Plant and Soil 111,241-244 (1988) (i) Kluwer Academic Publishers PLSO TR-41

Nitrogen and carbon utilization in shoots and roots of nitrogen-limited Pisum

PAULO DUARTE, PETTER OSCARSON, JAN-ERIC TILLBERG and CARL-MAGNUS LARSSON Department of Botany, University of Stockholm, S-106 91 Stockholm, Sweden

Key words: growth, nitrate, photosynthesis, Pisum, respiration

Abbreviations: DW, dry weight; FW, fresh weight; RGR, relative growth rate; RN, relative nitrogen addition rate

Introduction

Models of partitioning and utilization of C and N can be constructed by analysing xylem and phloem bleeding saps, as has been demonstrated e.g. in white lupin (Pate et al., t979), and in wheat grown with a split root system (Lambers et al., 1982). When studying these parameters in relation to N availability, it is of major importance to characterize the relationship between N supply and growth. In fact, the N (or nutrient) supply can be used to control growth effectively, by allowing the plants to adapt to a chosen relative rate of N addi- tion, which eventually leads to a linear relationship between the relative nitrogen (or nutrient) addition rate and the relative growth rate (reviewed by Ingestad 1982; Ingestad and Lund 1986). The ap- plicability of this method in terms of control of dry matter increment and other growth parameters has been demonstrated for several species (Ericsson 1981a; b; Ingemarsson et al., 1984; Ingestad 1980, 1981; Ingestad and Lund, 1979; Linder and Rook, 1983; Linder et al., 1981) including Pisum sativum (Oscarson and Larsson, 1986).

This communication concerns growth, and car- bon and nitrogen utilization in whole Pisum plants, growing with different relative rates of nitrogen addition. The aim is to form a basis for further application of this growth technique in detailed studies of fluxes, partitioning, and utilization of carbon in relation to long-term nitrogen nutrition.

Materials and methods

Plant material

Pisum sativum L. cv. Marina was grown as des- cribed elsewhere (Oscarson and Larsson, 1986).

Growth of the plants was monitored as dry weight increase in the interval 25 to 29 days after sowing. The relative growth rate (RGR) was calculated from the equation

W t = W0 eRGRt

where t is the time in days, and W0 and Wt the dry weights initially and after t days, respectively.

The seedlings were initially N-starved; however, from day 16 onwards, NO;- was added once daily in doses calculated to sustain relative rates of cul- ture N increments (RN; calculated analogous to RGR) of 0.06day -t, 0.10day ~, 0.12 day -1, and 0.14day J, respectively. The initial NO3 con- centration of the medium, immediately after addi- tion of the daily dose, ranged from 40 to 87 #M in the R N 0.06day -1 culture, and from 97 to 601 pM in the RN 0.14 day-~ culture, depending on plant age and culture density. All NO3 added was ass- umed to be taken up during the 24 h between NO3 - additions (see Oscarson and Larsson, 1986). The plants were cultured under continuous light, at a photon flux density of approximately 200/~mol m -2 s -l .

Net C02-fixation and root respiration

Gas exchange measurements were performed using a system schematically presented in Fig. 1. It consists of a two-compartment perspex box enclos- ing the whole plant. The upper (shoot) compart- ment was connected to an infrared gas analyser (Series 225 Gas Analyser, Analytical Development Co. Ltd, Hoddesdon, UK). Net CO2-fixation was measured at the same light conditions as mentioned above. The CO2 concentration was 330-370ppm

242 Duar te et al.

Reference

quently through a coil immersed in a thermostated water bath. Temperature of both compartments was maintained at 20~ The gas exchange was measured for between 30 and 60 min.

3ample

Chemica l analysis

Total C in the dried material was measured with a Carbo Erba elemental analyser.

e l e c t r o d e

Fig. 1. Schematic representation of the system used for the measurement of net photosynthesis and root respiration. Results and discussion

depending on the C02 concentration of the outside air which was used for the measurements. The lower compartment was completely filled with the daily air saturated NO3 solution at the beginning of the experiment. The two compartments were separated by an air tight rubber stopper which also held the plant in a fixed position. A pump circu- lated the nutrient solution over a Clark-type oxy- gen electrode mounted in a flow-cell, and subse-

Growth

The growth pattern of Pisum at different relative nitrogen addition rates is shown in Fig. 2. Over-all growth of the plants during the experimental period correlated well with RN (Fig. 2A). This ob- servation is in agreement with previous observa- tions on Pisum (Oscarson and Larsson, 1986; see also Oscarson et al., 1988). The root:shoot ratio in

6.0 IR N RGR r2 5.0 " - 0000901 / /0.10 0.107 0.99 �9 10.12 0.119 0.99 �9 �9 /0.14 0.135 0.99

r " "

shoot root

4 0 L 0.06 0 .036.0 .078.

,F 0.12 0.113A 0.125~

25 27 29 25 27 29

Days from sowing Days from sowing

Fig. 2. Growth of Pisum sativum at R N 0.06day ~, 0.10day -~, 0.12day -~ and 0.14day ~ during the time period 25 to 29 days. RGR values and square correlation coefficients are indicated. Each point represents mean value + SE for twelve plants. A: intact plants. B: roots and shoots.

Table 1. Net photosynthesis and root respiration in Pisum grown at different R N, expressed on a whole plant dry matter basis

R N Net photosynthesis Root respiration m g C g ~DWday ~ m g O 2 g - L D w d a y

0.06 32.2 + 16.3 16.5 + 4.3 0.12 72.6 +_ 3.5 48.3 _+ 12

27-day old plants was 1.08 in the RN 0.06day culture, but only 0.91 in the RN 0.12day -~ culture. This difference is explained both by uneven growth of root and shoot in the stage of adaptation to the chosen RN, and by the slightly differing growth rate observed also in the adapted stage, where the root grew consistently faster than the shoot (Fig. 2B). Nevertheless, for the purpose of measuring the daily processing of nitrogen and carbon, plants harvested in the interval 25 to 29 days after sowing could be considered adapted with respect to their growth rate to the chosen R N.

Gas exchange and over-all carbon-nitrogen budget

Gas exchange measurements performed on roots and shoots of plants cultured at RN 0.06 day-~ and 0.12day ~, respectively, are presented in Table 1. Both photosynthesis and root respiration of the plants adapted to different RN were practically con- stant on a weight basis throughout the experiment- al period (data not shown); not surprisingly, how- ever, the rates depended clearly on RN.

The data for the Rr~ 0.12day i culture were, together with previous data on N assimilation and

Growth [

Net photosynthesis

Root respiration <-w-1

C

26

213

176

Growth 23

t.2

10.2

13.5

1,8

Growth

SHOOT

N-uptake

C-w-1 ROOT

Growth

Fig. 3. Model for the assimilation of C and N and their growth related net distribution to shoot and root on day 27 after sowing.

N and C metabolism of N-limited Pisum 243

partitioning (Oscarson and Larsson, 1986), used to construct a model for assimilation of C and N, and growth-related net distribution of these elements to root and shoot on day 27 after sowing. The model contains the following elements: the observed initial C and N levels in roots and shoots on day 27; the expected partitioning of C and N to growth in the respective organ, assuming constant C and N levels and based on the observed RGR values; the actual amount of N received by the plant on day 27; the observed rates of C assimilation and 02 con- sumption. The N addition (as taken from the preset N addition scheme) to the culture corresponded to 2.8mgNg -~ DW plant. Taking the observed nitrogen content of the plant into account, this would result in an observed RN of 0.112day ~, which correlates well to the expected values. Similar considerations can be made concerning C assimilation. If net photosynthesis is compensated for root respiration (assuming a 1" 1 ratio between 02 consumed and CO2 evolved), the relative rate of carbon increment would be 0.141day 1, which compares with the predicted value of 0.12 day 1.

The scheme of Fig. 3 is deficient in certain im- portant aspects, e.g., the C and N exchange between root and shoot, and in determination of actual respiratory quotients in root tissue. How- ever, the obtained data demonstrate adaption of the C assimilation and growth processes to the chosen R N level. The method presented appears to be a promising tool for further investigations on carbon utilization under N limitation; two advan- tages that can be identified are the possibility to predict and control growth, and the possibility to relate the energy metabolism of the root to the processing of a known and controllable amount of NO3.

Acknowledgement

This work was supported by the Swedish Natural Science Research Council.

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