low-P treatment had greatly decreased thelevels of leaf acid extractable Pi (by 77%) and photosynthesisrates (by 68%). The detrimental effects of low-P treatment onthe rates of photosynthesis and the amounts of acid extractablePi became progressively greater with time. There was a strongcorrelation between levels of leaf acid extractable Pi and ratesof photosynthesis. The minimum level of Pi necessary to sustainthe maximum photosynthesis rate was 0.6 mmol m^–2. Belowthis minimum content of Pi, the rate of photosynthesis decreasedsharply with decreasing Pi. To investigate the direct effectof Pi depletion on photosynthate partitioning at equivalentrates of photosynthesis, the rates in controls were reducedto almost the same as those in 18 or 19 day old low-P plants(about 50% of those in controls) by lowering light intensityand/ or ambient CO₂ concentration. The data clearly indicatesthat low-P treatment had a direct effect in lowering photosynthatepartitioning into starch. Starch mobilization during the nightwas also inhibited under low-P conditions. (Received January 7, 1991; Accepted March 5, 1991)     

Red Light-Regulated Growth : I. Changes in the Abundance of Indoleacetic Acid and a 22-Kilodalton Auxin-Binding Protein in the Maize Mesocotyl

We examined the changes in the levels of indoleacetic acid (IAA), IAA esters, and a 22-kilodalton subunit auxin-binding protein (ABP1) in apical mesocotyl tissue of maize (Zea mays L.) during continuous red light (R) irradiation. These changes were compared with the kinetics of R-induced growth inhibition in the same tissue. Upon the onset of continuous irradiation, growth decreased in a continuous manner following a brief lag period. The decrease in growth continued for 5 hours, then remained constant at 25% of the dark rate. The abundance of ABP1 and the level of free IAA both decreased in the mesocotyl. Only the kinetics of the decrease in IAA within the apical mesocotyl correlated with the initial change in growth, although growth continued to decrease even after IAA content reached its final level, 50% of the dark control. This decrease in IAA within the mesocotyl probably occurs primarily by a change in its transport within the shoot since auxin applied as a pulse moved basipetally in R-irradiated tissue at the same rate but with half the area as dark control tissue. In situ localization of auxin in etiolated maize shoots revealed that R-irradiated shoots contained less auxin in the epidermis than the dark controls. Irradiated mesocotyl grew 50% less than the dark controls even when incubated in an optimal level of auxin. However, irradiated and dark tissue contained essentially the same amount of radioactivity after incubation in [¹⁴C]IAA indicating that the light treatment does not affect the uptake into the tissue through the cut end, although it is possible that a small subset of cells within the mesocotyl is affected. These observations support the hypothesis that R causes a decrease in the level of auxin in epidermal cells of the mesocotyl, consequently constraining the growth of the entire mesocotyl.