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The relationship between mobilization of N reserves and changes in translatable messages following defoliation in Lolium temulentum L. and Lolium perenne L
Abstract
It is well established that defoliation induces changes in numerous metabolic processes in forage species, including alterations in enzyme activities and in the size of corresponding substrate pools, and mobilization of C or N compounds, these being under the control of modified source/sink relationships. Such integrated physiological responses suggest that regulation occurs at the molecular level. This hypothesis was tested with a genetically uniform line of Lolium temulentum L, an annual species, after confirming with continuous ¹⁵N labelling that the same basic processes (N uptake, N mobilization) occurred during regrowth after defoliation as previously reported in L. perenne L.
Results showed that the initial availability of N reserves fully accounted for the difference in regrowth yield. In vitro translation products of RNA extracted from different tissues of L. temulentum and L. perenne after defoliation were compared with those from intact plants, and clearly showed that mRNA profiles were finely modulated, and dependent upon both the tissue (meristem, sheath or laminae) and the time elapsed after defoliation. Changes in the pattern of translation products were detected using 2-dimensional electro-phoresis as soon as 1 h after defoliation, but the major up-or down-regulations of transcript abundance occurred between 12 h and 72 h after shoot removal. The results suggested that the early appearance of new messages (< 1 h) may be involved in the regulation of medium-term (>12 h) physiological adjustments to defoliation. These medium-term modifications may be functionally related to post-defoliation changes in enzyme activities and kinetics reported in the literature.
... Defoliation (removal of leaves) is practiced by the Northern Indians since old times for the use of leaves as a green vegetable, condiment and spice without knowing its impact on subsequent plant morphological and physiological changes. Research on other crops has shown that defoliation affects growth and mobilization of carbon and N compounds, and influences the total carbon input and determines the biomass accumulation as new source-sink relationships are established between the organs remaining after defoliation and the regrowing leaves of Lolium temulentum and Lolium perenne (Ourry et al. 1996). Since the pathways of carbon and N assimilation are linked in higher plants (Evans and Terashima 1988; Pace et al. 1990), carbon assimilation provides the driving force for N assimilation. ...
... Since the pathways of carbon and N assimilation are linked in higher plants (Evans and Terashima 1988; Pace et al. 1990), carbon assimilation provides the driving force for N assimilation. Therefore, internal cycling of carbon and N compounds is of great importance during plant development , and also a pre-requisite for regrowth after defoliation (Ourry et al. 1996). Thus, the success of a plant within a given environment is dependent on the compromise within the carbon and N interaction (Foyer et al. 1995). ...
Mustard is an important oilseed crop of the tropical and subtropical regions of the world. The languid production of oilseeds has converged the attention of agricultural scientists to innovate and implement methods for boosting yield using agricultural, physiological and biotechnological tools. Among other factors, the use of plant growth regulators and management of plant canopy have a proven potential in modulating plant function and improving nutrient use and the source-sink relationship. The present review discusses the factors and the prospects of employment of gibberellic acid and removal of lower leaves as a tool for improving the source-sink relationship in mustard.
... Remobilization of N from shaded leaves at the bottom of the canopy to well illuminated leaves at the top occurs as the crop LAi develops (Lemaire et al., 1991;section 2.1.2). For regrowth after defoliation, plants remobilize organic N stored in roots and stubbles (Avice et al., 1996(Avice et al., , 1997Ourry et al., 1996). in these species, perenniality is related to the capacity to restore N root reserve during regrowth . ...
Les espèces prairiales sont soumises à des défoliations successives qui réduisent fortement la quantité de CO2 assimilé et l'absorption d'azote. Afin d'assurer sa croissance après la coupe, la plante mobilise vers les tissus aériens en croissance certains substrats carbonés et azotés stockés dans les tissus laissés en place par la coupe. Pour assurer la productivité et la pérennité du couvert par une bonne gestion, il importe de bien identifier ces mécanismes d'adaptation. Les substrats carbonés et azotés mis en réserve sont disponibles dans les tissus laissés en place par la coupe (racines, pivots, tiges, stolons, base des feuilles). Différentes expérimentations sur ray-grass anglais, trèfle blanc, luzerne ont permis d'évaluer le rôle de ces réserves mobilisées vers les tissus en repousse. Les incidences actuellement connues des modes de défoliation (selon leur rythme et leur intensité) et de certains facteurs environnementaux sur ces réserves sont abordées. Chez les légumineuses, il apparaît qu'une plus faible disponibilité en réserves azotées au moment de la coupe se traduit par une réduction du potentiel de repousse alors que chez certaines graminées une compensation intervient suite à une augmentation de l'absorption d'azote par la plante.
This study examined the effects of fertilizer application and plant density on the yield of the first and second-cut forage sorghum. The tiller development (tiller numbers and mean tiller weight) of two different cultivars, one has few large tillers (FS401 R) and the other has many small tillers (SX 11) was compared. The dry matter (DM) yield of the first-cut sorghum increased with high-density plot (14.28 plants m^<-1>) under high fertilizer application (360 kg N, 360 kg P_2O_5, 360 kg K_2O ha^<-1>) in cultivar FS 401 R, but DM yield incultivar SX 11 was not affected by density. In cv. FS401 R, the mean tiller weight of main stem in high-density plot was not different from that in low-density plot (4.76 plants m^<-2>), in spite of having more tiller numbers. In cv. SX11, the decrease in mean tiller weight occurred largely by an increase in the tiller numbers with high-density plot and higher fertilizer application. These results showed that the mean tiller weight of main stem affected on the DM yield of the first-cut sorghum. DM yield of the second-cut sorghum was higher in low-density plot than high-density plot in both cultivars. Much of the tiller in the low-density plot consisted of tillers originating from the lateral buds of daughter tillers in the stubble, and these survived tillers contributed to yield by increasing the mean tiller weight. On the other hand, much of the tillers in the high-density plot were of tillers originating from the lateral buds of main stem. More of these lateral buds were released at early period of regrowth, but most of them died before 43 days after cutting. The second-cut DM yield in the high-density plot was less than that in low-density plot. Therefore, the second-cut DM yield depended on the productivity of tillers originating from the lateral buds of daughter tillers in stubble. We considered that low density planting under high fertilizer application produced a higher yield in the forage sorghum's ratoon.
Post-defoliation carbohydrate stores, leaf area and the number of active meristems are important factors affecting the subsequent regrowth of grasses. Defoliation height affects the magnitude of all these factors. Timothy (Phleum pratense L.) and meadow fescue (Festuca pratensis Huds.) are the two most common pasture species in Finland, but little is known about their response to defoliation height. In this study the effect of three defoliation heights, 3, 6 and 9 cm, on the regrowth rates of timothy and meadow fescue in both the generative (June-July) and vegetative (August) phases of growth were examined in two one-year experiment in year 2000 and 2001. In addition, the main post-defoliation parameters were measured and their contributions to regrowth were studied. In June-July 2000 the regrowth rates, kg dry matter ha(-1) d(-1), of both species increased linearly by 10% by increasing the cutting height from 3 to 9 cm. In August 2000 the regrowth rates increased by 27% and the cumulative regrowth dry matter yield increased by 29%. In 2001 the defoliation height had no effect on the regrowth rates but the cumulative regrowth yield increased by 10% by increasing the cutting height. Meadow fescue produced 8-21% higher cumulative regrowth yields than timothy. In the reproductive phase, the regrowth rate of timothy is dependent on the population density of vegetative tillers but for meadow fescue population density did not have such importance. In vegetative phase there was no single factor essential for regrowth rates of either of the species.
Forage is the primary feed of the horse; it normally comprises more than 50% of the horse’s diet on DM basis and it may supply 100% of the ration of many horse categories. Grasses are well adapted to frequent defoliation and to the presence of large herbivores, and consequently they cover, globally, large areas of natural and seminatural vegetation, and are also widely used in intensive forage production. As the Graminae is one of the largest plant families — over 600 genera — there is a large variation in the physiological mechanisms by which grasses react to environmental variables and management factors. In addition, humans utilize grass yield in several ways, i.e. as grazed grass, silage, haylage or hay. Consequently, crop physiology behind the grass yield is vast. In this paper we first describe on outline of plant physiology of grasses and main differences between C3 and C4 grasses as well as difference between forage legumes and grasses. Secondly, we focus on plant physiology of grasses that is of most relevance in the context of equine nutrition. We present an outline of key processes affecting the digestibility of grasses, since digestibility is the most important single feature of forage affecting the nutritive value. In addition, we cover fructan metabolism in grasses and its consequences in producing pasture, silage, haylage and hay. We also present a summary of effects of nitrogen on the production and nutritive value of grasses. We conclude that knowledge of plant physiology provides tools to understand the changes in forage quality and quantity. Based on this knowledge we have tools to choose the most proper management options in order to produce high quality forage for different types of horses.
A data base was constructed of the % N and plant d. wts (W) in t ha⁻¹ of C3 and C4 crops that had been grown with sufficient nitrogen to permit maximum growth rate. The % N of all crops declined sharply with increase in W but this decline differed between C3 and C4 crops. When W was greater than I t ha⁻¹, 86% of the variance in In % N was removed by the model % N = aW−b with b = −0·5 for all crops, and a = 5·7% for C3 crops and 4·1 % for C4 crops. The same model gave a good description of data on C3 and C4 crops entirely independent of that used for developing the model. According to this relationship the fractional decline in % N with increase in plant mass was the same for both types of crops, but C4 crops contained about 72% of the nitrogen in C3 crops at equivalent d. wts. As approx. 32% more dry matter was produced per unit of intercepted radiation for C4 and C3 crops, the N uptake (or weight of plant protein produced) per unit of intercepted radiation was approximately the same for both types of crops.
A small improvement in the degree of fit to % N = aW−b was obtained by allowing both a and b to vary with the crop. Values of b obtained in this way for tall fescue, lucerne and winter wheat, but not for potato and sorghum, were consistent with Hardwick's ‘skin core’ hypothesis (Annals of Botany, 1989, 60, 439–46). The entire data set was, however, consistent with Caloin and Yu's model (Annals of Botany, 1984, 54, 69–76) in which there is a conceptual N pool for photosynthesis and another N pool for the other processes.
Two different methods of 15N labelling (continuous or pulse-chase) showed that during the first 6 days of perennial ryegrass re-growth, 70 % of foliar protein nitrogen comes from organic nitrogen mobilization of both roots and stubble, while it represents no more than 45 % after 2 weeks. The similarity of changes in 15N labelling of amino acid and protein pools suggests that amines and amino acids are the main transport compounds in nitrogen export from source organs. At the same time, proteolysis in roots and stubble is confirmed by the decrease of soluble protein content of about 40 and 45 %, respectively. Re-growth of new leaves is associated with a significant decrease of endopeptidase and aminopeptidase activities. It is suggested that decreasing protein turnover could be a means by which new re-growing leaves could restrict their energetic requirements. The rapid protein mobilization in source organs is associated with a significant increase of endopeptidase absolute activity in roots, while it remains unaffected in stubble. Specific activities of source organ peptidases (endo-, amino- and carboxypeptidases) increased during the first days of re-growth, with the exception of those of aminopeptidase in stubble. This particular pattern of proteolysis and protein mobilization is discussed in relation to other well known schemes involving source/sink relationships in plants.
Nitrogen re-mobilization during ryegrass (Lolium perenne L.) re-growth was studied as a function of time. Roots and stubble compounds were labelled with 15N before clipping. Experimental results clearly showed that ryegrass re-growth involves two periods of differing physiological
significance. The first occurs during the first 6 d when nearly all the nitrogen of the new leaves comes from organic nitrogen
re-mobilized from roots and stubble. At the same time, the uptake of nitrogen from the medium is very low, no reduction of
endogenous nitrate occurs and proteolysis and amino amido nitrogen re-mobilization in stubble are not fully compensated for
by synthesis.Thus, the change in protein-N content, expressed per plant, in stubble decreases from 998 μg (uncut plant) to
866 μg and 661 μg for 2 d and 4 d respectively after cutting. The second period (after the sixth day) may be more representative
of stable behaviour of ryegrass, with assimilation of mineral nitrogen from the medium providing the predominant source of
nitrogen for re-growth.
Methods used to construct whole-plant models of nitrogen translocation are reviewed. Translocation of nitrogen in phloem and xylem are examined in relation to the overall nutrition of a plant. Partitioning of nitrogen within shoots is influenced by the extent to which nitrogenous solutes are transferred laterally (xylem-to-phloem; xylem-to-xylem) and by the extent of nitrogen cycling in leaves and roots. Considered in the context of the whole plant these processes contribute to the maintenance of an appropriate balance of carbon and nitrogen for supply to developing apices and seeds. Floral structures also play an important role in seed nutrition. It is argued that control of nitrogen partitioning occurs within the shoot and that transfer of amides between xylem and phloem, and cytokinins translocated from roots may play an important role in partitioning of nitrogen.
Glucose, fructose, sucrose and fructans were the main non-structural carbohydrates of perennial ryegrass. Fructans accumulated predominantly in stubble, i.e. in the basal part of the leaves. Regrowth after defoliation involved two different phases. The first one (0–6 days) was characterized by the mobilization of carbohydrates in both roots and stubble to sustain foliage development in the absence of current photosynthate. During the second phase (6–29 days) carbohydrate content rose to the values observed prior to cutting. Concomitant with a decrease in soluble protein and in non-structural carbohydrate contents, sucrose-sucrose-fructosyltransferase (SST) activity decreased during the first 6 days in roots and stubble whereas fructan exohydrolase (FEH) activity increased and invertase activity was high. Prior to the subsequent increase in carbohydrate content, sucrose synthase activity increased sharply while sucrose phosphate synthase showed only slight variation. During the second period of regrowth FEH declined and SST increased. In regrowing leaves, however, fructan accumulation was not paralleled by an increase in SST activity. Results are interpreted and discussed in terms of source-sink relationships and carbohydrate partitioning.
Nitrogen re-mobilization and changes in free amino acids were studied as a function of time in leaves, stubble, and roots during ryegrass (Lolium perenne L.) re-growth. Experiments with N-15 labelling clearly showed that during the first days nearly all the nitrogen in new leaves came from organic nitrogen re-mobilized from roots and stubble. On the days of defoliation, stubble had the highest content of free amino acids with 23 mg per g dry weight against 15 mg and 14 mg in leaves and roots, respectively. The major amino acids in leaves were asparagine (23% of total content in free amino acids), aminobutyrate, serine, glutamine, and glutamate (between 7% and 15%) whereas in roots and stubble the contribution of amides was high, especially asparagine (about 50%). Re-growth after cutting was associated with a rapid increase of the free amino acid content in leaves, with a progressive decrease in roots while stubble content remained virtually unchanged. In leaves, asparagine increased from the first day of re-growth, while the aspartate level remained unchanged and glutamine increased strongly on the first day but decreased steadily during the next few days of re-growth. Asparagine in stubble and roots changed in opposite directions: in stubble it tended to increase whereas in roots it clearly decreased. In contrast, stubble and roots showed a similar decrease in glutamine. In these two plant parts, as in leaves, aspartate remained at a low level. Results concerning free amino acids are discussed with reference to nitrogen re-mobilization from source organs (stubble and roots) to the sink organ (regrowing leaves).
An existing system of flowing solution culture, in which pH and the concentration of several nutrient ions are automatically monitored and controlled, has been extended to include NH4+ at 10 mmol N m-3. A brief account is given of the use of the equipment with a simulated sward of perennial ryegrass (Lolium perenne L.). Uptake of NH4+ measured over three successive days in June, varied with daily solar radiation and exceeded 1000 mg N m-2 d-1. The uptake of NH4+ showed a pattern of diurnal variation similar to the variation in solar radiation, but with a lag period for uptake of 5 h. Hourly uptake rates ranged from 32 to 67 mg N m-2h-1 and solar radiation from 0 to 2.8 MJ m-2 h-1. During a 24 h period, additional measurements were made of K+ uptake and net H+ efflux, both of which showed patterns of diurnal variation with lag periods of 6 h hand 7 h, respectively. The stoichiometric ratio of the sum of NH4+ and K+ to the net efflux of H+ was 1.02: 1.
Experiments with simulated swards of perennial ryegrass (Lolium perenne L.) grown in flowing nutrient solution with NO3- held at 0.1 mg N I−1 show that the rate of NO3- uptake was related to diurnal, day-to-day, and seasonal changes in radiation. In summer the diurnal variation in NO3-uptake ranged from 25 to 50 mg N m−2 h−1 and the day-to-day variation ranged from 500 to 1500 mg N m−2 d−1. Mean daily rates of uptake over 12 d periods in summer and in winter averaged 908 and 44 mg N m−2, respectively. The pattern of NO3- uptake followed that of CO2 flux with the maximum rate of the former occurring 5 or 6 h after the maximum CO2 influx. After defoliation, NO3- uptake was severely curtailed for 2 d concomitant with a very small influx of CO2. Analysis of the changes that occurred in the rate of NO3- uptake immediately after the switching on or off of artificial light suggests that two reversible processes may be involved
in the relation between NO3-uptake and radiation, one with a longer and the other with a shorter time constant.
Miniswards of Lolium perenne, Festuca rubra, Agrostis casteUana and Poa trivialis were grown in a sand-limestone mixture and fed with a complete nutrient solution containing 3 mol m−3 NH4NO3. The miniswards were cut weekly to a height of 4 cm until a steady state of growth, based on weight of clippings removed, was achieved. After clipping (day 0) the nutrient solution was replaced by one in which all the nitrogen (N) was enriched with 15N to 5 atom per cent, but was otherwise identical. This allowed discrimination of remobilization from current root uptake when considering the supply of N for regrowth of laminae. Destructive harvests were made over the following 28-d period, while unharvested plants continued to receive a weekly clip.
The proportion of the total uptake of labelled N which appeared in the clipped material was of the order L. perenne > P. trivialis > A. castellana > F. rubra. The change in unlabelled N in the roots plus leaf bases over time was modelled as a proportion of the amount present on day 0. A simple exponential model with a non-zero asymptote was found to model the data; the asymptote was assumed to represent the proportion of unlabelled N not readily available for remobilization. Species differences in the asymptotes were found, with F. rubra having a significantly larger asymptote than A. castellana and P. trivialis, which in turn both had significantly larger asymptotes than L. perenne.
All species used both root uptake and remobilization to supply N for post-defoliation regrowth of laminae. However, the relative contribution of each source was species-dependent. The proportion of N in the clipped material derived from remobilization was significantly greater for F. rubra than A. castellana, which in turn had a significantly greater proportion derived from remobilization than P. trivialis and L. perenne.
A system is described for growing plants in flowing solution in which the concentration (activity) of some nutrient ions is
continuously monitored and held almost constant by means of ion-selective electrodes which control the operation of nutrient
pumps. By recording the concentration of a given ion in the flowing nutrient solution and the amount of that ion which is
delivered by the nutrient pumps, it is possible to follow nutrient uptake over long and short periods. A brief account is
given of the operation of the system using a nitrate ion-selective electrode to examine aspects of uptake by perennial ryegrass (Lolium perenne L.).