effects of soil temperature on root and shoot growth traits and iron deficiency chlorosis in sorghum...

Plant and Soil 130: 97-103, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands. PLSO IRONll

Effects of soil temperature on root and shoot growth traits and iron deficiency chlorosis in sorghum genotypes grown on a low iron calcareous soil

R.B. CLARK and NICOLE REINHARD US Department of Agriculture, Agricultural Research Service, Department of Agronomy, University of Nebraska, Lincoln, NE 68583, USA

Key words: chlorosis severity, dry matter yields, growth chamber, leaf area, root length, shoot/ root ratio, Sorghum bicolor

Abstract

Iron deficiency chlorosis (FeDC) is a common disorder for sorghum [Sorghum bicolor (L.) Moench] grown on alkaline calcareous soils. Four sorghum genotypes were grown in growth chambers on a low Fe (1.3/xg/g DTPA-extractable), alkaline (pH 8.0), calcareous (3.87% CaCO 3 equivalent) Aridic Haplustoll to determine effects of different soil temperatures (12, 17, 22 and 27°C at a constant 27°C air temperature) on various root and shoot growth traits and development of FeDC. As soil temperature increased, leaf chlorosis became more severe, and shoot and root dry weights, root lengths, and leaf areas increased markedly. Shoot/root ratios, shoot weight/root length, leaf area/shoot weight and leaf area/root weight and root length also increased while root length/root weight decreased as soil temperature increased. Severe FeDC developed in all genotypes even though genotypes had previously shown different degrees of resistance to FeDC. Genotypes differed in most growth traits, especially dry matter yields, root lengths, and leaf areas, but most traits did not appear to be related to genotype resistance to FeDC. The most FeDC resistant genotype had the slowest growth rate and this may be a mechanism for its greater resistance to FeDC.

Introduction

Iron deficiency chlorosis (FeDC) is a common disorder for sorghum [Sorghum bicolor (L.) Moench] grown on low Fe, alkaline, calcareous soils (Clark, 1982; Fisher and Reyes, 1954; Myers and Johnson, 1933; Withee and Carlson, 1959). A common expression about the sorghum FeDC problem in the Great Plains of the USA is 'wait until the soil temperature increases and the soil moisture decreases and the problem will go away'. This commonly occurs unless the chlorosis has been so severe that plants cannot over- come the original symptoms. The reasons for this response are not well understood.

FeDC in sorghum has normally been associ- ated with low soil availability of Fe (Brown and

Holmes, 1956; Fisher and Reyes, 1954; Myers and Johnson, 1933). As the growing season pro- gresses, the availability of Fe does not necessari- ly change, but plants regreen even though no Fe has been added. A hypothesis suggested to ac- count for regreening of plants is enhanced root growth, especially at warmer soil temperatures, so that roots make contact with more Fe, and that the enhanced number of roots make more Fe available for uptake by producing phyto- siderophores, reducing more Fe > t o F e 2+

through proton and reducing agent release, and/ or greater root capacity to reduce Fe (Marschner et al., 1986).

Cool soil temperatures can induce FeDC in

98 Clark and Reinhard

plants (Wallace and Lunt, 1960). Cool soil temperatures are also known to reduce root growth in many plant species (Cooper, 1973; Drew, 1979; Nielson, 1974; Nielsen and Humphries, 1966; Russell, 1977; Taylor and Kaspar, 1985), and reduced root growth could possibly be a reason for the higher incidence of FeDC in sorghum grown at cool soil temperatures.

The objective of this study was to determine the effects of soil temperature on root and shoot growth traits of sorghum genotypes known to differ in resistance to FeDC when grown on a low Fe calcareous soil.

Materials and methods

Table 1. Chemical properties of Ulysses silt loam soil used to grow sorghum genotypes at varied soil temperatures

Property/test

pH (1:1 soil:water) 8.03 Organic matter (%) 0.95 CaCO 3 equivalent (%) 3.87 Total N (mg kg-') 730 P (NaHCO 3 extract) (/xg g 1) 7.97 CEC (+cmol g-a) 24.3

Ca (% of CEC) 81.6 Mg (% of CEC) 13.2 K (% of CEC) 4.4 Na (% of CEC) 0.7

Micronutrients (DTPA extract) Fe (/zg g-l) 1.3 Mn (/zg g-a) 3.7 Zn (/xg g a) 0.29 Cu (/xg g-a) 0.60

Four sorghum genotypes [SC33-9-8-E4 and TX7000 considered to be relatively resistant and SCl18-15E and TX428 considered to be relatively susceptible to FeDC (McKenzie et al., 1984; Williams et al., 1982)] were grown on a low Fe, alkaline, calcareous soil at varied soil temperatures under controlled conditions. Growth chamber conditions were 16 /8h light/dark at 27°C and a light intensity of 300/zmol m -2 s -I. Light was provided by high pressure sodium and metal halide lamps. Relative humidity in the chamber was 50 -+ 3% during the light and 60 - 3% during the dark cycles.

Plants were grown in 2.65 kg of Ulysses silt loam (fine-silty, mixed mesic, Aridic Haplustoll) in vessels where soil temperature could be controlled. Table 1 gives the chemical properties of the soil before fertilizer was added. The soil received fertilizer amendments of 33.9 N, 32.6 K, and 17.3 P in mg kg -1 soil as Ca(NO3)2, KCI, and Ca(H2PO4)2, respectively.

The vessels to control soil temperature con- sisted of an inner plastic cylinder (10cm inside diameter and 30 cm tall) to contain soil and an outer cylinder (16 cm inside diameter and 30 cm tall) to create a cavity between cylinders. This cavity was sealed at the top and bottom by 6.2 mm thick plastic plates; the top plate was cut open over the soil compartment and the lower plate had small holes below the soil compartment for draining excess water. Port holes were drilled on opposite sides of the outer cylind~

and plastic stems inserted to allow for water flow when tubing was connected to the stems.

Sorghum seeds treated with captan [(N- trichloromethylthio)-cyclohex-4-ene-1, 2-dicar- boximide] were germinated in rolled paper to- wels kept moist with CaSOa-enriched water. Four 3-day-old seedlings were transferred to the soil in each vessel. Only one genotype was grown per vessel. Seedlings were allowed to grow 10 days before soil temperature treatments were administered. Water at regulated temperatures from refrigerated baths was pumped through tubing to the vessels (five to six vessels per each set of tubing) to control soil temperature within -0 .5°C for water flowing through the system. Soil temperature was reduced gradually over 36 to 48 hours when treatments were initiated so that roots would not be shocked by rapid temperature changes. Vessels had thermometers inserted into the soil to monitor soil temperature daily. Deionized water was added as needed to keep water near field capacity in the soil.

Plants were grown at 12, 17, 22, and 27°C soil temperatures at an ambient 27°C air temperature for each experiment. Plants were grown for 12 days at the given temperature before experiments were terminated. Because of technical reasons, only one soil temperature was main- tained per experiment. Four replications of each genotype were randomized within the series of vessels. Additional experiments were conducted in which each genotype was grown at each of the

soil temperatures at the same time (results not reported).

Visual FeDC ratings (1 =green to 5=very severe FeDC) and general appearance of the plants were recorded at the time experiments were terminated. Plants were harvested by cut- ting shoots 0.5 cm above the soil, fresh weights of leaves determined, and leaf areas measured using a leaf area meter (Li-Cor, Lincoln, NE). Leaves were dried in a forced air oven at 70°C and weighed.

The soil in each vessel was removed as an unbroken core by soaking the vessels in water. Roots were washed free of soil and representa- tive fresh root samples (1 to 2g) were stained black and total root length determined using a 2 x 2 cm grid (Tennant, 1975). Stained and re- maining roots were dried separately at 70°C and weighed.

The data were analyzed statistically using a factorial design. Linear and quadratic measures of the temperature variable were not calculated because of the heterogeneous variability around the means. That is, the variability around the means for plants grown at 12 and 17°C was considerably lower than the variability around the means of plants grown at 22 and 27°C.

Results

The level of significance for each growth trait of

Iron deficiency chlorosis in sorghum 99

plants grown over the four soil temperatures was greater than P = 0.01 (Table 2). The level of significance for the sorghum genotypes was greater than P = 0.06 for all growth traits except shoot weight/root length, leaf area/shoot weight, and leaf area/root length which were less than P = 0.20. All temperature x genotype inter- actions were significant at levels greater than P=0.05 except leaf area/shoot weight which was not significant at P = 0.20.

Visual FeDC symptoms in all genotypes ten- ded to be more severe as soil temperature increased (Table 3). SC33-9-8-E4 had the least severe and TX428 generally had the most severe FeDC symptoms at each soil temperature.

Shoot and root dry matter yields increased as soil temperature increased (Table 3). Shoot weights increased more than root weights as soil temperature increased from 12 to 27°C. SC33-9- 8-E4 had the lowest dry matter yield of the genotypes and SCl18-15E had the highest. Shoot/root ratios showed less dramatic increases than either shoot or root dry matter yields.

Root lengths of the four genotypes were similar at 12°C and increased as soil temperature increased (Table 4). Two of the genotypes (TX7000 and SCl18-15E) had significant increases in root length as soil temperature increased from 12 to 17°C, but the other two genotypes did not. The greatest increase in root length for all genotypes was between 17 and 22°C. Except for SCl18-15E which had a large

Table 2. Levels of significance (F test) for growth traits of sorghum genotypes grown at varied soil temperatures

Trait Source a

Temperature (T) Genotype (G) T x G

Probability level (P)

Shoot dry matter yield <0.01 <0.01 <0.01 Root dry matter yield <0.01 <0.01 <0.01 Whole plant dry matter yield <0.01 <0.01 <0.01 Shoot / root dry matter ratio <0.01 0.03 <0.01 Root length <0.01 <0.01 <0.01 Root length/root weight <0.01 0.06 0.05 Shoot weight/root length <0.01 >0.20 <0.01 Leaf area <0.01 <0.01 <0.01 Leaf area/shoot weight <0.01 >0.20 >0.20 Leaf area/ root weight <0.01 0.02 0.01 Leaf area / root length <0.01 >0.20 <0.01

a Degrees of freedom for each test were: T = 3, G = 3, G x T = 9, and total = 63.


Table 3. Visual chlorosis ratings, shoot and root dry matter yields, and shoot/root at varied soil temperatures in a low Fe calcareous soil

dry weight ratios of sorghum genotypes grown

Trait Genotype Root temperature (C)

12 17 22 27

Chlorosis ratings" SC33-9-8-E4 2.0 3.2 (visual) TX7000 3.0 3.0

SCl18-15E 4.0 3.8 TX428 4.0 4.3

Shoots SC33-9-8-E4 0.127 0.186 (g/4plant~ TX7000 0.165 0.294

SCl18-15E 0.200 0.350 TX428 0.200 0.231

Mean 0.173 0.265 LSD(P<0.05)

Roots SC33-9-8-E4 0.086 0.115 (g/4plants) TX7000 0.133 0.146

SCl18-15E 0.134 0.243 TX428 0.143 0.182

Mean 0.119 0.171 LSD (P < 0.05)

Whole plant SC33-9-8-E4 0.212 0.300 (g/4 plants) TX7000 0.298 0.440

SCl18-15E 0.314 0.593 TX428 0.344 0.413

Mean 0.292 0.437 LSD (P < 0.05)

Shoot/root ratio SC33-9-8-E4 1.51 1.67 TX7000 1.32 2.03 SCl18-15E 1.75 1.48 TX428 1.49 1.31

Mean 1.35 1.52 LSD (P < 0.05)

0.083

0.089

0.164

0.25

3.5 3.5 4.1 5.0 4.2 4.4 4.1 4.5

0.574 0.519 0.804 0.898 0.924 1.199 0.570 0.632 0.718 0.182

0.350 0.316 0.727 0.362 0.707 0.936 0.516 0.364 0.575 0.494

0.924 0.835 1.531 1.260 1.631 2.135 1.086 O.996 1.293 1.307

1.65 1.70 1.26 2.60 1.36 1.30 1.15 1.9.__.~4 1.63 1.88

a = green to 5 = very severe chlorosis.

increase , r oo t lengths t e n d e d to dec rea se in the geno types g rown at 27°C.

Specific r oo t lengths ( root l e n g t h / r o o t weight ) d e c r e a s e d as soil t e m p e r a t u r e inc reased with the g rea tes t change occur r ing b e t w e e n 17 and 22°C

(Tab l e 4). SC33-9-8-E4 had the h ighest specific r oo t length while S C l 1 8 - 1 5 E had the lowest . Shoo t weight tha t a given roo t l ength had to sus ta in was h ighes t for p lan ts g rown at the highest soil t e m p e r a t u r e .

L e a f a rea i nc reased d rama t i ca l l y as soil t em- p e r a t u r e inc reased (Tab l e 5). S C l 1 8 - 1 5 E had the larges t increase and SC33-9-8-E4 had the small- est. T h e ra te of leaf a r ea inc rease for SC33-9-8- E4 and TX428 was re la t ive ly low b e t w e e n 22 and

27°C, bu t c o n t i n u e d to inc rease a lmos t l inear ly

for S C l 1 8 - 1 5 E and TX7000 above 17°C. Specific leaf a r ea ( leaf a r e a / s h o o t weight ) and leaf a r e a / r oo t weight or leaf a r e a / r o o t l ength also in- c r ea sed in the g e n o t y p e s as soil t e m p e r a t u r e inc reased (Tab l e 5). D i f fe rences a m o n g geno types for specific leaf a rea and lea f a r e a / r oo t l ength were re la t ive ly smal l and overa l l d i f fe rences a m o n g g e n o t y p e s were insignificant . O n the o t h e r hand , g e n o t y p e s d i f fe red in leaf a r e a / r o o t weight ove r the t r ea tmen t s ; TX7000 had the h ighest va lue and S C l 1 8 - 1 5 E the lowest . The increases in these l a t t e r g rowth t ra i ts wi th inc reased soil t e m p e r a t u r e were no t as g rea t as for leaf a rea increases .


Table 4. Total root length, specific root length, and shoot dry weight per root length of sorghum genotypes grown at varied soil temperatures in a low Fe calcareous soil


12 17 22 27

Root length SC33-9-8-E4 2180 2520 7560 6610 (cm/4 plants) TX7000 2690 4380 13090 6660

SCl18-15E 2450 5300 9960 18170 TX428 2830 3430 8340 6680

Mean 2540 3910 9740 9530 LSD(P <0.05) 1920

Root length/ SC33-9-8-E4 25.7 22.1 21.9 20.1 Root weight TX7000 21.0 30.8 18.0 17.8 (cm/mg dry wt) SCl18-15E 21.5 21.9 14.9 19.8

TX428 20.9 19.4 16.7 19.0 Mean 22.3 23.6 17.9 19.1

LSD (P < 0.05) 3.0

Shoot weight/ SC33-9-8-E4 0.059 0.077 0.078 0.088 Root length TX7000 0.062 0.066 0.071 0.149 (rag dry wt/cm) SCl18-15E 0.081 0.067 0.096 0.066

TX428 0.072 0.067 0.079 0.103 Mean 0.068 0.069 0.081 0.102

LSD (P < 0.05) 0.014

Table 5. Leaf area, specific leaf area, and leaf area per root length and dry weight of sorghum genotypes grown at varied soil temperatures in a low Fe calcareous soil


12 17 22 27

Leaf area SC33-9-8-E4 24 48 126 146 (cm2/4 plants) TX7000 30 80 157 236

SCl18-15E 34 83 199 315 TX428 44 56 14_~2 16___55

Mean 33 67 156 215 LSD (P < 0.05) 21

Leaf area/ SC33-9-8-E4 183 263 215 286 Shoot weight TX7000 186 266 203 263 (cm2/g dry wt) SCl18-15E 171 235 214 264

TX428 21___29 24_~3 25_._._! 263 Mean 190 252 221 269

LSD (P < 0.05) 27

Leaf area / SC33-9-8-E4 277 440 355 482 Root weight TX7000 249 558 266 686 (cm2/g dry wt) SCl18-15E 299 345 285 344

TX428 32._._9_9 31"7 28_44 52__._33 Mean 289 415 297 509

LSD (P < 0.05) 75

Leaf area/ SC33-9-8-E4 107 201 167 256 Root length TX7000 114 179 149 395 (cm2/1000 cm) SC118-15E 139 157 204 174

TX428 15__._fi 16___33 19__.~2 278 Mean 129 175 178 276

LSD (P < 0.05) 42


Discussion

The soil used in this study induced such severe FeDC on all sorghum genotypes that genotype differences were not easily detected. Neverthe- less, the visual FeDC ratings noted for the more FeDC resistant SC33-9-8-E4 could be separated from the more FeDC susceptible genotypes like SCl18-15E (Table 3). The differences between the other two genotypes did not show clear visual separations. The amount of DTPA- extractable Fe in this soil was lower than that considered to be adequate for sorghum and many other plants grown on low Fe calcareous soils (Lindsay and Schwab, 1982). This soil induced such severe FeDC symptoms on field- grown sorghum in 1988 that plants died or pro- duced little or no grain (R. B. Clark, personal observation).

The soil temperature for optimum root growth was about 22°C for three of the sorghum genotypes, but appeared to be higher for SCl18- 15E (Table 4). Results from other experiments (data not reported) indicated that the optimum temperature for SCl18-15E was between 27 and 32°C. The temperature for optimum shoot growth of three of the four genotypes was 27°C or above. Soil temperatures below 22°C reduced both root dry matter yield and root length.

Optimum soil temperature for maize (Zea mays L.) seedling root growth was 25°C, and root elongation of seedlings grown at 17 and 21°C was 5 and 20%, respectively, of those grown at 25°C (Logsdon et al., 1987). Reduced root growth at 18 compared to 25°C was the major factor restricting nutrient (P) uptake by maize (Mackay and Barber, 1984). Reduced root growth similar to that noted in this study has been noted for various plant species and genotypes grown at cooler than normal temperatures (Cooper, 1973; Cutforth et al., 1986; Gre- gory, 1986; Schwartz et al., 1987; Stone and Taylor, 1983).

Visual observations of the roots of sorghum plants grown at 12°C showed few fibrous or lateral roots, and roots were larger in diameter and shorter than plants grown at higher (22 and 27°C) temperatures. The main seminal root had elongated extensively, was whiter, had no bran- ches, and was normally larger than roots grown

at the higher root temperatures. Similar results were noted for barley (Hordium vulgate L.) and pearl millet [Pennisetum typhoides S. and H.) roots grown at low soil temperatures (Gregory, 1983; Macduff et al., 1986; Schwartz et al., 1987).

Because of decreased root diameters and increased number of branch roots as soil temperature increased, specific root lengths (root length/ root weight) would likely increase. However, these values decreased in our study (Table 4). This indicated that root length likely decreased more than root weight as soil temperature increased. In contrast to specific root length, specific leaf area (leaf area/shoot weight), leaf area/root length, and leaf area/root weight increased as soil temperature increased (Table 5). This indicated that shoot growth probably took priority over root growth as soil temperature increased.

It was hypothesized that FeDC resistant genotypes might have lower shoot weight or leaf area per unit of root weight or root length or have higher root lengths per plant to give them an advantage over FeDC susceptible genotypes to keep FeDC symptoms from occurring or being as severe. Although the sorghum genotypes considered to be more resistant to FeDC had higher leaf area/root weight values than the genotypes considered to be more susceptible to FeDC, the genotypes generally showed few differences in most root and shoot growth traits. The largest difference between the most resistant and susceptible genotypes was the slow growth of the FeDC resistant genotype SC33-9-8-E4. This may be a mechanism whereby this genotype shows relatively high resistance to FeDC. The higher FeDC resistance of SC33-9-8-E4 in the field than many other sorghum genotypes may also be related to its small plant size at maturity (Wil- liams, 1987).

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effects of soil temperature on root and shoot growth traits and iron deficiency chlorosis in sorghum...

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