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FORAGE & GRAZING LANDS

Phosphorous Uptake and Concentration of Timothy Genotypes under Varying N Applications

Agriculture and Agri-Food Canada, 2560 Hochelaga Bd., Sainte-Foy, QC, G1V 2J3 Canada

* Corresponding author (belangergf{at}agr.gc.ca)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Improving P uptake in timothy (Phleum pratense L.) would reduce excess soil phosphate while producing forage with greater P nutritional value. A strong relationship exists between N and P concentration in plants, and we hypothesized that genotypes characterized for contrasting N concentration or uptake may also exhibit contrasting P concentration and uptake. Two timothy populations derived from divergent selection for high and low forage N concentration were studied in Experiment 1. These two populations were also studied in Experiment 2, along with seven half-sib families identified in a field study as having contrasting dry matter (DM) yield and N concentration. In both experiments, a reference population, ‘Champ’, was included and plants were grown in a growth room with varying N rates. Independent of applied N, the genotypes differed for P concentration, P uptake, and P to N ratio (P/N), in forage and total biomass, as well as for leaf P concentration, leaf weight ratio (LWR), root weight ratio (RWR) and efficiency of P uptake for each unit of root biomass (PUPE_root). Averaged across N rates in Experiment 2, forage P concentration among genotypes ranged from 4.1 to 5.1 g P kg^-1 DM, while forage P uptake ranged from 10.0 to 14.6 mg P per plant. Variations in P uptake and concentration were related to genotypic differences in DM yield, leaf P concentration, LWR, and RWR. Phosphorus concentration and uptake decreased under N stress. We conclude that variation exists among timothy genotypes for P concentration and uptake.

Abbreviations: DM, dry matter • LWR, leaf weight ratio • P/N, P to N ratio • PUPE_root, P uptake efficiency by root • RWR, root weight ratio

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
EXCESS PHOSPHATE IN THE SOIL is a common agricultural problem that can lead to P pollution of water bodies, primarily via soil erosion. Plants with greater P concentration, when associated with high DM yield, can uptake more P, thus decreasing the amount in soil. Through genetic selection, the forage P concentration of high-yielding genotypes could be increased to help reduce the risk of P pollution. Forages with increased P concentration can also be beneficial for high-producing dairy cows (Minson, 1990).

A strong relationship exists between N and P concentration in forage plants. The positive effect of N fertilization on P uptake by timothy was reported (Kamprath, 1987; Bélanger and Richards, 1999). Ercoli et al. (1996) observed a similar increase of P and N uptake in sorghum with P fertilization. Bélanger and Richards (1999) concluded that the optimal N concentration could be used to estimate the optimum P concentration in timothy, the most commonly grown forage species in eastern Canada.

Previous work concluded that some genetic variability exists for N concentration and uptake among high-yielding timothy genotypes, and some of this variability was associated with changes in biomass partitioning between leaves, stems, and roots (Brégard et al., 2000, 2001). To our knowledge, there are no similar studies that investigate the possibility of genetic variability of P uptake in timothy. In this study, we hypothesized that variability for P concentration and uptake exists among timothy genotypes in forage and total biomass. Our objectives were (i) to determine the P concentration and uptake of timothy genotypes known for their contrasting N concentration or uptake and grown under limiting to nonlimiting N nutrition, and (ii) to establish the relationship of P concentration and uptake with biomass partitioning in timothy. This study was conducted in a controlled environment to allow complete root recovery.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Plant Material and Growth Conditions
This study includes two experiments conducted on timothy in growth rooms. The plant material, growth conditions, and sampling procedures for these experiments are as described in Brégard et al. (2000)( 2001). The first experiment (Exp. 1) studied two populations of timothy derived from two cycles of divergent selection for high (N+) and low (N-) forage N concentration; the cultivar Champ was included as a control (Brégard et al., 2000). The second experiment (Exp. 2) involved seven contrasting half-sib families, the two previously mentioned contrasting populations (N+ and N-) and the cultivar Champ (Brégard et al., 2001). Populations and half-sib families are hereafter called genotypes. In Exp. 1, four replications of a split-plot experimental design were used with four N rates as main plots, and three genotypes randomized to subplots. In Exp. 2, four replications of a split-plot experimental design were used with two N rates as main plots, and 10 genotypes were grown in subplots.

The plants were grown in pots filled with Turface (AIMCOR, Deerfield, IL), a medium similar to vermiculite but with a higher cationic retention. Plants were grown in a growth room with one plant per pot (surface area of 100 cm²; volume of 0.4 L) in Exp. 1, and two plants per pot (surface area of 150 cm²; volume of 1 L) in Exp. 2. Lights simulated a 16-h day with a quantum flux of 350 µmol m^-2 s^-1 (photosynthetically active radiation). Day and night temperatures were kept at 25 ± 5°C and 15 ± 5°C, respectively. Twice a week, the plants received 50 mL of a complete nutrient solution. The solution contained 4.4 mM MgSO₄, 2.2 mM K₂HPO₄, 2.2 mM KH₂PO₄, 2.2 mM CaCl₂, 0.45 mM ferric citrate, and micronutrients (102 µM H₃BO₃, 20 µM MnCl₂·4H₂O, 1.683 µM ZnSO₄·7H₂O, 0.705 µM CuSO₄·5H₂O, 1.202 µM H₂MoO₄·H₂O, 0.037 µM CoCl₂·6H₂O). In Exp. 1, the four N rates were obtained by means of nutrient solutions with NH₄NO₃ at 0.35 mM (N0), 1.75 mM (N5), 3.5 mM (N10), and 7 mM (N20). This corresponded to 1 (N0), 5 (N5), 10 (N10), and 20 (N20) mg N plant^-1 wk^-1. In Exp. 2, N was limiting with 1.75 mM NH₄NO₃ and nonlimiting with 8.75 mM NH₄NO₃, corresponding to 5 and 25 mg N plant^-1 wk^-1, respectively. From seeding to the end of the first regrowth cycle, plants received a fertilization equivalent to 20 mg N per plant per week. In both experiments, plants were cut at a 5-cm height 28 d after seeding and at the end of the first regrowth cycle (60 d after seeding); in Exp. 2, plants were cut again at the end of the second regrowth cycle (90 d after seeding).

Sampling and Analyses
Plants were separated into leaves, stems (including sheaths of leaves surrounding the stems), and stubble at the end of the second regrowth cycle in Exp. 1 (90 d after seeding) and the third regrowth cycle in Exp. 2 (120 d after seeding); the majority of plants were then at the heading stage of development (stages 54–58; Simon and Park, 1981). The DM yield of each set of plant parts was determined. Total P and N concentrations of leaves, stems, stubble, and roots were determined with QuikChem Automated Analyzer (Zellweger analytics, Inc., Lachat Instruments, Milwaukee, WI), after the H₂SO₄-H₂O₂ digestion method of Kjeldahl (adapted from Richards, 1993). The forage DM yield was calculated by adding leaf and stem DM yields. The total biomass DM yield was calculated by adding forage, stubble, and root DM yields. Forage and total biomass DM yield were previously reported (Brégard et al., 2000, 2001).The forage N concentration and P concentration were estimated from stem and leaf concentrations weighted on the basis of their respective DM yields. The total biomass P concentration was estimated from forage, stubble, and root concentrations weighted on the basis of their respective DM yields. The forage P uptake was calculated as forage P concentration multiplied by forage DM yield, while the total biomass P uptake was calculated as total biomass P concentration multiplied by the total biomass DM yield. Root to total biomass DM yield ratio (RWR) was calculated as root DM yield divided by total biomass DM yield. Leaf weight ratio was calculated as leaf DM yield divided by forage DM yield. The P/N was calculated as forage or total biomass P concentration divided by their respective N concentrations. The PUPE_root was calculated as total biomass P uptake divided by the root DM yield.

Statistical Analysis
All variates were subjected to analyses of variance following the experimental design by means of the GLM procedure of the SAS Institute (Cary, NC). In both experiments, the effects of the genotypes were tested by means of orthogonal contrasts. In Exp. l, the effect of N was partitioned into linear and quadratic contrasts. Orthogonal contrasts were defined according to genotype DM yield characteristics reported in previous studies (Brégard et al., 2000, 2001). Statistical significance was postulated at P < 0.05.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Genetic Variability
Phosphorus Uptake
The P uptake differed among genotypes (Tables 1 and 2) . In Exp. 1, N+ had significantly greater total biomass P uptake than N-, although the difference in forage P uptake was not significant (Table 1). In Exp. 2, N+ had greater forage P uptake and total biomass P uptake than N-, and Genotypes 1254, 1258, 1263, 1313, and 1318 had greater forage P uptake and total biomass P uptake than 674, 709, and Champ (Table 2). However, the contrast comparing Genotypes 1254, 1258, 1263, 1313, and 1318, with Genotypes 674, 709, and Champ interacted significantly with N, both for forage P uptake and total biomass P uptake. The contrast comparing N+ with N- interacted also with applied N in Exp. 2 for forage P uptake, while the interaction was not significant in Exp. l.

The PUPE_root also differed among genotypes (Tables 1 and 2). The genotype N+ had a significantly lower PUPE_root than N- in Exp. 1, and a similar trend was observed in Exp. 2. The PUPE_root of Genotypes 1254, 1258, 1263, 1313, and 1318 did not differ significantly than those of 674, 709, and Champ in Exp. 2. The greater P uptake of N+, 1254, 1258, 1263, 1313, and 1318 compared with the other genotypes are therefore not explained by a greater ability of roots to take up P.

Phosphorus Concentration
As with P uptake, the forage P concentration and total biomass P concentration differed significantly among genotypes in both experiments (Tables 1 and 2). The genotype N+ had significantly greater forage P concentration and total biomass P concentration than N- in both experiments, independent of applied N. In Exp. 2, Genotypes 1254, 1258, 1263, 1313, and 1318 had significantly greater forage P concentration and total biomass P concentration than 674, 709, and Champ. These results indicate that variability for P concentration exists in timothy.

In addition to differences in forage P concentration among genotypes, we also noted significant differences among genotypes in the P concentration of leaves in both experiments (Tables 1 and 2). The genotype N+ had greater leaf P concentration than N-, and Genotypes 1254, 1258, 1263, 1313, and 1318 had greater leaf P concentration than 674, 709, and Champ. Therefore, the greater forage P concentration of some genotypes is partly due to a greater leaf P concentration. The stem P concentration of N+ was significantly greater than that of N- in Exp. 2, and a similar trend was observed in Exp. 1. However, the stem P concentration of Genotypes 1254, 1258, 1263, 1313, and 1318 did not differ than that of 674, 709, and Champ in Exp. 2.

The forage P concentration measured in this controlled study, primarily in Exp. 2, was markedly higher than values reported in field studies of timothy grown under nonlimiting P (Minson, 1990; Bélanger and Richards, 1999). We speculate that the soluble form of P fertilization, applied under controlled conditions, led to a greater P availability than under field conditions; this maximized the P uptake in plants. Consequently, it is worth noting that the results of our study apply to nonlimiting P conditions. Differences in P uptake among genotypes might differ under limiting P conditions. Further research on the interaction between genotypes and levels of P nutrition is required.

Relationship between P Concentration, N Concentration, and DM Yield
The P/N in forage and total biomass differed significantly among genotypes (Tables 1 and 2). The forage P/N and total biomass P/N were greater for N+ than N- in both experiments, and for Genotypes 1254, 1258, 1263, 1313, and 1318 than for 674, 709, and Champ in Exp. 2. Therefore, N+ and Genotypes 1254, 1258, 1263, 1313, and 1318 took up more P than the other genotypes for each unit of N taken up. We expected the P/N to remain relatively constant for genotypes with contrasting N concentration because of the strong relationship between N and P concentration observed in previous studies (Kamprath, 1987; Bélanger and Richards, 1999). Overall, genotypes with high N concentration also had high P concentration as expected, and this confirms the adjustment of N and P concentration in plants. The stoichiometry of both elements, however, is not constant since the P/N increased for genotypes with greater N concentration.

During a growth cycle, forage P concentration decreases with increasing DM yield (Bélanger and Richards, 1999). This suggests that genotypes with greater forage P concentration may have lower DM yield. We previously reported that N+ tended to have lower forage DM yield than N- (Brégard et al., 2000, 2001); this confirms that genotypes with greater P concentration might have lower DM yield. However, Genotypes 1263 and 1254 had both greater P concentration and forage DM yield than other genotypes tested in the same experiment. Hence, some high-yielding genotypes may also have high P concentration.

Relationship between P Concentration and Uptake, and Biomass Partitioning
After noting the genotypic variability for P concentration and uptake, we attempted to relate the differing P status to morphological differences. The RWR significantly differed among genotypes (Tables 1 and 2), indicating a greater proportion of root in total biomass for Genotypes 1254, 1258, 1263, 1313, and 1318, and for N+ (Brégard et al., 2000, 2001).

Genotypes differed significantly in PUPE_root (Tables 1 and 2), which indicates that some genotypes took up more P than others for each unit of root biomass. The genotypes with the greatest PUPE_root, however, were not necessarily those with the greatest P uptake. We conclude that differences of P uptake among genotypes were more directly related to differences in RWR than to differences in the physiological capacity of the root to absorb P as estimated by PUPE_root. Phosphorus uptake is commonly reported as being closely related to root characteristics, such as biomass, morphology, or density (Itoh and Barber, 1983; Caradus and Snaydon, 1986; Otani and Ae, 1996; Grant and Robertson, 1997). Haynes and Ludecke (1981), however, found no relationship between P uptake and total root mass in pot-grown plants of lotus (Lotus pedunculatus Cav.) and white clover (Trifolium repens L.).

In addition to RWR differences, LWR significantly differed among genotypes (Tables 1 and 2). Genotypes 1254, 1258, 1263, 1313, and 1318 had greater LWR than 674, 709, and Champ, and N+ had greater LWR than N-. Consequently, since the leaves had greater P concentration than the stems, genotypes with greater LWR tended to have greater forage P concentration. Therefore, the variability of forage P concentration among genotypes may be due in part to leaf to stem ratio variability. Similarly, Brégard et al. (2000) reported that the greater forage N concentration of some timothy genotypes was associated with a greater proportion of leaves in the forage biomass.

In this study, LWR and leaf P concentration were related to the varying P status of timothy forage. Therefore, the forage leaf proportion and P concentration may be closely related to the P uptake in timothy; that is, shoot attributes may determine P uptake in plants. Caradus and Snaydon (1986) reported that P influx in white clover is controlled as much by shoot attributes as by root attributes. Also, Salette and Huché (1991) reported that the P absorption may be directly related to the dynamic of N and C absorption and shoot metabolism.

Nitrogen Stress
Nitrogen fertilization is known to affect P uptake and concentration (Bélanger and Richards, 1999). In this study, the P availability was similar for all N rates. As we expected, N fertilization significantly increased forage P concentration, total biomass P concentration, forage P uptake, and total biomass P uptake in both experiments (Tables 1 and 2). Nitrogen fertilization also increased leaf P concentration and stem P concentration in both experiments. Bélanger and Richards (1999) and Kamprath (1987) reported an increase in P uptake when the applied N was increased in timothy and maize (Zea mays L.), respectively. Clarke et al. (1990), working on wheat cultivars, observed simultaneous modifications of N and P concentrations depending on plant maturity or environmental changes.

The N stress had a greater effect on P uptake than on DM yield. The forage DM yield under limiting N (N0) in Exp. 1 was 51% of that under the nonlimiting N (N20) (Brégard et al., 2000), but the forage P uptake was 24% of that under N20 (Table 1). In Exp. 2, the forage DM yield under limiting N averaged 38% of that under nonlimiting N (Brégard et al., 2001). The forage P uptake, however, averaged 26% of that under nonlimiting N (Table 2). The total biomass DM yield under limiting N in Exp. 2 was 65% of that under nonlimiting N, which was equivalent to Exp. 1 where total biomass DM yield at N0 was 66% of that at N20 (Brégard et al., 2000, 2001). The total biomass P uptake under limiting N was 36% in Exp. l, and 37% in Exp. 2, of those under nonlimiting N (Tables 1 and 2). Consequently, the greater effect of N stress on P uptake than on DM yield indicates that N fertilization stimulated P absorption.

The N stimulation of P absorption by plants can be attributed to numerous factors, such as an increased root growth and a greater physiological capacity of the root to absorb P (Kamprath, 1987; Wilson, 1988). Nitrogen fertilization significantly decreased RWR in both experiments (Tables 1 and 2) and root DM yield in Exp. 1 (data not shown). Under nonlimiting N, the root DM yield was 73% in Exp. l and 97% in Exp. 2 of those under limiting N. Therefore, the greater P uptake under nonlimiting N cannot be attributed to increased root growth. The PUPE_root, that is, the P uptake per unit of root biomass, increased with increasing N rates. Under limiting N, the PUPE_root was 35% in Exp. l and 27% in Exp. 2 of those under nonlimiting N. This indicates that the amount of P transferring through each unit of root biomass increased with increasing N rates. The stimulating effect of N fertilization on P uptake can therefore be attributed to the increased physiological capacity of the root to absorb P.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Timothy genotypes, under conditions of nonlimiting P availability in our study, differ in P concentration and uptake for forage and total biomass from limiting to nonlimiting N. This genetic variation in P uptake and concentration is related to genotypic differences in DM yield, leaf P concentration, the proportion of leaves in the forage, and the proportion of roots in the total biomass. Under N stress, the P concentration and uptake of timothy are reduced as a result of reduced DM yield and physiological capacity of the root to absorb P.

	ACKNOWLEDGMENTS

 
We wish to thank Dr P.G. Jefferson from Agriculture and Agri-Food Canada for critical review of the manuscript, and C. McRae from Editworks for editorial assistance.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Contribution no. 727 Agriculture and Agri-Food Canada.

Received for publication May 1, 2001.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Bélanger, G., and J.E. Richards. 1999. Relationship between P and N concentrations in timothy. Can. J. Plant Sci. 79:65–70.
• Brégard, A., G. Bélanger, and R. Michaud. 2000. Nitrogen use efficiency and morphological characteristics of timothy populations selected for low and high forage N concentrations. Crop Sci. 40:422–429.[Abstract/Free Full Text]
• Brégard, A., G. Bélanger, R. Michaud, and G.F. Tremblay. 2001. Relationships between yield, N concentration, biomass partitioning, and forage nutritive value of contrasting genotypes of timothy. Crop Sci. 41:1212–1219.[Abstract/Free Full Text]
• Caradus, J.R., and R.W. Snaydon. 1986. Plant factors influencing phosphorus uptake by white clover from solution culture. Plant Soil 93:165–174.
• Clarke, J.M., C.A. Campbell, H.W. Cutforth, R.M. DePauw, and G.E. Winkleman. 1990. Nitrogen and phosphorus uptake, translocation, and utilization efficiency of wheat in relation to environment and cultivar yield and protein levels. Can. J. Plant Sci. 70:965–977.
• Ercoli, L., M. Mariotti, A. Masoni, and F. Massantini. 1996. Effect of temperature and phosphorus fertilization on phosphorus and nitrogen uptake by sorghum. Crop Sci. 36:348–354.
• Grant, R.F., and J.A. Robertson. 1997. Phosphorus uptake by root systems: Mathematical modelling in ecosys. Plant Soil. 188:279–297.
• Haynes, R.J., and T.E. Ludecke. 1981. Yield, root morphology and chemical composition of two legumes as affected by lime and phosphorus applications to an acid soil. Plant Soil. 62:241–254.
• Itoh, S., and S.A. Barber. 1983. Phosphorus uptake by six plant species as related to root hairs. Agron. J. 75:457–461.[Abstract/Free Full Text]
• Kamprath, E.J. 1987. Enhanced phosphorus status of maize resulting from nitrogen fertilization of high phosphorus soils. Soil Sci. Soc. Am. J. 51:1522–1526.[Abstract/Free Full Text]
• Minson, D.J. 1990. Phosphorus. p. 230–264. In Forage in ruminant nutrition. Academic Press, San Diego, CA.
• Otani, T., and N. Ae. 1996. Sensitivity of phosphorus uptake to changes in root length and soil volume. Agron. J. 88:371–375.[Abstract/Free Full Text]
• Richards, J.E. 1993. Chemical characterization of plant tissue. p. 115–139. In M.R. Carter (ed.) Soil sampling and methods of analysis. Lewis Publ., Boca Raton, FL.
• Salette, J., and L. Huché. 1991. Diagnostic de l'état de nutrition minérale d'une prairie par l'analyse du végétal: principes, mise en oeuvre, exemples. Fourrages 125:3–18.
• Simon, U., and B.H. Park. 1981. A descriptive scheme for stages of development in perennial forage grasses. p. 416–418. In J. Allan Smith and V.W. Hays (ed.) Proc. Int. Grassland Congr., 14th, Lexington, KY. 15–24 June 1981. Westview Press, Boulder, CO.
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This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in Crop Science Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Bélanger, G. Articles by Michaud, R. Search for Related Content PubMed Articles by Bélanger, G. Articles by Michaud, R. Agricola Articles by Bélanger, G. Articles by Michaud, R. Related Collections Forage Management Phosphorus Other Forage Crops