Roles of the fructans from leaf sheaths and from the elongating leaf bases in the regrowth following defoliation of Lolium perenne L

The study of carbohydrate metabolism in perennial ryegrass (Lolium perenne L. cv. Bravo) during the first 48 h of regrowth showed that fructans from elongating leaf bases were hydrolysed first whereas fructans in mature leaf sheaths were degraded only after a lag of 1.5 h. In elongating leaf bases, the decline in fructan content occurred not only in the differentiation zone (30-60 mm from the leaf base), but also in the growth zone. Unlike other soluble carbohydrates, the net deposition rate of fructose remained positive and even rose during the first day following defoliation. The activity of fructan exohydrolase (FEH; EC 3.2.1.80) was maximal in the differentiation zone before defoliation and increased in all segments, but peaked in the growth zone after defoliation. These data strongly indicate that fructans stored in the leaf growth zone were hydrolysed and recycled in that zone to sustain the refoliation immediately after defoliation. Despite the depletion of carbohydrates, leaves of defoliated plants elongated at a significantly higher rate than those of undefoliated plants, during the first 10 h of regrowth. This can be partly attributed to the transient increase in water and nitrate deposition rate. The results are discussed in relation to defoliation tolerance.

A rapid and reliable method for NO quantification and 15NO/14NO determination using isotope ratio mass spectrometry: an application for the detection of NO synthesis in propionibacteria

For the last decade, numerous studies have focused on the positive or toxic effects of nitric oxide (NO) in procaryotic and eucaryotic cells. This gas has fundamental roles in neurotransmission, vasodilatation, cytotoxicity, and intestinal motility. The ability to produce NO by intestinal microflora or probiotic bacteria is unknown. In this preliminary study, we present a rapid and reproducible procedure for NO quantification and 15NO/14NO determination (based on the reaction between nitrite and acidic potassium iodide) by isotope ratio mass spectrometry. Using this method, we have demonstrated for the first time in vitro production of NO by a dietary bacterium (Propionibacterium acidipropionici, Pa 1) under anaerobic culture conditions. Using different sources of nitrogen, we have clearly shown that propionibacteria can synthesize NO from reduction of nitrate or nitrite. In our experimental conditions, NO synthase was not involved in NO production by propionibacteria.

Nitrogen and Carbon Flows Estimated by 15N and 13C Pulse-Chase Labeling during Regrowth of Alfalfa

The flow of 15N and 13C from storage compounds in organs remaining after defoliation (sources) to regrowing tissue (sinks), and 13C losses through root or shoot respiration were assessed by pulse-chase labeling during regrowth of alfalfa (Medicago sativa L.) following shoot removal. A total of 73% of labeled C and 34% of labeled N were mobilized in source organs within 30 d. Although all of the 15N from source organs was recovered in the regrowing tissue, much of the 13C was lost, mainly as CO2 respired from the root (61%) or shoot (8%), and was found to a lesser extent in sink tissue (5%). After 3, 10, or 30 d of regrowth, 87, 66, and 52% of shoot N, respectively, was derived from source tissue storage compounds; the rest resulted from translocation of fixed N2. Overall results suggest that most shoot C was linked to photosynthetic activity rather than being derived from mobilization of stored C in source organs. Furthermore, isotopic analysis of different chemical fractions of plant tissue suggests that between 14 and 58% of the shoot C derived from source tissues was linked to the mobilization of N compounds, not carbohydrates.

Effects of low temperature on nitrate uptake, and xylem and phloem flows of nitrogen, in Secale cereale L. and Brassica napus L

The effect of low temperature on nitrate uptake and subsequent N translocation and cycling was investigated in Secale cereale L. and Brassica napus L. transferred to 7 °C for 9 d or kept at a warm temperature (20 °C). Some plants were grown with a split root system and ¹⁵ NO₃ ^- , labelled to measure NO₃ ^- uptake, root-to-shoot and shoot-to-root translocation of N from NO₃ ^- , taken up. Other plants with single-root system were subjected to ¹⁵ N pulse-chase labelling to quantify endogenous N remobilization. Lowering growth temperature from 20 to 7 °C reduced nitrate uptake more strongly in rye (-59%) than in winter rape (-28%). A very large proportion of the nitrate taken up was further translocated to shoots in both species. However, lowering the temperature decreased, xylem N translocation by about 60 and 30% in S. cereale and B. napus, respectively. Most root N in the latter species came directly from root absorption, while in rye roots the proportions ascribe able to direct root allocation and phloem flow were well balanced. Cold treatment did not significantly modify the imbalance between the two origins. Mobilization of endogenous N from roots to shoot, estimated by pulse-chase labelling, was limited in B. napus and significant in S. cereale despite a depressive effect of low temperature. In general, low temperature led to an increase in root N concentration in both species. It is Suggested that low temperature may directly affect the nitrate uptake system, and also that N accumulation in the roots (resulting from greater inhibition of N xylem flow than of NO₃ ^- uptake) might increase the inhibition of uptake by higher cellular NO₃ , and/or amino acid content. The usefulness (for spring growth) of root N accumulation triggered by low temperature is discussed.

Nitrogen Reserve Mobilization during Regrowth of Medicago sativa L. (Relationships between Availability and Regrowth Yield)

An experiment was designed to study the role of N and C reserves on regrowth of the shoots following defoliation of forage species. Starch and N accumulation in root and crown tissue of nonnodulated Medicago sativa L. were modified during regrowth by applying different levels of N and different cutting heights. Plants were obtained with similar crown and root dry weights, but having either low starch and high tissue N or high starch and low tissue N. The plants were then submitted to a second defoliation and supplied with optimal N nutrition, and N flow from reserve was quantified using pulse-chase 15N labeling. Maximum yields following the second regrowth were obtained from those plants having a high tissue N, despite their low level of nonstructural carbohydrate. When N in the roots and crown exceeded 5 mg N plant-1 at the beginning of regrowth, about 68% was translocated to regrowing shoots. Highly significant correlations were also found between the amounts of N available in roots and crown at the beginning of regrowth and (a) the amount of N that was mobilized to new tissues, (b) the amount of N taken up during the regrowth period, and (c) the final shoot yield after 24 d of regrowth. No similar correlations were found for plants that varied in their initial starch content of roots and crown. It is suggested that N reserves were used mainly during the first 10 d after defoliation, and that the resulting aerial growth during this period should be sufficient to restore N2 fixation and/or N uptake to levels equal to those prior to defoliation. These data emphasize (a) the importance of root N reserves in initiating and sustaining new shoot growth, and (b) the need for a re-evaluation of the contribution of C reserves to shoot regrowth.

Fructan and cryoprotection in ryegrass (Lolium perenne L.)

Changes in carbohydrate content and composition of ryegrass (Lolium perenne L.) cv. Reveille field-grown in Normandy were investigated from May 1987 to May 1988. During the winter period, the concentrations of high DP (degree of polymerization) fructan were the lowest whilst those of ethanol-soluble carbohydrates were the highest. Ethanol-soluble carbohydrates also accounted for most of the non-structural carbohydrates of four cultivars of Lolium perenne field-grown in central France in January 1989. The accumulation of osmotically active carbohydrates during winter leads only to a small depression of freezing point and consequently, these sugars cannot be considered as efficient cryoprotectants in ryegrass.

Modifications électrophorétiques des protéines solubles foliaires de Suaeda maritima var. macrocarpa, Atriplex hortensis et Phaseolus vulgaris en relation avec la teneur en NaCl du milieu de culture

The study of the growth and metabolism of young plants grown in the presence of salt enables their classification according to their increasing sensitiveness to NaCl: Suaeda maritima (L.) Dum. var. macrocarpa Moq., Atriplex hortensis L. and Phaseolus vulgaris L. The influence of salinity on the evolution of soluble leaf proteins was compared in the three plants grown on culturing solutions more or less rich in salt. Examination of electrophoregrams emphasizes the qualitative stability of the protein macromolecules. The protenome of the obligate halophyte Suaeda maritima var. macrocarpa is characterised by its richness in high molecular weight proteins. With Atriplex hortensis, a salt-resistant plant, accumulating Na+ in its aerial organs, the salt induces a progressive increase of the synthesis of a high molecular weight proteinaceous fraction. In contrast, Phaseolus vulgaris, a plant very sensitive to NaCl, does not transport Na+ into the tissues analyzed. In this species the salt does not induce a detectable change in the soluble proteins. The increased synthesis or the preservation of the high molecular weight proteins in the resistant or halophilic plants could reflect an adaptation mechanism to hypersaline conditions.