Modifications of Root Geometry in Winter Wheat by Phosphorus Deprivation

J.PlantPhysiol. Vol. 135.pp. 513-517(1989}

Modifications of Root Geometry in Winter Wheat by Phosphorus Deprivation SVEINN ADALSTEINSSON

and PAUL]ENSEN

Dept. of Plant Physiology, Univ. of Lund, P.O. Box 7007, S-22007 Lund, Sweden Received March 16, 1989· Accepted July 21, 1989

Summary The effects of phosphorus (P) deprivation on root geometry in winter wheat (Triticum aestivum) were studied. In the first week the plants were grown in complete nutrient solutions at 2 P levels (10 IlM and 1000 IlM phosphate), and in the second week both concentrations of P were omitted in the nutrient solution for half of the plants. The plants were harvested on days 7, 9,11,13 and 14, dried and analysed for P. Lateral and seminal root length and number were determined. The low P plants had lower dry weights and lower P contents in the dry matter than the high P plants. P was retranslocated from shoot to root in the P deprived plants. In the deprived high P plants, a high P level in the root could be sustained until day 11, whereafter it decreased to about the same level as in the root of the low P plants. P affected mainly the number of laterals. After two weeks most of the parameters (lateral root number and length, dry weight, P content) determined for the different categories of plants could be ranked in the order: deprived low P < low P < deprived high P < high P. It is argued that the internal P concentration plays a major role in root geometry and lateral root partition. The effects of remobilized P on lateral root formation are discussed.

Key words: Root geometry, lateral roots, phosphate deprivation, Triticum aestivum. Introduction Root development is controlled by various endogenous and exogenous factors such as hormones, temperature (Abbas Al-Ani and Hay 1983) and soil texture (Veen and Boone 1981). The effects of nutrients on root growth may be caused by either abundance or absence of the particular mineral nutrient in the root environment. Wiersum (1958) proposed a kind of ion hierarchy in the degree that ions affect root growth. In a series of experiments, Drew and collaborators elegantly demonstrated that localized supply of nutrients, mainly Nand P, caused localized proliferation of root growth (Drew et al. 1973, Drew 1975, Drew and Saker 1975, Drew and Saker 1978). It has also been shown that nitrate exerts different effects on laterals depending on its concentration and interaction with ions such as phosphate (Aaalsteinsson and Jensen 1988). Shortage of especially less mobile nutrients, such as P and Fe, is considered common in soils (Bieleski 1973, Marschner 1986). As root geometry is relevant in P acquisition from © 1989 by Gustav Fischer Verlag, Stuttgart

soils (Bieleski 1973), the plant's benefit of local root growth control is obvious. The question arises whether the effects of nutrients such as P (Hackett 1968, Drew and Saker 1978) on lateral root development are mainly caused by the external ion concentration (Drew 1975) or by elevated internal ion concentration. Locally high internal P concentrations invoke, in turn, the issue of interacting control mechanisms for root growth and negative feedback of P influx (Lefebvre and Glass 1982, Schjl'Jrfing and Jensen 1984). In P deficient plants, P is retranslocated from shoot to root (Clarkson et al. 1978). At a cellular level, P is remobilized by a phosphatase system that is repressed at sufficient P levels (Szabo-Nagy et al. 1987 and references therein). For understanding the specific roles and effects of nutrients, quantitative root studies are necessary which distinguish between the different root components, for example, length and number of seminals and laterals. Up to now such measurements have been time-consuming. Less tedious but precise root analysis techniques, like those described by Aaalsteinsson and Jensen (1988), may serve this purpose.

514

SVEINN ADALSTEINSSON and PAUL JENSEN

Table 1: Experimental design.

0.05

Cultivation, days

0.04

Low P Control

0- 7 7-14

Shoot

External P concentration, Jl.M

10 10

Deprived 10 not provided

High P Control 1000 1000

Deprived 1000 not provided

...--

..

I

c .!!!

Q.

The aim of the present study was to investigate the effects of P deprivation on various root parameters using intact winter wheat plants grown at different P levels and quantitative root analysis methods.

..

.....CI >-

0

Seeds of winter wheat (Triticum aestivum L. cv. Starke II) were germinated in darkness at about 20°C in Petri dishes on filter paper moistened with distilled water. After 3 days, the seedlings were transferred to floating discs made of black foam rubber (100 mm in diameter, 10 mm thick), each disc holding 6 seedlings. Three such plant groups were placed in each of 21 black-painted plastic beakers containing 1.81 nutrient solution with 10 Jl.M or 1000 ~ phosphate (50% Na2HP0 4, 50% NaH2P04) according to Table 1. Where not otherwise stated, the terms low and high P are used for plants grown at the two P levels. In addition, all nutrient solutions contained O.lmM KCI, 0.2mM KN0 3, O.lmM Ca(N0 3h, 0.1mM MgS0 4, 20Jl.M Fe-EDTA, 0.48J1.M CuCh, 8nM Na2Mo04, 4nM ZnS04, 3.7 Jl.M H 3B03 and 3.64 Jl.M MnS04 (initial pH 6.0). The nutrient solutions were continuously aerated and changed on days 3, 7, 9, 11 and 13. Cultivation was conducted at 15 ± 1 °C and 50 ± 5 % relative humidity. Light was provided 16 h per day [ca 70 W m - 2 (ca 35 W m- 2 400-700nm), Osram Power Star HQI 400W metalhalogen lamps]. On day 7 after transplanting, half of the plants were transferred to P-free solution according to Table 1. Plants were harvested on days 7, 9, 11, 13 and 14,6 replicates (= 6 plants) for each treatment. Roots were spread on a plexiglass sheet and photocopied for length determinations. The photocopies were analysed by a digitizer connected to a computer with appropriate software as described by Ai1alsteinsson and Jensen (1988). Seminal and nodal (adventitious) roots were collectively termed as seminal roots. Second order laterals were not distinguished from first order laterals. Roots were carefully blotted between filter papers, and roots and shoots were weighed for fresh weights and dried at 70°C for 2 days. After dry weight determinations, roots and shoots were wet-combusted in a 2: 1 (v/v) mixture of concentrated HN0 3 and HCI04. After dilution, total P in plant digests was determined according to Lindeman (1958), modified by substitution of hydrazine sulfate with ascorbic acid.

Results Dry weights of low P and high P plants were about the same on day 7 (Fig. 1). Thereafter, the high P roots grew faster than the low P roots. Deprivation of P reduced shoot and root growth of both low P and high P plants already around day 9. The drop of shoot and root dry weights for low P plants between days 13 and 14 was probably due to individual differences inherent in the starting material. Root to shoot dry weight ratio increased with age in all treatments from 0.36 on day 7 to around 0.5 on day 14 (Table 2).

rj

:, ,\ :, ,\ :,

1\ 1\

f-

1\ ,\

.,.

0.01

V V

V V V

V V ,\ V 1\ V 1\ V

:, :,

Q

Material and Methods

.I. ....

1\ 1\ 1\

0.02 0.01

~

l~

.I.

.c CI 'iii ~

~

0.03

1\ 'j ~

1\ l,a :, V 1\ V

~

1\ V ,\ V 1\ /

~

j

'j

4

1\

1\ V

'j

1\ V 1\ V 1\

1\ ....

T~

0.02

/

1\ V 1\ V 1\ V 1\ V 1\ V V 1\ V

V

\

~ /

'j

l\ 'j 1\ l\ V 1\ V :, V

1\ V 1\ V ,\ V :, V :, V 1\ V 1\ 'j

'j

)~

:, V

1\ 'j

:, 'of.

l~

T

~

~

Root 0.03

7

9

13

11

14

Age,days Fig. 1: Dry weights of shoots and roots of winter wheat. Plants grown at different P levels; low P (0), deprived low P (~), high P (_) and deprived high P (~). Cultivation and P levels according to Table 1. Values are means of 6 plants ± SE.

Table 2: Root to shoot dry weight ratios of winter wheat plants grown at different P levels according to Table 1. C = controls; D = deprived. Values are means of 6 plants ± SE. Age, days 7 9 11 13 14

Root/shoot DW ratio 10 Jl.M P C 0.36±0.01 0.38±0.02 0.44±0.03 0.5S±0.01 0.S4±0.03

1000 Jl.M P

D 0.38±0.01 0.41±0.03 0.S6±0.02 0.54±0.03

C 0.36±0.01 0.38±0.01 0.48±0.02 O.S2±O.Ol 0.53±0.01

D 0.34±0.02 0.46±0.01 O.S3±O.Ol 0.48±0.01

In shoots of low P plants, P content in controls increased slightly between days 7 and 14, while it remained at the same level in deprived plants (Fig. 2). Similarly, P content was almost unchanged (around 2 J.tmol) in low P roots during the Table 3: Root to shoot P ratios of winter wheat plants grown at different P levels according to Table 1. C = controls; D = deprived. Values are means of 6 plants ± SE. Age, days

Root/shoot P ratio 10 Jl.M P C

7 9 11 13 14

0.36±Q.02 0.41±0.04 O.SO±0.02 O.SO±0.01 O.SO±O.03

1000 Jl.M P

D

C

D

0.36±0.01 0.53±0.03 0.5S±0.02 0.63±0.02

0.33±0.02 0.31±0.01 0.36±0.02 0.31 ±0.02 0.32±0.01

0.28±0.01 0.38±0.01 0.34±0.01 0.31±0.O3

P effects on lateral roots

20

--..

,.. I

Lateral roots

15

...

5

(,)

0

c

r'

G)

c 0

Q.

...

.... i'r

II [I

14

4

.c E

~

t'

1/ 1/ 1/ 1/ 1/ ..... t"I 1/ 1\ 1/

E

..

A

120

J

G)

'0

..

160

Shoot

c .!!! c. 10

..3-

515

:::I

~ ,..~

[\ [\

[\

.... ~

~ ... f.I

1/

f7'~

II II !/

1/ 1/ 1/

~

[I

I'-

[\ [\ ~

....

~

[I

II

rl

14

~/

~ r\ V;

1\ ~

E .£

~~

Cl

c ..!

Root

7

9

11

13

14

B

0

E .£ 0.02

Cl

c ..!

Cl

...

~~

c ,.l~

,...

r\ r\ r\ \ \ \

0.01

G)

>


rl II

J. II

0 r"l.J.

1"11

nI~lfi

[I

II

..LR

ft

[;

0

7

9

~

....

~

l! l!

II

~ :\

/

!/

k ~\

t~ II 1/ II

[\

11 Age, days

II II II 1/ II

1/

~

~

~ 1\ \ 1\ \ \ 1\

~

~

1/ 1/ ~ II ~

~

1\ 1\ V

l! 1\ II

..... ~

~

1\ II [\ 1/

.-l

~ V II

II II

!oJ

~

r1

II r\ II r\ II r\ II r\ [; [\ II

3

1

G)

deprivation period (Fig. 2). The P content in roots of low P controls increased slightly at the same time. Compared with low P plants, in high P plants the P contents in shoots and roots were higher on day 7. The shoot P content decreased in deprived high P plants from about 10 Jtmol on day 9 to about 7 Jtmol on day 14. The root P content decreased by about 1.5 Jtmol during the same period. The root to shoot ratio of P was highest in deprived low P plants and lowest in high P plants (Table3). On day 9, the number of lateral roots in high P plants was higher than in low P plants (Fig. 3 A). This difference was maintained throughout the investigation period. Compared with controls, the root number in deprived high P plants was higher on days 9 and 11, but lower thereafter. The laterals of deprived low P plants followed the controls quite closely in number, but on day 14 they were considerably fewer than the controls. Overall lateral root length (Fig. 3 B) followed roughly a similar pattern as the lateral root number. From day 7 to day 11 average lateral root length was around 1.8 cm and it increased to 2.2 cm on days 13 and 14 (Fig. 3 C). Average lateral root length was unaffected by P deprivation, with a possible exception of deprived low P ones. The seminal roots accounted for about two-thirds of the total root length on day 7 (Table 4). Their relative proportion of the root system decreased subsequently with the onset of lateral root formation. In the deprived plants this pattern was less clear. The proportion of lateral root length remained unchanged between days 9 and 11 (around 40 %) in deprived low P plants but increased to around 70 % on days 13 and 14. In deprived high P plants, the proportion of lateral root length was 74% on day 11 and changed little thereafter. The order [deprived low P
4

>

Age, days

Fig. 2: Total P contents of shoots and roots of winter wheat grown at different P levels according to Fig. 1. Values are means of 6 plants ± SE.

In.

... G)

II

J.

0

2

iij

8

l!

40

~

,..L

II

Z

~

~

[j

80

~

V 1/ 1/ II II

~ II 1/ 1\ II

~

[\

V

~ ~ rl

[\

/ /

13

14

'I 'I

1\ II 1\ 1/ 1\ V II I

[\

Fig. 3: Lateral root number (A), overall (B) and average (C) lateral root length of winter wheat grown at different P levels according to Fig. 1. Values are means of 6 plants ± SE. Table 4: Proportion of lateral roots [m overall lateral root length (total overall root length)-l] in winter wheat plants grown at different P levels according to Table 1. C = controls; D = deprived. Values are means of 6 plants ± SE. Age, days

Proportion of lateral roots

c 7 9 11 13 14

10 JLM P

0.34±0.02 0.46±0.08 0.S3±0.04 0.68±O.06 0.74±O.07

1000 JLM P

D

C

D

0.42±0.07 0.39±0.04 O.68±O.06 0.71±O.04

0.32±0.11 O.51±O.OS 0.67±0.10 0.77±0.03 O.S1±0.03

0.58±0.07 0.74±0.06 0.76±O.03 0.7S±O.07

P < high P] in the proportion of lateral roots on day 14 (Table 5) was also similar for other parameters, e.g. dry weight and P content, as well as overall lateral root length and number. The root to shoot P ratio showed a reverse pattern. Discussion Phosphorus concentrations of soil solutions are usually below 100 JtM; 0.2-10 JtM are typical values (Asher and Lo-

516

SVEINN ADALSTEINSSON and PAUL]ENSEN

Table 5: Comparison of various growth, P and lateral root parameters for winter wheat grown for 14 days at different P levels according to Table 1. Relative values, deprived low P plants = 100 %. [P] values are based on /Lmol P (g fresh weight) - 1; other P values are basedon/LmoIP(plant)-I.C = controls;D = deprived. Parameter

D

10 /LM P

C

1000 D

P

100 100

109 111

124 106

133 133

100 100 100 100 100 100

79 161 130 149 104 131

49 257 123 205 165

51 634 320 513 212 300

100 100 100 100

172 139 122 104

203 173 117 106

272 219 122 114

~

C

Dry weight

Shoot Root Phosphorus R/S P ratio P in shoot P in root P in whole plant [Ploot [P]shoot Lateral roots Overall length No. of laterals Average length Partition

88

neragan 1967). High P concentrations (0.5 - 2 mM) in static nutrient solutions with limited volume may yield similar growth rates as lower concentrations (1-100 ttM) in soils or flowing culture solutions (Loneragan and Asher 1967, Bieleski 1973). The external P concentrations used in this study (Table 1) are quite high, but for examining the dynamics of P in the plants over a period of two weeks, they were considered suitable. Similar root to shoot dry weight ratios in the different treatments (Table 2) indicate that P deprivation did not cause severe P deficiency, which is typically associated with a low ratio (e.g. Chapin and Bieleski 1982). Delays of up to 8 days in P deprivation effects on growth have been reported (Clarkson et al. 1978).

Partial P provision

During P deprivation, P was translocated from the shoot to the root (Fig. 2 and Clarkson et al. 1978). On a cellular level, P in the vacuoles is found to be mobilized slowly (Crosset and Loughman 1966, Brodelius and Vogel 1985). However, sufficient cytoplasmic P levels in the root cells could be sustained during P deprivation for maintained lateral root formation and elongation (Fig.3 and Anghinoni and Barber 1980). The increase in lateral root number in deprived high P plants on days 9 and 11, compared with controls, can possibly be considered as a response to decreased external P levels (Fig. 3 A). However, inspite of efflux from the high P roots, the external P concentration on day 8 was low (around 5ttM) and not detectable with ion chromatography (Dionex 2010i) on subsequent days. Decreased P efflux rates during P deficiency have been reported (Bieleski and Ferguson 1983 and references therein). The deprived high P plants had higher P content than the controls (Fig. 2) on day 9, possibly due to lower efflux. The overall lateral root length was mainly dependent on the lateral root number (Fig. 3). In all treatments, the formation of laterals is characterized by an initial lag period followed by more vigorous growth. This may be due to two phases in average lateral root length (Fig. 3 C), which is generally affected by the formation of new laterals. On day 14, deprived roots had lower average lateral root length than controls (Fig. 3 C). This could be caused by an insufficient P supply, i.e. the lack of P suppressed root growth (Anghinoni and Barber 1980). It was demonstrated early that ions, particularly the metabolized ones, can substantially modify lateral root growth of cereals (Bose mark 1954). Root dry weights increased locally in zones supplied with P, which was interpreted as an interaction between translocated carbohydrates from the shoot and external P supply (McClure 1972). In a more detailed study, Drew and Saker (1978) reported local proliferation of lateral root growth in reponse to localized P supply, despite increased P translocation to other parts of the root deprived

P deprivation

Shoot

Shoot

c----'

r-./-f-L---"I"'---~-T-V:":"l p

P uptake

Root (+P)

c--------' Root (-P)

Root (-P)

Fig.4: Possible pathways of P transport in roots partially (left) and entirely (right) deprived of P, emphasizing the role of vacuolar P in lateral root formation. During partial P provision, P is translocated from P fed roots to the shoot and to the P starved roots where the vacuoles buffer the cytoplasmic P and few laterals are formed. During complete P deprivation, P is translocated from the shoot (distal P pool) or the vacuoles (proximate P pool), giving rise to similar number of lateral roots on the axes. P = Phosphorus; C = Cytoplasm; V = Vacuole; T = Transport; L = Laterals. For further details, see Discussion.

P effects on lateral roots of external P. On day 14, lateral root partition and many other lateral root parameters were positively related to the internal P status of the plant (Tables 4, 5). Thus, since external P concentration was low and about the same around the roots of deprived low and high P plants on day 14, lateral root formation and partition are probably related to the internal P status of the plants (Table 5). Hence, as the vacuolar provision of P buffers cytoplasmic P, it can be speculated whether lateral root initiation (an energy demanding process) is positively connected with vacuolar P status (Fig. 4). The size of the vacuolar P buffer is in turn affected by the «local» P uptake. Low capacity of this proximate P buffer could explain low lateral formation in barley roots deprived of P as reported by Drew and Saker (1978), despite increased P input from distal roots fed with P (Fig. 4). Other nutrients (primarily N) may also be involved. It is more doubtful to relate average lateral root length to internal P levels, but in the case of low P status (deprived low P, TableS) this parameter can also be affected. However, a high lateral root number in the deprived high P plants could possibly be a trait from the former nutrient status. The existence of nutrient sensors and «strategies» in P acquisition in winter wheat cannot be supported by the data presented here. The relevance of external P concentration in local root growth control is possibly only limited to the extent that it provides P to the shoot or the vacuole, which in turn buffers cytoplasmic P (Fig. 4). The current data point to temporarily elevated P levels in the root (or in the plant as a whole), which favours lateral root formation. Once formed, the rate of lateral root elongation is independent of the P status of the root (Fig. 3 C and Drew 1975). Similar results have been reported for N (Drew et al. 1973). Acknowledgements We would like to thank Mrs. Ann-Margret Svensson for skilful technical assistance. This work was supported by grants from the Swedish Council for Forestry and Agricultural Research (P.J.), Nordic Board for Research Courses (Styrelsen for nordiska forskarkurser; S.A.), the Botanical Society in Lund (Lunds Botaniska Forening; S.A.) and the Hierta-Rietzius foundation (S.A.).

References ABBAS Al-ANI, M. K. and R. K. M. HAy: The influence of growing temperature on the growth and morphology of cereal seedling root system. J. Exp. Bot. 34, 1720-1730 (1983). ADALSTEINSSON, S. and P. JENSEN: Root development in winter wheat grown at different NIP supply: Root length patterns and N-P interactions in phosphate uptake. Physio!. Plant. 72, 271-278 (1988). ANGHINONI, 1. and S. A. BARBER: Phosphorus influx and growth characteristics of corn roots as influenced by phosphorus supply. Agron. J. 72, 685-688 (1980). ASHER, C. J. and J. F. LONERAGAN: Response of plants to phosphate concentration in solution culture: I. Growth and phosphorus content. Soil Sci. 103, 225-233 (1967). BIELESKI, R. L.: Phosphate pools, phosphate transport, and phosphate availability. Annu. Rev. Plant Physio!. 24, 225-252 (1973). BIELESKI, R. L. and I. B. FERGUSON: Physiology and metabolism of phosphate and its compounds. In: Encyclopedia of Plant Physiol-

517

ogy, New Series (LAUCHLI, A. and R. L. BIELESKI, eds.), Vo!' 15 A, pp. 422 - 449. Springer-Verlag, Berlin. ISBN 3-540-12103-X (1983). BOSEMARK, N. 0.: The influence of nitrogen on root development. Physio!' Plant. 7, 497-502 (1954). BRODELIUS, P. and H. J. VOGEL: A 31p NMR study of the phosphate mechanism of cultured Catharanthus roseus and Daucus carota plant cells. J. Bio!. Chern. 260, 3556-3560 (1985). CHAPIN, III. F. S. and R. L. BIElESKI: Mild phosphorus stress in barley and a related low-phosphorus-adapted barleygrass: Phosphorus fractions and phosphate absorption in relation to growth. Physio!. Plant. 54, 309-317 (1982). CLARKSON, D. T., J. SANDERSON, and C. B. SCATTERGOOD: Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley). Planta 139, 47 ~ 53 (1978). CROSSET, R. N. and B. C. LOUGHMAN: The absorption and translocation of phosporus by seedlings of Hordeum vulgare (L.). New Phyto!' 65, 459-468 (1966). DREW, M. c.: Comparison of the effects of a localized supply of phosphate, nitrate, ammonium and potassium on the growth of the seminal root system, and the shoot, in barley. New Phyto!' 75,479-490 (1975). DREW, M. C. and L. R. SAKER: Nutrient supply and the growth of the seminal root system in barley. II. Localized, compensatory increases in lateral root growth and rates of nitrate uptake when nitrate supply is restricted to only part of the root system. J. Exp. Bot. 26, 79-90 (1975). - - Nutrient supply and the growth of the seminal root system in barley. III. Compensatory increases in growth of lateral roots, and in rates of phosphate uptake, in response to a localized supply of phosphate. J. Exp. Bot. 29, 435 - 451 (1978). DREW, M. C., L. R. SAKER, and T. W. ASHLEY: Nutrient supply and the growth of the seminal root system in barley. I. The effect of nitrate concentration on the growth of axes and laterals. J. Exp. Bot. 24, 1189-1202 (1973). HACKETT, c.: A study of the root system of barley. I. Effects of nutrition on two varieties. New Phyto!. 67, 287 -299 (1968). LEFEBVRE, D. D. and A. D. M. GLASS: Regulation of phosphate influx in barley roots: Effects of phosphate deprivation and reduction of influx with provision of orthophosphate. Physio!. Plant. 54, 199-206 (1982). LINDEMAN, W.: Observation on the behavior of phosphate compounds in Chlorella at the transition from dark to light. - In: Proceedings of the 2nd International Conference on the Peaceful Uses of Atomic Energy. Vo!' 24, pp. 8-15. United Nations Pub!., Geneva (1958). LONERAGAN, J. F. and C. J. ASHER: Response of plants to phosphate concentration in solution culture: II. Rate of phosphate absorption and its relation to growth. Soil Sci. 103, 311-318 (1967). MARSCHNER, H.: Mineral Nutrition in Higher Plants. - Academic Press, London. ISBN 0-12-473540-1 (1986). MCCLURE, G. W.: Nutrient distribution in root zones. Further studies of the effects of phosphorus distribution on corn and wheat. Can. J. Bot. 50, 2275 - 2282 (1972). SCHJI1lRRING, J. K. and P. JENSEN: Phosphorus nutrition of barley, buckwheat and rape seedlings. II. Influx and efflux of phosphorus by intact roots of different P status. Physio!' Plant. 61, 584-590 (1984). SZAB6-NAGY, A., Z. OlAH, and L. EWEI: Phosphate induction in roots of winter wheat during adaptation to phosphorus deficiency. Physio!. Plant. 70, 544-552 (1987). VEEN, B. W. and F. R. BOONE: The influence of mechanical resistance and phosphate supply on morphology and function of corn roots. Plant Soil 63, 77-81 (1981). WIERSUM, L. K.: Density of root branching as affected by substrate and separate ions. Acta Bot. Neerl. 7, 174-190 (1958).