Plant architecture of Paspalum vaginatum schwartz modified by nitrate and ammonium nutrition

A arquitetura de Paspalum vaginatum schwartz modificada pela nutrição de nitrato e amônio

FISIOLOGIA VEGETAL

Plant architecture of Paspalum vaginatum schwartz modified by nitrate and ammonium nutrition¹

José Beltrano², Marta Guillermina Ronco³, Roberto Barreiro⁴ and Edgardo Raúl Montaldi⁵

ABSTRACT - Paspalum vaginatum Schwartz plants were grown under greenhouse conditions in a continuous-flow hydroponic culture, containing NO₃^- or NH₄⁺or NH₄NO₃ as nitrogen source. After 30 days, the size of aerial biomass and root system decreased significantly when plants were supplied with NH₄⁺as exclusive nitrogen source. Compared to NO₃^- treatment, reducing and non-reducing sugars were decreasing together with a significant increase in amino acids content. NH₄⁺-nutrition caused tillers to grow toward an orthogravitropic position (average angle of 68° with respect to the horizontal), and with NO₃^--nutrition, tillers tended to become diagravitropic (average angle of 23°). With NH₄NO₃ all the parameters measured had values in between those of the other two sources. Thus, the morphologic differences among plants growing in NO₃^- or NH₄⁺ nutrition confirm the hypothesis that nitrogen source determines the growth habit of tillers in P. vaginatum by modulating the endogenous levels of reducing-non-reducing sugars.

Index terms: nitrogen source, orthogravitropism, diagravitropism, sucrose.

A arquitetura de Paspalum vaginatum schwartz modificada pela nutrição de nitrato e amônio

RESUMO - Plantas de Paspalum vaginatum Schwartz foram cultivadas em casa de vegetação, em cultura hidropônica de fluxo contínuo, contendo NO₃^-, NH₄⁺ ou NH₄NO₃ como fontes exclusivas de nitrogênio. Após 30 dias, o peso da matéria seca da parte aérea e da raiz diminuiu significativamente quando as plantas receberam NH₄⁺. Comparando esse tratamento com NO₃^-,ocorreu decréscimo no teor de açúcares redutores e não-redutores e aumento significativo no conteúdo de aminoácidos. A aplicação de NH₄⁺ causou crescimento das hastes na direção ortogravitrópica (ângulo médio de 68° em relação à horizontal), e nas plantas que receberam NO₃^-_, as hastes tendiam a tornar-se diagravitrópicas (ângulo médio de 23°). Com NH₄NO₃, todos os parâmetros medidos apresentaram valores entre os das outras duas fontes. As diferenças morfológicas entre as plantas nutridas com NO₃^- ou NH₄⁺confirmam a hipótese segundo a qual a fonte de nitrogênio determina o hábito de crescimento das hastes em Paspalum vaginatum ao regular os níveis endógenos dos açúcares redutores e não-redutores.

Termos para indexação: fonte de nitrogênio, ortogravitrópico, diagravitrópico, sucrose.

INTRODUCTION

The growth habit of grasses such as Cynodon dactylon, C. plectostachyum and Paspalum vaginatum is determined by their endogenous sucrose content. This means that high concentrations of non-reducing sugars determine the diagravitropic growth of tillers; moreover plants tested in environments that diminish their sucrose content, the growth habit of tillers tends to become orthotropics (Montaldi, 1969, 1970, 1973, 1974; Willemoës et al., 1988).

Gnanam et al. (1980) demonstrated that completely inhibits light activation of two enzymes of photosynthesis, resulting in an accumulation of photosynthetic PGA which is metabolized via pyruvate and Krebs cycle. This leads to an increase in the availability of carbon skeletons for amino acid biosynthesis. Ammonium ions that enter the plant must be immediately assimilated to avoid toxic effects on plant metabolism.

In higher plants, enrichment increases the flow of newly fixed carbon into TCA-cycle intermediates and amino acids while decreasing the carbon flux into sucrose and starch (Elrifi & Turpin, 1986). In seedling plants supplied with ^, Goyal et al. (1982) and Mehrer & Mohr (1989) observed that the carbohydrate reserves quickly disappeared, so that proteins and lipids are used as respiratory substrates.

Different forms of inorganic nitrogen elicit distinct morphogenic effects on plants (Aspinall, 1961; McIntyre, 1971, 1972; Betria & Montaldi, 1976; Leakey et al., 1978; Montaldi et al., 1984) and also act as important regulators of photosynthetic carbon flow (Elrifi & Turpin, 1986).

McIntyre (1965) found in plants of Agropiron repens that, by varying the nitrogen supply, it was possible to control the behavior of rhizome buds. In this regard the buds ceased growth at low nitrogen concentration, whereas at high nitrogen concentration, the buds grew out as branches.

Nitrogen supplied as NH₄NO₃ to the rhizomes of A. repens, attached to the parent plant, caused the apical bud to develop as a tiller instead of rhizomes (McIntyre, 1972). Moreover, rhizomes growing with low nitrogen supply showed higher carbohydrate content than those growing in a high supply. Montaldi (1970) has shown that or urea opposed the morphogenic effect of sucrose on diagravitropism of C. dactylon.

In this work was tested the hypothesis that nitrogen nutrition determines the growth habit of tillers in P. vaginatum by modulating the endogenous concentrations of reducing-non-reducing sugars.

MATERIAL AND METHODS

Plant material and growth condition

Plants of P. vaginatum were obtained from phytomers (node with attached leaf, the subtending internode and the axillary bud) rooted on moist vermiculite in a growth chamber at 22±2°C. During 10 days pretreatment period, seedlings were grown into continuously aerated hydroponic culture units that received a half-strength nutrient solution containing 1.0 mM , 0.25 mM H2, 1.25 mM K⁺, 0.5 mM SO₄^2-, 0.25 mM Ca²⁺, 0.25 mM Mg²⁺, 19 mM B, 7.2 mM Cl, 3.7 mM Zn, 0.13 mM Cu, 0.05 mM Mo and 10 mM Fe as Fe-Sequestrene (Chaillou et al., 1994) (complete nutrient solution) until the start of the treatments. Seedlings with roots between 5 and 10 cm long were transferred into 200 mL tubes with continuously-flow hydroponic culture (1 plant per tube) and placed in opaque boxes to avoid the effect of light on roots.

The experiment consisted of four treatments on 12 plants each, arranged in a completely random design; the treatments were as follows: (a) Complete nutrient solution; (b) Complete nutrient solution (without ) containing 1.0 mM , as NH₄Cl ; (c) Complete nutrient solution (without ) containing 1.0 mol m^-3 , as (NH₄)₂SO₄ ; (d) Complete nutrient solution containing 0.5 mM plus 0.5 mM , as NO₃NH₄. Except for SO₄²⁺, which was used as the variable ion, concentrations of other nutrients were the same for treatments solutions as for the pretreatment solution, and the pH was adjusted to 6.4. The culture solution was replaced with fresh at 2-day intervals.

The plants were grown in a greenhouse with a photoperiod of 14 hours and day/night temperatures of 24±5/17±5°C. The photosynthetic photon flux at plant height was about 1,400 µmol m^-2 s^-1, measured at noon with a Licor Li-1000 Data Logger.

The plants were grown for 4 weeks after starting the treatments.

Measurements

Plant parts were separated into shoots (stems and leaves) and roots. Shoots and roots were dried at 80°C, 72 hours, for dry weight (DW). The number of tiller, stolons and internode stolon lengths were determined. Average tiller angle with respect to the horizontal was measured with a protractor.

Each sample (1 g fresh weight) was ground with mortar and pestle and extracted in 5 mL of methanol-chloroform-water (MCW). Total amino acids were evaluated by the method of Yemm & Cocking (1955). The method of Bradford (1976) was used for protein determinations using bovine serum albumin as the protein standard. Total nitrogen was measured by micro-Kjeldahl digestion and the determined by the Nessler method, as described by Linder & Harley (1942). Reducing and total sugars were analyzed by the Somogy method (Cronin & Smith, 1979), in 95% (v/v) ethanol extracts. Respiration rates were measured in shoots, with a Gilson differential respirometer.

The effects of the different treatments were determined by analysis of variance (ANOVA). Differences among treatments means were analyzed by the LSD at 0.05 probability level.

RESULTS AND DISCUSSION

The aerial and root biomass showed a sharp reduction in plants treated with , mostly by decreasing shoot growth compared to root growth. Plants suppling with NH₄Cl and (NH₄ )₂SO₄ decreased the shoot growth by 65% with respect to , whereas, NH₄NO₃ decreased the shoot growth by 57%. Roots growth decreased by 56%, 56% and 49%, respectively (Fig. 1). In a young leaf, where rapid photosynthesis occurs, reduction consumes a large quantity of reducing power. In view of the increased requirements for photochemical energy in reduction, one might expect -N plants to grow better than -N plants. However, the -supplied plants generally exhibit less growth than -N plants. Chaillou et al. (1994); Raab & Terry (1994); Rideout et al. (1994), have also observed this detrimental effect of compared to nutrition in Beta vulgaris and in soybean. Other researchers have found that nutrition decrease dry matter (Lewis & Chadwick, 1983; Lindt & Feller, 1987; Salsac et al., 1987; Ota et al., 1988; Lewis et al., 1989).

Fig. 2 shows that in our experiments dark respiration was 56% less in -N plants than it was in -N ones. Elrifi & Turpin (1986) showed that resulted in a large stimulation of dark respiration, so they proposed that the suppression of photosynthetic carbon fixation, in response to nitrogen supply, was the result of a competition for metabolites between the Calvin cycle and nitrogen assimilation. On the other hand, Gnanam et al (1980) and Giengenberger & Stitt (1991) conclude that an increase in the respiration may be due to a faster turnover of pyruvate and Krebs cycle, so speeding up carbohydrate breakdown and, as a consequence, there are more carbon chains to form amino acids.

In our experiments -N caused a significant increase in amino acids content (80.29 mmol g^-1 DW) as compared with -N plants (33.17 mmol g^-1 DW) (Fig. 3). Similar results were encountered in other species, that is the amino acids content was higher in -N plants compared to -N ones (Chaillou et al., 1986a,1986b; Rideout et al., 1994; Osaki et al., 1995). Our results are also consistent with those of Mohamed & Gnanam (1977), Platt et al. (1977), Bassham et al. (1981) and Chaillou et al. (1986a) who proposed that ion diverted the assimilated carbon mainly towards amino acids.

Although plants grew less under than in solution, their leaves accumulated nitrogen at almost equal rate. Fig. 4 shows no significant differences in the total nitrogen content. On the other hand, -N supply increased the synthesis of soluble protein (Fig. 3). These results agree with those of Raab & Terry (1994), who found that soluble leaf protein increased significantly in -N plants compared to -N.

In the experiments of this work, plants treated with , as exclusive nitrogen source, showed lower level of total sugars (8.13% shoot DW) than those treated with (11.30% shoot DW) (Fig. 4). The rapid assimilation of in glutamine led to a consumption of carbon skeletons, causing depletion of the foliar starch, sucrose and maltose (Raab & Terry, 1995), likewise Mehrer & Mohr (1989) concluded that the consumption of fixed carbon during the assimilation of could appreciably reduce carbohydrates storage. Also with -N supply, the endogenous content of nonreducing sugars decreased significantly, the Fig. 5 shows that the accumulation of non-reducing sugars was substantially less than in -fed plants (1.04% DW and 2.87% DW, respectively). Raab & Terry (1994) showed that -N plants had higher activities of sucrose synthase than -N plants, in the latter system the activity of acid invertase doubled that of -N plants.

The decrease in non-reducing sugars content with -N treatment caused the tillers to grow tending toward an orthogravitropic position (NH₄Cl = 68°; (NH₄)₂SO₄ = 45°) with longer internodes (18.9 and 19.5 mm, respectively) and less stolons. On the other hand, plants treated with -N that reached a higher content of endogenous non-reducing sugar, their tillers tended to become diagravitropic (23°), with shorter internodes (11.3 mm), and a significant increase in the stolon number per plant (2.91, -N versus 0.45, -N) (Figs. 6 and 7).

The lower sucrose content of erect shoots of -N plants, as compared with prostrate stolon of -N plants, is consistent with the hypothesis that the nitrogen nutrition and endogenous levels of reducing-non-reducing sugar determine the growth habit of tiller in P. vaginatum. Therefore, the canopy architecture of plants supplied with was strikingly different from that of plants treated with (Fig. 8). Morphogenic differences among plants as a consequence of different nitrogen sources, result from a variation in the orthogravitropic/diagravitropic ratio of the tiller growth, the length of internodes and the growth angles of shoots and stolons with respect to the horizontal. Those characteristics were found also in C. dactylon by Montaldi (1970, 1974) and in P. vaginatum by Willemoës et al. (1988). Moreover, in agreement with Kojima & Sonoike (1985) and Osaki et al. (1995), it was found that the nitrogen form affects the development and growth pattern of the plant.

CONCLUSIONS

1. The morphogenic impact of different nitrogen sources absorbed by P. vaginatum results from changes in the endogenous sucrose content.

2. Such changes determine the plant architecture, modifying the orthogravitropic/diagravitropic ratio of tiller growth.

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Publication Dates

• Publication in this collection
  14 Oct 2011
• Date of issue
  July 1999