Growth and Senescence of the Successive Leaves on a Cocksfoot Tiller. Effect of Nitrogen and Cutting Regime

The effects of nitrogen supply and cutting regime on the morphologicalcharacteristics (leaf appearance and expansion rates, leaf growthduration, leaf lifespan) of a cocksfoot sward were studied overthree growing seasons to gain a better understanding of thechanges in tiller characteristics (length and age of laminae,number of leaves per tiller) over time and in different seasons.We show that, for a given regrowth, the lamina expansion rateat the tiller level depended on herbage nitrogen status, butthe time course of its components differed according to nitrogensupply. When nitrogen was supplied, leaf appearance was fasterbut also decreased faster. In other words, the length of successivelaminae increased faster when nitrogen was supplied. The samewas true for the growth duration of the laminae and their lifespan.These changes resulted from the length of the sheaths from whichthe successive laminae emerged. As nitrogen increased cell number,it changed the ratio of lamina length_n+1/sheath length_nmorethan the ratio lamina length_n/sheath length_nat the same insertionlevel. Therefore sheath length increased faster and leaf appearancedecreased faster when nitrogen was supplied. This finding helpsto explain the effects of different heights and frequenciesof cutting in terms of their effects on sheath length. Copyright2000 Annals of Botany Company Nitrogen, defoliation, cocksfoot, sheath     

The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

M26 apple rootstocks were grown in sand culture and suppliedwith three rates of nitrogen (N) with the irrigation: none,0·8 mol N m^–2 or 8·0 mol N m^–2. Allthe N supplied to the trees was labelled with ¹⁵N at 5·0atom percent enrichment. The effect of N supply on tree growth,N uptake and the remobilization of N from stems for the annualgrowth of the trees was measured. Increasing the N supply increasedleaf growth, but had no effect upon root mass and so alteredthe root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon storedreserves of N for their seasonal growth. Initially this N wasused for leaf growth, which stopped after a few weeks. Thereafterthe N-deficient plants retranslocated some of the N from theirleaves to support root growth. Increasing the N supply had littleeffect upon the amount of N remobilized for growth, althoughwell-fertilized plants accumulated N in their leaves and didnot retranslocate any to support root growth. The partitioningof N between roots and shoots was, therefore, altered by increasingthe N supply. Amino acid analysis of stems showed that the majorforms of N remobilized during growth were protein rich in asparagineand arginine The results show the importance of internal N cycling for thegrowth of young apple trees, and are discussed in relation toother studies of N cycling in deciduous trees Malus domestica Borkh., nitrogen, remobilization, growth, partitioning, storage     

Effect of Nitrogen Supply on the Growth and Senescence of Leaves of Lolium perenne with Contrasting Rates of Leaf Respiration

Respiration rates remain high during senescence which is anenergy demanding process. Different rates of senescence mayexplain the contrasting respiration rates of mature leaves oftwo populations of perennial ryegrass cv. S23. This hypothesiswas tested by measuring the lengths of elongating and senescingleaves of plants of two populations (GL72, a slow and GL66,a fast respiring population) for 76 d following the transplantationof the seedlings into soil-filled pots in a growth room. Nitrogenwas supplied at two different rates at the beginning of theexperiment and again at day 36. At high levels of nitrogen supply the slow respiring populationhad a faster elongation rate and so a greater leaf length thanthe fast respiring population. There was no difference betweenthem in the rate of leaf senescence or in the duration of leafgrowth. Consequently, the fast respiring population is consideredprofligate in its use of carbon. By contrast, under low nitrogensupply both elongation and senescence rates were higher in thefast respiring population, although there was no differencein final leaf length. Therefore the longevity of leaves of thefast respiring population was reduced. It is argued that theleaves of the fast respiring population may turn-over mineralsmore rapidly than those of the slow, which may be more advantageousin conditions of low nitrogen supply and offset the benefitsof a more conservative use of carbon normally seen under conditionsof high nitrogen supply. Lolium perenne L., perennial ryegrass cv. S23, nitrogen supply, respiration rate, leaf senescence rate, leaf elongation rate, leaf longevity, population     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

Turnover of Carbohydrates in Relation to Growth in Apple Trees. II. Distribution of 14C Assimilates Labelled in Autumn, Spring and Summer

One-year-old plants of the apple rootstocks MM. 106 and M.7were allowed to assimilate ¹⁴CO₂ in autumn, spring and summerand the distribution of the tracer within the plant over a growingseason was followed. In MM.106 distribution of the tracer intwo fractions of extractable carbohydrate and the residues representingstructural material, was determined. The results of the radioactivityassay are discussed in relation to seasonal patterns of assimilatesupply and demand in the different regions of the plant. Malus sylvestris L, apple, ¹⁴C assimilates, distribution of carbohydrates     

Spring Mobilization of Protein Nitrogen in Apple Bark

Mobilization of protein nitrogen in the spring was studied in bark of stems of unringed and double stem-ringed apple rootstocks M.7 given different nitrogen treatments. A ready protein hydrolysis occurred; the proteins contributed the greater part of the storage nitrogen exported to the growing parts. Protein hydrolysis was little affected by the supply of newly absorbed nitrogen. Movement of nitrogen out of the bark between the rings could not be demonstrated. Protein breakdown in the isolated bark sections was slightly reduced. Arginine was the predominant amino acid in the proteins of the trees with a high level of storage nitrogen but was not conspicuous in the low-nitrogen trees. The protein composition changed little during hydrolysis. Only the share of arginine in the high-nitrogen trees dropped appreciably. It is suggested that the high-nitrogen trees possess a special storage protein characterized by a high arginine content. Analysis of the nitrogen fraction of isolated bark sections showed that the composition of the soluble nitrogen was characterized by a high level of asparagine and especially of arginine, and was quite different from the composition of the proteins. The data suggest that the asparagine in particular originated largely from transformation of the various amino acids set free during protein hydrolysis.     

Studies in the Comparative Physiology of Apple Rootstocks: I. The Effect of Nitrogen on the Growth and Assimilation of Malling Apple Rootstocks

The growth and net assimilation rates of four Malling applerootstocks of varying vigour were observed under a wide rangeof cultural conditions, viz. soil, sand, and nutrient solutionat three different hydrogen ion concentrations. The mean netassimilation rates of the stocks were found to be in order ofvigour, irrespective of cultural medium, greater vigour alwaysaccompanying higher assimilation rate. There was a marked differentialresponse to nitrogen, Crab C, a vigorous stock, being more efficientthan M. IX at low nitrogen supply, whereas at higher levelsthe relative differences were less pronounced. Nitrogen deficiencyalso reduced very markedly the net assimilation rate of bothstocks.     

Effect of Soil Water Potential on Growth of Apple Trees Infected with Pratylenchus penetrans

Malling-Merton 106 apple rootstocks inoculated with Pratylenchus penetrans, or uninoculated, were grown in a growth chamber in pots of loamy sand maintained at two moisture levels, 0 to -0.4 bar or 0 to -10 bars. Either inoculation or low soil moisture suppressed shoot growth and increased root necrosis. However, the nematode-soil moisture interaction was not significant.     

Effect of Nitrogen on Growth, Yield and Photorespiratory Activity in Spring Wheat

Spring wheat plants growing in pots in controlled environmentrooms were given extra nitrogen after flag leaf emergence. Theeffect of nitrogen on growth, yield, the activity of ribulose1,5–bisphosphate carboxy–lase/oxygenase and thedistribution of¹⁴C in photorespiratory intermediates and indifferent parts of the plants was determined. Extra nitrogenincreased the movement of ¹⁴C to the ear and increased grainyield by 29 per cent, mainly because of an increase in grainnumber. Though extra nitrogen delayed senescence of the leaves,the growth of the ear in the later stages was not increasedin proportion to the extra green area. The relative inefficiencyof leaf area with extra nitrogen, which has also been foundin the field, was not due to a reduction in photosynthesis perunit leaf area. Nor was there evidence of an increase in photorespirationas reflected by a greater flow of carbon into the photorcspiratorymetabolites glycine and serine, or an increase in the activityof ribulose 1,5–bisphosphate oxygenase relative to thecarboxylase. We suggest that there may be an increase in theloss of carbon in dark respiration. Triticum aesttvum, nitrogen, growth, yield, photorespiration     

Studies in the Nutrition of Apple Rootstocks The Effect on Growth and Mineral Composition of Supplying Iron Through the Leaves

Apple rootstocks were grown with either 0.02 ppm Fe (Fe₀) or5 ppm (Fe₃), to give very chlorotic or dark-green plants. Toinvestigate whether iron can be supplied through leaves insteadof roots the shoots of half the plants in each treatment weredipped periodically in solutions of iron. This prevented chlorosisin Fe₀ plants and increased their growth, which did not, however,equal that of Fe₃ plants supplied with iron through the roots.Growth of Fe₃ plants was reduced by dipping. Iron was not translocated from leaves to roots, although theconcentration in leaves was greatly increased by dipping. Dippingreduced the amount of manganese in Fe₀ roots to one-quarterof that in roots of undipped Fe₀ plants. Effects of treatmentson nitrogen, potassium, calcium, magnesium, and copper levelsare also described.     

Growth and Senescence of the Successive Grass Leaves on a Tiller. Ontogenic Development and Effect of Temperature

The tiller characteristics (length and age of laminae, numberof leaves per tiller) which depend on morphogenetic characterssuch as leaf appearance and expansion rates, leaf growth durationand leaf lifespan were studied in the field over four growingseasons to gain a better understanding of the progressive changesin leaf digestibility over time, and to facilitate the developmentof predictive mathematical models. We show that, for a givenregrowth, only the number of leaves per tiller and the laminaexpansion rate remain constant throughout growth. In other words,the length of successive laminae, their growth duration andlifespan increased while their rate of appearance decreasedin such a way that the lamina expansion rate at the tiller levelremained constant. These changes were associated with an increasein sheath length which governs both the lamina appearance rateand its growth duration. As temperature increased, the averagelamina expansion rate and the number of laminae which grew bothincreased simultaneously. Therefore, high temperature acceleratesthe changes in tiller characteristics which occur as growthprogresses. Copyright 2000 Annals of Botany Company Leaf, senescence, phyllochron, lifespan, digestibility, temperature, cocksfoot, Dactylis glomerata L     

The Metabolism of Oat Leaves during Senescence: II. Senescence in Leaves Attached to the Plant

The course of senescence in the first leaves of light-grown Avena seedlings when attached to the plant has been compared with that previously studied in detached leaves and leaf segments. Proteolysis in the leaf, whether attached or detached, is accompanied by markedly polar basipetal transport of amino acids. This polar transport can be superimposed on the known transport of amino acids towards a locally applied cytokinin. In the intact plant, it results in a strong movement into the roots. The reducing sugars, which are set free in senescence, do not participate appreciably in this polar transport phenomenon.     

The Behaviour of Peroxidase and Polyphenol Oxidase during the Growth and Senescence of Tobacco Leaves

The activities of polyphenol oxidase and peroxidase per unitarea of attached tobacco leaves increased as the leaves expanded,and reached a stable level in the mature leaves. After the onsetof senescence the enzyme activities fell rapidly, but at a laterstage they showed a small rise. Enzyme activities in tissuestaken from non-senescent leaves increased during incubationat 20 °C. These increases were sensitive to inhibitors ofprotein synthesis. Enzyme activities in tissues taken from leavesin early senescence increased during incubation at 35 °Cbut not at 20 °C. These increases were very largely insensitiveto inhibitors of protein synthesis and were not apparently relatedto de novo protein synthesis. There were no increases in enzymeactivities in tissues taken from leaves in late senescence andincubated at 35 °C or 20 °C.     

Spring Mobilization of Storage Nitrogen in Isolated Shoot Section of Apple

The process of mobilization of nitrogenous compounds in trees during spring development was studied in short isolated shoot sections (usually bearing one bud each) of Golden Delicious apple trees. During leafing-out of the bud, changes in the amounts of total, protein and soluble nitrogen and of soluble amino acids and amides in bark and wood were followed. The nitrogen required by the growing parts came mainly from protein breakdown in the tissues below the bud; in the tissues above the bud, total nitrogen decreased little, whereas the drop in protein nitrogen was considerable. In de-budded sections and in internode sections where total nitrogen remained almost unchanged, protein hydrolysis occurred as well. It is concluded that the protein breakdown is not strongly dependent on the demand of the bud for nitrogen. Inversion of the sections did not result in any change in the pattern of nitrogen mobilization: a marked drop occurred in the nitrogen content of the physiologically basal part of the section and only a slight decrease in the apical part. The translocation of stored nitrogenous compounds to the growing parts seems to occur in the phloem, at least over short distances. Asparagine and arginine were found to be the major components of the soluble amino-nitrogen fraction throughout. The relative importance of asparagine was reduced in tissue where a substantial loss of nitrogen occurred during leafing-out of the bud. This is explained in terms of a preferential export of asparagine to the bud.     

The Control of Autumn Senescence in European Aspen

The initiation, progression, and natural variation of autumn senescence in European aspen (Populus tremula) was investigated by monitoring chlorophyll degradation in (1) trees growing in natural stands and (2) cloned trees growing in a greenhouse under various light regimes. The main trigger for the initiation of autumn senescence in aspen is the shortening photoperiod, but there was a large degree of variation in the onset of senescence, both within local populations and among trees originating from different populations, where it correlated with the latitude of their respective origins. The variation for onset of senescence with a population was much larger than the variation of bud set. Once started, autumn senescence was accelerated by low temperature and longer nights, and clones that started to senescence late had a faster senescence. Bud set and autumn senescence appeared to be under the control of two independent critical photoperiods, but senescence could not be initiated until a certain time after bud set, suggesting that bud set and growth arrest are important for the trees to acquire competence to respond to the photoperiodic trigger to undergo autumn senescence. A timetable of events related to bud set and autumn senescence is presented.     

Effect of Enterobacter aerogenes on the Rhizosphere Microflora of Apple Trees*

Fifty–seven fungi and 59 bacteria were isolated from soils collected from two apple orchards located in Kelowna and Summerland, British Columbia, Canada. Sixteen fungi and 35 bacteria, respectively, were significantly inhibited by the biological control agent Enterobacter aerogeness in a dual culture test. These results indicate that the antibiotic(s) produced by this organism has a wide spectrum of activity. This ability of E. aerogenes may protect apple trees from infection by other, soil microorganisms.     

An Analysis of Growth Rates in the Early Life of Apple Trees

A long-term continuing experiment to study the effect of rootstockand pruning on composite apple trees is described. Cox's OrangePippin was worked on to either M·26 or MM·104rootstocks. Two pruning regimes were applied to give a totalof four treatments. The course of whole plant growth expressed by the cumulativedry weight during the first 9 years is described by means ofgrowth curves. Special reference is made to the growth and functionof the perennial structure, and its contribution to whole plantgrowth is quantified by means of the parameter Unit ProductionRate. This expresses the rate of growth per unit weight of theperennating structure. Unit Production Rate declined with increasingtree age; in the first 5 years spur pruning and invigoratingrootstock treatments gave the highest values whilst later thissituation was reversed. The efficiency of the process of perennation is quantified interms of the production rate of the deciduous fraction per unitweight of the perennating structure. In this respect dwarfingrootstock treatments were appreciably superior to invigoratingrootstock treatments only after the fifth year. Spur pruninggave lower values than minimal pruning at 7 years with spur-prunedCox/MM·ro4 alone showing a decline with increasing ageafter 5 years.     

The Responses of Young Apple Trees to Supplementary Nitrogen and their Relation to Carbohydrate Resources

Supplementary nitrogen was supplied as ammonium nitrate to pottedtrees of MM.106 apple rootstock during the summer period ofextension growth or during autumn after the end of shoot extension.Effects of autumn nitrogen in the current year were small butsummer nitrogen stimulated shoot growth markedly and increasedleaf-area duration. Neither total root growth nor relative carbohydratecontent were reduced by the treatments. Indeed no direct effectof treatment on relative carbohydrate content was observed.Although a smaller fraction of the total photosynthate was utilizedfor root growth when shoot growth was stimulated by extra nitrogen,total dry-matter production on the basis of available lightenergy intercepted was greater or more efficient when growthwas increased. Additional nitrogen, however, did not preventa seasonal decline in the apparent efficiency of utilizationof incident light energy.     

A Role for Glutamine Synthetase in the Remobilization of Leaf Nitrogen during Natural Senescence in Rice Leaves

Changes in the levels of cytosolic glutamine synthetase (GS1) and chloroplastic glutamine synthetase (GS2) polypeptides and of corresponding mRNAs were determined in leaves of hydroponically grown rice (Oryza sativa) plants during natural senescence. The plants were grown in the greenhouse for 105 days at which time the thirteenth leaf was fully expanded. This was counted as zero time for senescence of the twelfth leaf. The twelfth leaf blade on the main stem was analyzed over a time period of −7 days (98 days after germination) to +42 days (147 days after germination). Total GS activity declined to less than a quarter of its initial level during the senescence for 35 days and this decline was mainly caused by a decrease in the amount of GS2 polypeptide. Immunoblotting analyses showed that contents of other chloroplastic enzymes, such as ribulose-1,5-bisphosphate carboxylase/oxygenase and Fd-glutamate synthase, declined in parallel with GS2. In contrast, the GS1 polypeptide remained constant throughout the senescence period. Translatable mRNA for GS1 increased about fourfold during the senescence for 35 days. During senescence, there was a marked decrease in content of glutamate (to about one-sixth of the zero time value); glutamate is the major form of free amino acid in rice leaves. Glutamine, the major transported amino acid, increased about threefold compared to the early phase of the harvest in the senescing rice leaf blades. These observations suggest that GS1 in senescing leaf blades is responsible for the synthesis of glutamine, which is then transferred to the growing tissues in rice plants.