PLANT ARCHITECTURE OF PASPALUM VAGINATUM SCHWARTZ MODIFIED BY NITRATE AND AMMONIUM NUTRITION

Paspalum vaginatum Schwartz plants were grown under greenhouse conditions in a continuous-flow hydroponic culture, containing − 3 NO or + 4 NH or NH4NO3 as nitrogen source. After 30 days, the size of aerial biomass and root system decreased significantly when plants were supplied with 4 NH as exclusive nitrogen source. Compared to − 3 NO treatment, reducing and non-reducing sugars were decreasing together with a significant increase in amino acids content. 4 NH -nutrition caused tillers to grow toward an orthogravitropic position (average angle of 68° with respect to the horizontal), and with -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 − 3 NO or + 4 NH 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.


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 − 3 NO tested in environments that diminish their sucrose content, the growth habit of tillers tends to become orthotropics (Montaldi, 1969(Montaldi, , 1970(Montaldi, , 1973(Montaldi, , 1974Willemoës et al., 1988). Gnanam et al. (1980) demonstrated that + 4 NH 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, + 4 NH 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 + 4 NH , 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, 1971McIntyre, , 1972Betria & 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 NH 4 NO 3 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 + 4 NH 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.

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 NO , as NO 3 NH 4 . Except for SO 4 2+ , 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-chloroformwater (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 + 4 NH 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 + 4 NH , mostly by decreasing shoot growth compared to root growth. Plants suppling with NH 4 Cl and (NH 4 ) 2 SO 4 decreased the shoot growth by 65% with respect to − 3 NO , whereas, NH 4 NO 3 decreased the shoot growth by 57%. Roots growth decreased by 56%, 56% and 49%, respectively ( Fig. 1) (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 − 3 NO -N plants than it was in + 4 NH -N ones. Elrifi & Turpin (1986) showed that + 4 NH 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.
Although plants grew less under + 4 NH than in − 3 NO solution, their leaves accumulated nitrogen at almost equal rate. Fig. 4 shows no significant differences in the total nitrogen content. On the other hand, + 4 NH -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 In the experiments of this work, plants treated with + 4 NH , as exclusive nitrogen source, showed lower level of total sugars (8.13% shoot DW) than those treated with − 3 NO (11.30% shoot DW) (Fig. 4). The rapid assimilation of + 4 NH 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 + 4 NH could appreciably reduce carbohydrates storage. Also with + 4 NH -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 − 3 NO -fed plants (1.04% DW and 2.87% DW, respectively). Raab & Terry (1994) showed that  (Figs. 6 and 7).
The lower sucrose content of erect shoots of  (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 (1970Montaldi ( , 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.   2. Such changes determine the plant architecture, modifying the orthogravitropic/diagravitropic ratio of tiller growth.    Plant Physiology, v.107, p.575-584, 1995.