Drought effects on growth and carbon partitioning in a tall fescue sward grown at different rates of nitrogen fertilization

Eur. J. Agron .. 1995. 4(1). 91-99

Drought effects on growth and carbon partitioning in a tall fescue sward grown at different rates of nitrogen fertilization B. Qnillon, J.-L. Durand *, F. Gastal and R. Tournebize

1

1

Station d'Ecophysiologie des Plantes Fourrageres, INRA Centre Poitou-Charentes, 86600 Lusignan, France Present address: Station Agropedoclimatique, INRA, Petit Bourg BP 1232, 97185 Pointe Ii Pitre, Guadeloupe, France

Accepted 9 September 1994

* Abstract

To whom correspondence should be addressed.

The effects of drought on plant water and nitrogen status, shoot growth and carbon partitioning between root and shoot, were studied during two summer regrowth cycles of a tall fescue sward grown at different rates of application of nitrogen fertilizer. Nitrogen fertilizer was the main factor influencing the relative partitioning of the newly assimilated 14c. Drought did not prevent the expression of the effect of nitrogen fertilization on carbon partitioning. The highest carbon partitioning coefficients to roots (15-30 per cent) were measured in swards having the lowest nitrogen status. With the high nitrogen fertilization rate, water shortage increased the carbon partitioning to roots from 6 to 13 per cent (means for both regrowths, before rewatering). This increase mainly occurred in the surface horizon of soil (0-20 cm). However, an estimate in absolute terms showed that root growth was not increased by drought. Water shortage impaired the sward nitrogen status. It is therefore proposed that at least part of the change in the carbon allocation pattern induced by drought was mediated via an effect on nitrogen nutrition. The importance of the drought-induced modification of carbon partitioning is discussed. Key-words: 14C labelling, carbon partitioning, Festuca arundinaceae Schreb., leaf area index, leaf water potential, nitrogen status, root, shoot.

INTRODUCTION Crop yield is function of (1) the amount of carbon fixed by the plants, which depends on the quantity of photosynthetically active radiation (PAR) absorbed by foliage and the efficiency of photosynthesis, and (2) its subsequent allocation to plant organs of economic interest (Monteith, 1977 ; Gosse et aI., 1986). Drought may strongly affect both processes, resulting in a reduced crop yield. Water shortage often reduces growth of plant organs in different proportions. In various species, the negative effect of water shortage on shoot growth is proportionately more important than the effect on root growth, resulting in an increase of the root/shoot ratio (El Nadi et aI., 1969; Meyer and Boyer, 1981 ; Wesgate and Boyer, 1985 ; Sharp et aI., 1988; Durand et ai., 1989; Jefferies, 1993). Because biomass consists mainly of carbon (C) compounds and considering that water treatments do not greatly change the C concenISSN 1161-030/195101/$ 4.001© Gauthier-Villars - ESAg

tration in the dry matter of the different plant organs, these modifications of mass ratios should reflect a shift in the allocation pattern of assimilated C. Using 14C labelling methods, it has been demonstrated that soil drought, in addition to its negative effect on photosynthesis, modified the distribution of C between the different plant organs (Wardlaw, 1967; Munns and Pearson, 1974; Silvius et aI., 1977; Constable and Rawson, 1982; Robinson et ai., 1983; Deng et aI., 1990). Mineral nutrients' must be in solution to be absorbed by roots. Therefore, soil water depletion might decrease nutrient availability to plants. This was clearly demonstrated for mineral N by Garwood and Williams (1967) and D'aoust and Tayler (1968). Consequently, soil drought will impair the actual N status of the crop as was shown by Lemaire and Denoix (1987) in tall fescue. From these observations, it was proposed that part of the decrease in shoot growth under soil drought was mediated via a negative effect

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on N nutrition. It has also been suggested that the low mineral availability under water shortage and the resulting plant deficiencies in nutrients such as P or N will increase the root/shoot ratio (Gales, 1979). Lemaire and Salette (1984) demonstrated that during the regrowth of a tall fescue sward under nonlimiting water and N conditions, there was a stable relationship between the shoot biomass (DMa) and its N concentration. The actual N nutrition status for a given experimental treatment can then be determined by comparing the actual N concentration to the optimal N concentration for a given shoot biomass. We used this approach to compare different agronomic treatments. A large root biomass remains in the soil during successive regrowth cycles. Because this root biomass corresponds to the difference between growth and senescence, root growth cannot be estimated by root biomass measurements. Instead, a 14C02 labelling technique can be used to quantify the carbon allocation to roots (Belanger et ai., 1992b). Our objectives were to study the effects of drought on N status and carbon partitioning for an established tall fescue sward, and to see whether this could explain the effect of drought on herbage yield.

MATERIALS AND METHODS Growth conditions The study was conducted at the Station d'Ecophysiologie des Plantes Fourrageres, INRA Lusignan, France (OOI5'E, 46°26'N). Tall fescue (cv. Clarine) was sown in April 1988. The experimental sward was divided into plots (2.5 x 8.75 m), arranged in 4 replicates. One summer regrowth cycle was studied in 1989 and one in 1990. Prior to their use for the study, all plots were managed uniformly. Plots received 120 kg N ha- ' during the period from sowing to the first regrowth cycle in 1989 and 25 kg N ha- ' in April 1990. Different plots were used in 1989 and 1990. The measurement period started when the plants were defoliated to a height of 5 cm (10 July 1989, 12 July 1990) referred to as day O. On day 0, two levels of N application, denoted NO (0 kg N ha- 1 in 1989 and 30 N ha- 1 in 1990) and Nl (180 N ha- 1 in both years) were factorially combined with two water treatments, irrigated (IRR) and non-irrigated (DRY), except for 10 mm after the nitrogen fertilization. Thus, 4 treatments denoted NO-IRR, NI-IRR, NO-DRY and NI-DRY were studied during 5 weeks. After that period, the dry plots were rewatered once (50 mm on 18 August, 1989 and 30 mm on 20 August, 1990) and studied for 3 and 2 more weeks, in 1989 and 1990, respectively.

B. Onillon et al.

Global solar radiation, air temperature and humidity at 2 m above ground were measured at an automatic weather station located at 300 m from the experimental site. Leaf water potential Predawn ('I)fd) and midday ('I)f".> water potentials were measured at regular intervals during the regrowth cycles on mature leaf blades with a pressure chamber, following the recommendations of Turner (1981). Three or four leaves were sampled per treatment. Plant biomass The above-ground biomass was determined weekly by harvesting shoots in two sward areas of 582 cm 2 in each plot. The leaf area of the sample was then measured with a planimeter (Li Cor 31(0). Samples were dried at 80°C for 48 hours, and weighed. The specific leaf area was computed and used for the determination of leaf area index (LAI). Using a Kjeldahl procedure, two determinations of N concentration in the harvested above ground biomass (N% : g of nitrogen for 100 g of dry matter) were made per treatment on each sampling day separately on blade and sheath fractions. The shoot N concentration was then computed combining N percentage and weight of sheath and blade fractions . Tiller density was determined at the beginning of each regrowth cycle from the above-ground harvest used for biomass determination. Carbon partitioning Carbon partitioning between shoot and root was measured by a 14C labelling technique adapted for sward conditions by Warembourg and Paul (1973) and described by Belanger el al. (I 992b). Briefly, a 0.25 m2 sward area was isolated under a closed chamber and exposed to 14C during one day. One chamber was used throughout the study so that one replicate was labelled each day. Within the chamber, the temperature was kept close to the ambient temperature. A CO 2 generating unit maintained the CO 2 concentration at 330 ± 25 ppm and produced 14C02 . The Na2C0 3 solution used to generate the CO 2 had a specific activity dependent on the expected photosynthetic activity of the sward. The specific activity of the Na 2 C0 3 solution was 4.6, 9.2, 18.5 MBq g-l C for the NlIRR, NO-IRR and both non-irrigated treatments, respectively. Seven days after labelling, shoot and root were harvested. Shoot samples were taken from two areas of 14 x 40 cm, and six soil cores down to a 60 cm depth and of 8 cm in diameter were sampled. Three soil horizons of 20 em were defined. The roots were washed and separated from the soil in each Eur. J. Agron.

93

Drought effects on growth and carbon partitioning in tall fescue

sample. In the upper horizon (0-20 cm), the roots were separated from the underground part of the tillers. All plant samples were dried at 80 DC for 48 hours, weighed and ground. Two measurements of radioactivity were made per sample, using two sub-samples of known weight. The sub-samples were burned in a oxidizer (Oxymat IN 401, Intertechnique, France), and the 14C radioactivity was determined with a liquid scintillation counter (SL30, Intertechnique). Knowing the counting efficiency, the activity was computed for each part of the plants on a land area basis. The coefficient of carbon partitioning to the roots was defined as the activity recovered in roots divided by the activity recovered in the whole plant, and multiplied by 100. The C partitioning measured in our study is the net partitioning, and does not take into account respiration .

tion of the first value in both years (Figure 1). In contrast, in non-irrigated plots, 'I'd was variable and always lower than in the irrigated plots. It ranged from - 0.2 to - 1.3 MPa, rainfalls inducing transient increases of 'I'd followed by rapid declines (Figure 1). Rewatering of DRY treatments resulted in a complete recovery of the water potential only in 1989. 'I'd remained high in the N1-DRY plots for five days after rewatering, and rapidly declined afterwards. In the NO-DRY plots, however, 'I'd remained high until the end of the experiment. In 1990, when only 30 mm were applied for rewatering, 'I'd recovery was transient in both treatments.

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1989


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For the 1990 regrowth cycle, the sum of temperatures and the cumulated incident radiation were similar to those measured in 1989 (Table 1). During the same periods however, the average maximum temperatures and incident global radiation were 3 DC and 1 MJ m- 2 higher, respectively, in 1990 than in 1989, which resulted in a 1 mm higher potential evapotranspiration.

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The predawn water potential ('I'd) in irrigated plots was stable and greater than - 0.2 MPa with the excep-

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Table 1. Mean temperatures and ETP during the dry periods studied in 1989 and 1990. Tmax, Tmin, and Tm, in DC, are daily maximum, minimum and average ([Tmax+TminJI2) respec· tively. Rg is the solar global radiation (Mf m-2 ) and ETP is the Penman evapotranspiration (mm).

dry period duration (days) Mean Tmax Mean Tmin Mean Tm Mean Rg Mean ETP Sum of Tm (0 days) Sum of Rg (MJ m,2 Vol. 4, nO 1 ' 1995

1989

1990

37

33

27.4 14.4 20.8 22.2 4.63 770 820

30.6 15.3 23. 1 23.6 5.62 762 779

Figure I. Predawn leaf water potential during the 1989 and 1990 summer regrowth cycles of a tall fescue sward, for N1-IRR (.), Nl-DRY (O), NO-1RR (e), and NO-DRY (O) (N1 : 180kg N ha- i ; NO: 0 kg N ha- i in 1989 and 30 kg N ha- i in 1990; IRR: irrigated; DRY: non-irrigated). Day 0 is the day of defoliation. The time of rewatering (Rw; 50 mm in 1989. 30 mm in 1990) is indicated by a vertical dotted line. Vertical continuous lines indicate rainfall. Values are means of three or four leaves and bars indicate one standard error.

Midday leaf water potentials ('I'm) varied according to year and treatment (Figure 2). 'I'm was always lower in the DRY than in the IRR treatments. The drought-induced difference over the whole regrowth period was approximately 0.9 MPa, irrespective of N treatment or year. In contrast, 'I'm values were lower

B. Onillon et al.

94

in 1990 (- 2.1 MPa averaged on all treatments and dates) than in 1989 (- 1.8MPa). N deficiency tended to induce lower 'I'm in both years, the average difference between Nl and NO being 0.15 MPa in the DRY treatments and 0.23 MPa in the IRR treatments.

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Days of regrowth Figure 2. Midday leaf water potential during the 1989 and 1990 summer regrowth cycles of a tall fescue sward. Treatments and symbols as in Figure 1.

Shoot growth Shoot growth was strongly affected by water and N regimes (Figure 3). In 1989, the mean shoot net growth (dry matter) was aproximately 13.0, 4.6, 4.5, 1.2 g m- 2 d-' for the NI-IRR, NO-IRR, NI-DRY and NO-DRY treatments respectively during the first 37 days of regrowth. In 1990, the net shoot biomass production was 9.1, 1.0, 2.2 g d-', for NI-IRR, NO-IRR and NI-DRY treatments. The net growth rate was near o after 20 days after cutting in the NO-IRR treatment and throughout the regrowth cycle in the NO-DRY treatment. In 1989, rewatering induced a significant shoot growth recovery in both nitrogen treatments with similar rates to those measured in previously irrigated

treatments (Figure 3). In 1990 the response of the crop to irrigation was less pronounced, the net growth of NO-DRY treatment staying close to zero. In 1990, the tiller density on day 0 was 75 per cent of that in 1989 (Table 2). This corresponded with a greater above-ground biomass on day 0 in 1989 than in 1990, the dry matter per tiller being similar in both years (Table 2). Following defoliation, this was associated with a faster LAI increase in 1989 than in 1990, when conditions were optimum (Figure 4). It took 14 days in 1989 and 26 in 1990 to reach a LAI of 4 in the NI-IRR treatment. In DRY treatments the LAI remained always lower than or equal to the LAI of the NO-IRR treatment. At the time of rewatering, it never exceeded the value of 2.6. Nitrogen nutrition status In all plots, the initial N concentrations (Figure 5) were very low (1 per cent in 1989 and 0.7 per cent in 1990). The N concentration then increased for all treatments during the first two weeks in 1989 and durEur. 1. Agron.

Drought effects on growth and carbon partitioning in taU fescue

Table 2. Tiller density. above-ground dry matter (DMa) and individual tiller weight measured at the beginning of the 1989 and 1990 summer regrowth cycles. Values are means of four treatments ± standard errors (n = 4).

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1989

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3891 ± 76 2 900± 82

1989 1990

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Figure 5. Characterization of the sward N status in the 1989 and 1990 summer regrowth cycles of tall fescue swards using the relationship between above-ground dry matter and its N concentration (g of N for 100 g of dry matter). Symbols as in Figure 1. Each year, measurements were made every 7 days from day 0 after cutting. Fo r the DRY treatments, the las! three or two values in 1989 and 1990, respectively, were obtained after rewatering. N percentage was measured on separate fractions of sheath and blade. N percentage of shoot was then computed combining N concentration with fraction weight. The average standard error of N concentration was 0.5 for leaf and 0.3 for sheath.

Days of regrowth Figure 4. Leaf area index (measured for unrolled blades) during the 1989 and 1990 summer regrowth cycles in tall fescue swards. Treatments and symbols as in Figure 1 .. bars indicate one standard error (n = 4).

ing the first three weeks in 1990 and stabilized then to a level dependent on the fertilization rate (Figure 5). For the treatments where substantial shoot growth occurred, N concentration then decreased. Before rewatering, for a given fertilization rate, the maximum N concentration was generally lower in the DRY than in the IRR treatments. In 1989 only, rewatering allowed the N concentration of the N1-DRY plots to reach the level of that in the NI-IRR plots. This occurred before any significant shoot growth. Vol. 4, n° 1 - 1995

14C

partitioning

The percentage of 14C found in roots seven days after labelling changed with time and was affected by the treatments (Figure 6). Nitrogen deficiency increased C partitioning to the roots under both irrigated and non-irrigated treatments. At the high N fertilization rate, drought increased the proportion of carbon allocated to the roots. With the NO treatment, however, there was no apparent difference between the irrigated and non-irrigated treatments. The effect of N or water deficiency on C partitioning to the roots was essentially an increase in the C allocation to the roots of the upper soil horizon (0-20 cm) (Figure 7). Following rewatering, the percentage of 14C found in roots was lower, except for the NO-DRY treatment in 1989.

96

B. Onillon et al.

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DISCUSSION The aim of the research was to analyse the combined effects of N deficiency and drought on sward productivity. The dependence of plant water status on N status and vice versa are first considered. Nitrogen nutrition effects on plant water status For the irrigated plots, no significant differences were observed in 'I'd' indicating that soil water availability was similar in both nitrogen treatments. In contrast, N deficiency lowered the leaf water potential at midday when plants transpired intensely. However, these differences were limited in time and relatively small: between 0.15 and 0.20 MPa at leaf water potentials never higher than -1.5 MPa. A similar result was reported in cotton by Radin and Parker (1979). This could be related to a decrease in root water conductance induced by N deficiency. This has been demonstrated for different species in experiments conducted under controlled conditions (Morizet and

Days of regrowth Figure 7. i4e partitioning coefficient to tall fescue roots at three depths of soil (0-20 em. 20-40 em. 40-60 em) in the 1989 summer regrowth cycle. Treatments and symbols as in Figure 1.

Mingeau, 1976; Radin and Boyer, 1982; Chapin et aI., 1988; Radin and Matthews, 1989). Drought effect on sward N status During the first 2-3 weeks of regrowth, the N concentration increased with N fertilization rate. The low initial N concentration was partly the consequence of the low N fertilization during the period preceding the measurement period. Furthermore, following defoliaEur. J. Agron.

Drought effects on growth and carbon partitioning in tall fescue

tion, the sward was composed of a high proportion of sheath with low N concentration. Water deficit negatively affected the N status. For a given above-ground biomass, N concentration was much lower for the NI-DRY treatment than for the NI-IRR treatment in both years. This was consistent with the findings of Lemaire and Denoix (1987), and was probably due to a lower N availability to the plants when the soil was dry. Following rewatering in 1989, the rapid increase in N concentration for the N1-DRY treatment before any significant increase in shoot growth supported this hypothesis. In 1990, the amount of water applied at rewatering was probably insufficient to increase the N availability. Differences of such magnitude in nitrogen status have been reported to bring about large alterations of sward productivity (Belanger et al. 1992a). Carbon partitioning between shoot and root

The NO-IRR and NO-DRY treatments induced the highest percentages of 14C found in roots. A similar observation was previously reported for different grasses under irrigated conditions, using measurements of shoot and root biomass, or 14C labelling techniques (Lemaire, 1975; Powell and Ryle, 1978; Caloin et aI., 1980; Gastal and Saugier, 1986; Belanger et aI., 1992b). Our results showed that this effect also occurred for plants characterized by low predawn leaf water potential. In accordance with this observation, Heitholt (1989) also showed that nitrogen was the main factor influencing the root :shoot ratio of winter wheat under water deficit in a controlled environment. The NI-DRY treatment increased the relative C partitioning to roots from 6 to 13 per cent (means of regrowths and before rewatering). These values are low in comparison with those reported for plants having storage organs: the taproot of lucerne (Hall et aI., 1988; Durand et aI., 1989), the stalk of sugarcane (Hartt, 1967), the grains of wheat (Wardlaw, 1967), the corm of Gladiolus grandifiorus (Robinson et aI., 1983), and the tubers of potato (Munns and Pearson, 1974). Photoassimilate translocation from source leaves is generally delayed when plants are submitted to water deficit (Wardlaw, 1967, 1969; Hartt, 1967; Munns and Pearson, 1974; Sung and Krieg, 1979; Robinson et aI., 1983 ; Deng et aI., 1990). But an alteration of the transport system delaying the translocation of assimilates toward the roots was unlikely. The fact that N modified the relative 14C-allocation to roots almost in the same proportions under non-limiting and limiting water conditions tended to support the view that water deficit did not affect photoassimilate transport. Phloem transport per se is usually not considered as sensitive to water deficit (Wardlaw, 1969; Sung and Krieg, 1979, Schurr 1991). The seven days chase Vol. 4, n° 1 - 1995

97

period was longer than in most of the experiments dealing with kinetics of assimilate export from 14C_fed leaves, which range generally. from a few hours to three days, and the export appears delayed mainly in experiments with short chase periods (Hartt, 1967; Wardlaw, 1969). In grasses and under non-limiting water conditions, photoassimilates reach their final destination during a period of three to four days after their synthesis (Michulnas et aI., 1985; Danckwerts and Gordon, 1987; Prud'homme et aI., 1993). Consequently, it was assumed that the chase period of seven days used in our study was sufficient to determine the pattern of distribution of the newly assimilated C. Therefore, the effect of drought on C partitioning in this experiment was limited and probably due to a simultaneous reduction of root and leaf growth. Indeed, the high sensitivity of grass roots to drought was earlier reported in ryegrass by Gales (1979) and Van Loo (1992). This conclusion was reinforced by the fact that the sward N status strongly depended on water regime. Consequently, the increase of C allocation to roots under drought was at least partly due to the drought-induced N deficiency, as earlier proposed by Gales (1979). Shoot growth and estimate of total biomass and root growth

An estimate of total biomass and root growth using shoot biomass and C partitioning measurements clearly showed that variation in C partitioning had a limited impact on herbage yield. In 1989 for instance, during the first five weeks of regrowth, the shoot regrowth was 482 and 168 g m- 2 for the NI-IRR and NI-DRY treatments, respectively. The average coefficients of C partitioning to the above-ground biomass for the same treatments were 93 per cent and 87 per cent. Assuming that the biomass partitioning was close to the 14C partitioning, the growth of the total plant was estimated (shoot growth divided by the coefficient of partitioning to above ground biomass) to be 512 g m-2 for the NI-IRR treatment and 193 g m-2 for the NI-DRY treatment. With the same assumption, root growth was calculated to be 31 and 25 g m-2 for the NI-IRR and the NI-DRY treatments, respectively. Similar trends appeared in 1990. Hence, despite an increase in the relative C partitioning to roots, root growth expressed in absolute terms was decreased by drought. Furthermore, the increase in the relative C partitioning to roots took place mainly in the upper soil horizon. Therefore, under our conditions, tall fescue did not produce more root biomass or deeper roots when exposed to water deficit. The sward productivity (total biomass) depended on incident radiation, interception efficiency (mainly depending on LAI) and radiation use efficiency. As the global incident radiation accumulated during the two

98

periods studied was similar (Table 1), the main processes affected were leaf area expansion and solar energy conversion. There were large differences between the two years in terms of leaf area growth. The lower growth of the NI-IRR treatment in 1990 was partly due to differences in the initial sward status. Both initial nitrogen concentration and tiller density were lower in 1990. Davies (1966) emphasized the importance of considering tiller density to explain different growth rates following a defoliation. Furthermore, climatic data clearly indicated drier atmospheric conditions in 1990, also revealed by lower -qrm' Such conditions are known to strongly limit leaf expansion and carbon assimilation (Bunce 1978, Sheehy et al. 1979). Each year N and soil water deficiencies greatly affected the leaf area expansion rate, causing the LAI (estimated for unrolled blades) to be limiting for light absorption throughout the regrowth periods. Drought induced a further decrease in functional leaf area due to leaf rolling. Indeed the leaf area exposed to radiation was reduced by as much as 50 per cent under the most severe water deficit (data not shown). The data presented here showed that under our conditions, the drought-induced reduction in herbage yield was primarily due to the reduction of the leaf area expansion and only secondarily to the changes in C partitioning to roots. This second alteration was partly a consequence of N deficiency. It was only a relative increase and did not make deeper soil resources more available.

ACKNOWLEDGEMENTS This research was supported in part by a grant from the Region Poitou-Charentes to B.Onillon. The authors would like to thank Professor. J. L. Bonnemain for allowing us to use equipment for 14C analysis, and Dr G. Belanger for helpful comments on the manuscript.

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