Response of cassava, sunflower, and maize to potassium concentration in solution I. Growth and plant potassium concentration

Field Crops Research, 1 (1978) 347--361

347

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

RESPONSE OF CASSAVA, S U N F L O W E R , AND MAIZE TO POTASSIUM C O N C E N T R A T I O N IN S O L U T I O N I. G R O W T H A N D P L A N T P O T A S S I U M C O N C E N T R A T I O N

S.N. SPEAR, C.J. A S H E R and D.G. E D W A R D S

Department of Agriculture, University of Queensland, St. Lucia, Brisbane, Qld 4067

(Australia) (Received 29 August 1977)

ABSTRACT

Spear, S.N., Asher, C.J. and Edwards, D.G., 1978. Response of cassava, sunflower, and maize to potassium concentration in solution. I. Growth and plant potassium concentration. Field Crops Res., 1 : 347--361. Twelve cultivars of cassava and one cultivar each of sunflower and maize were grown for 27 days at eight constant solution potassium concentrations ranging from 0.5 to 8024 uM. All species and cultivars required a minimum solution concentration of 2 to 6 uM K to produce maximum yield. At solution potassium concentrations greater than 122 uM the yield of most cassava cultivars declined and at 8024 uM K all cultivars displayed magnesium deficiency symptoms. A limited variation among species and cultivars was observed in the minimum tissue concentration necessary for healthy growth. Large differences in dry matter percentage existed among species, plant parts and potassium treatments. Because of these differences, trends in potassium concentration expressed on a dry weight basis in some instances did not accurately reflect trends in potassium concentration in the fresh tissue. Although all three species produced maximum dry matter yield at similar solution potassium concentrations, at suboptimal concentrations cassava regulated its growth more successfully in relation to potassium supply than did sunflower or maize. At 0.5 uM K cassava continued to grow slowly with less reduction in relative yield, less pronounced deficiency symptoms and a smaller gradient in tissue potassium concentrations from older to younger leaves than the other species. Continued slow growth of cassava at low solution potassium concentrations was aided by greater increase in the relative size of the root system.

INTRODUCTION Cassava (Manihot esculenta Crantz) is an i m p o r t a n t c a r b o h y d r a t e source for h u m a n c o n s u m p t i o n , livestock feed, and a variety o f industrial uses. It is e s t i m a t e d t h a t the c r o p serves as a staple f o o d f o r s o m e 200 t o 300 million p e o p l e in the t r o p i c s (de Vries et al., 1 9 6 7 ; Nestel, 1 9 7 3 ) . T h e success o f cassava in subsistence f o o d p r o d u c t i o n s y s t e m s has b e e n a t t r i b u t e d to a n u m b e r o f f a c t o r s including its c o m p a r a t i v e l y high biological e f f i c i e n c y in the p r o d u c t i o n o f calories (de Vries et al., 1 9 6 7 ; C o u r s e y and Haynes, 1 9 7 0 ) ,

348

its drought tolerance, its ability to produce a crop on soils of low fertility, and the fact that harvest can be deferred until an acceptable yield of thickened roots has accumulated (Rogers and Appan, 1972). It has been suggested that there exists great potential for yield improvement in cassava and with an appropriate research input, comparable to that devoted to other major crops, substantial increases in productivity may be possible (Coursey and Haynes, 1970; Araullo et al., 1974}. Relatively little information is available concerning the mineral nutrition of cassava and there is a lack of precise quantitative data on external and internal nutrient requirements. Recent work at the Department of Agriculture, University of Queensland, has been aimed at providing such information using flowing solution culture techniques to ensure adequate characterization and control of the root environment. This paper describes effects of a wide range of constant solution potassium concentrations on the growth and tissue potassium concentrations of 12 cultivars of cassava, and one cultivar each of sunflower and maize. Following papers will describe effects on potassium absorption, distribution and utilization, and interactions between potassium, calcium and magnesium. MATERIALS AND METHODS Plant culture

Tip cuttings of 12 cassava cultivars were propagated in mist chambers according to the method of Forno et al. (1976). Sunflower (cv Hysun 10) and maize (cv Q617) seeds were surface sterilized in 0.3 per cent sodium hypochlorite solution, rinsed in deionized water, and germinated on moist paper towels in an incubator at 30 °C. Cassava, sunflower and maize were grown for 27 days at eight constant solution potassium concentrations in flowing culture units of 2250 1 capacity (Edwards and Asher, 1974). Solution flow rates were adjusted to 1.6 1/pot/rain, all 60 pots within each unit being arranged in parallel with respect to solution flow. Filtered air was bubbled continuously through the solution in each pot. Solution pH and temperature were maintained at 6.1 + 0.1 and 25.1 -+ 0.2 °C, respectively. Three replications were used for cassava cultivars M Aus 7, M Aus 17, Nina and Ceiba, while four replications were used for the remaining cassava cultivars, sunflower and maize. Nutrient solutions were analyzed daily for potassium, calcium and magnesium, drip feed systems being used to add these elements at rates approximately equal to rates of uptake. The mean daily concentrations together with their 95 per cent confidence limits were as follows (~M): potassium, 0.5 + 0.1, 1.8 + 0.2, 6.4 + 0.7, 31.9 + 2 . 8 , 1 2 2 +- 4, 525 +-19, 2064 + 64, 8024 + 191; calcium 753 ± 7; magnesium 147 + 3. At the lowest three potassium concentrations, analyses were made after a concentration process in which 200 ml of solution was passed through a 10 cm X 1 cm column of

349

Dowex-50 (H ÷ form) and eluted with 15 ml of 2N HNO3. Following the addition of NaC1 (1000 ppm sodium in the final volume) to decrease ionization interferences (Parker, 1970), potassium was determined by atomic absorption spectroscopy. Concentrations of basal nutrients other than calcium and magnesium were (/~M): N, 1000; S, 400 to 4400; Na, 170; P, 50; Fe, (as sequestrene-138) 20; C1, 15; Si, 10; B, 3; Zn, 0.5; Mn, 0.25; Cu, 0.10; Mo, 0.02. The range of sulphur concentrations arises from the use of K2SO4 as the potassium source. Basal sulphur was provided in all treatments as a mixture of calcium and magnesium sulphates (254 pM and 147 pM, respectively). An adequate supply of basal nutrients was maintained by completely renewing the nutrient solution once weekly. Nitrogen was supplied in the nitrate form (Ca(NO3)2.4H20) thereby avoiding the reported interferences of ammonium with the absorption of potassium (Tromp, 1962), calcium and magnesium (Cox and Reisenauer, 1973; Claassen and Wilcox, 1974). The phosphorus concentration (50 pM) was somewhat higher than is commonly used in flowing solution culture experiments (cf. Asher and Loneragan, 1967) because it has been found that cassava requires a higher solution phosphorus concentration for maximum growth than many other species (S. Jintakanon, personal communication, 1977).

Harvesting and chemical analysis At harvest, 27 days after imposing the treatments, roots were desorbed in 0.01M NaC1 and then separated from the shoots. Shoots were divided into stems and leaves (blades and petioles). In cassava and maize the leaves were further subdivided into immature leaves, the youngest fully expanded leaf (YFEL), the second, third and fourth fully expanded leaves, (L 2 3 4), the fifth, sixth and seventh leaves (L 5 6 7), the eighth, ninth and tenth leaves (L 8 9 10), and any remaining leaves below leaf position ten. In sunflower a similar procedure was followed, except that leaves more mature than the youngest fully expanded leaf were grouped in pairs instead of in threes. Fresh weights of each plant part were obtained and the samples were dried at 70 °C. After weighing, the samples were ground and subsamples digested in a HNO3--HC104 mixture (Johnson and Ulrich, 1959). After dilution potassium was determined by flame photometry. Tissue potassium concentrations were calculated on both fresh and dry weight bases. RESULTS

Symptoms and dry matter yield At 0.5/~M K all species and cultivars showed substantial yield depression (29 to 69 per cent). In sunflower and maize symptoms characteristic of potassium deficiency were observed (Wallace, 1951). With cassava some un-

350

certainty exists concerning typical potassium deficiency symptoms. Thus Cours (1953) reported that potassium deficient plants were small and showed inward curling of the leaves. Bronzing or purpling of the leaf, prior to tip and marginal chlorosis, were the symptoms found by Krochmal and Samuels (1968). However, Howell (1974) reported that potassium deficiency symptoms were seen on the lower leaves as a yellowing, followed by browning of the tips and adjacent margins. In the present study potassium deficiency symptoms first appeared as purple spotting of the older leaves, upward curling of the leaf margins and downward curling of the leaf tips. Later, chlorotic areas developed along the tips and margins of the leaf lobes eventually joining and developing into a necrotic strip. The older leaves and petioles senesced prematurely, with the exception of the leaves present on the cutting prior to the imposition of the treatments, which were retained and tended to be largely free of symptoms. Stem internodes were shortened markedly, and in cultivars M Aus 10, Ceiba and Mameya, longitudinal grooves developed in the upper stem internodes with fine cracks adjacent to these grooves. In general leaf symptoms were much milder in cassava than in the other two species. In addition, cassava continued to produce new photosynthetic area slowly under conditions of potassium deficiency. However, in sunflower and maize there appeared to be a rapid initial phase of growth after which gains in leaf area due to the expansion of new leaves were offset by senescence and death of the older leaves. The mean growth reduction due to potassium deficiency in cassava at 0.5 ~M K was less than in the other two species. Thus the growth of cassava was reduced on average by 48 per cent compared with 57 per cent for sunflower and 69 per cent for maize {Table I). However, among cassava cultivars there were large differences in susceptibility to potassium deficiency, the yield reductions at 0.5 ~M K ranging from as little as 29 per cent in M Aus 10 to as much as 68 per cent in Ceiba. Increasing the solution potassium concentration to 2 pM improved growth and lessened the severity of deficiency symptoms in all species and cultivars. Growth reductions were 43 and 30 per cent from the maximum yield for maize and sunflower, respectively, while in cassava the mean reduction in yield was 32 per cent. For one cassava cultivar, Mameya, and for sunflower, yield increases at solution potassium concentrations greater than 2 pM were not statistically significant. At 6 pM K, all species and cultivars were free from potassium deficiency symptoms and had either reached or closely approached maximum yield (Table I). In three cassava cultivars, M Aus 14, M Aus 17 and Ceiba, yields tended to increase over the range 6 to 122 pM K, but these increases were not statistically significant. With cassava there was a tendency in most cultivars for yields to decline at potassium concentrations above 122 ~M K, the respective mean yields of all cultivars at 525, 2064 and 8024 uM K being 82, 77 and 62 per cent of the maximum yield. At 8024 pM K, all cassava cultivars displayed leaf symptoms

351

TABLE

I

Total dry weights of cassava, sunflower, and maize (g/plant) grown in flowing culture at eight constant potassium concentrations Potassium concentration in solution (~M)

0.5

2

Species

Dry weight* of tops + roots (g/plant)

Cassava M Aus 3 M Aus 7 M Aus 10 M Aus 12 M Aus 14 M Aus 17 Nina Seda Amarillo Ceiba Pata de Paloma Mameya Mean of 12 cv. Sunflower Maize

8.37 5.75 7.87 6.37 8.44 5.76 6.52 3.78 7.13 4.49 5.09 6.34 6.33 2.76 5.85

11.73 7.96 8.92 7.87 9.19 7.10 8.56 3.53 7.99 5.87 6.81 9.39 7.91 4.37 10.92

6

17.28 13.84 11.03 9.74 11.96 11.27 11.85 6.62 12.77 10.55 8.56 11.91 11.53 5.14 19.07

32

13.27 9.07 9.70 9.50 12.87 10.46 8.28 5.33 9.86 9.87 8.06 9.08 9.61 5.68 14.49

122

15.56 12.12 10.85 9.24 14.81 16.01 9.90 5.33 11.30 14.22 8.81 12.86 11.77 6.30 16.24

525

2064

8024

15.18 9.42 10.54 9.12 11.59 10.26 9.34 6.45 10.94 8.08 5.47 9.73 9.68 5.36 14.09

13.40 9.31 10.56 9.07 10.43 10.05 8.15 6.43 11.03 6.54 5.11 10.75 9.24 5.47 13.71

8.10 7.73 10.43 10.49 6,12 8.18 7.92 4,19 7,61 4,13 5.80 8.09 7.40 6,42 15.51

*For species and cultivars, values not joined by the same horizontal line differ significantly (P = 0.05) by Duncan's Multiple Range Test. c h a r a c t e r i s t i c o f m a g n e s i u m d e f i c i e n c y ( H o w e l l , 1 9 7 4 ) while t h e s e s y m p t o m s w e r e o b s e r v e d also o n M Aus 7 at 2 0 6 4 p M K. A t 8 0 2 4 p M K r o o t s o f M Aus 7 and both roots and shoots of maize showed symptoms of calcium deficiency (Wallace, 1 9 5 1 ; F o r n o et al., 1 9 7 6 ) . H o w e v e r , n o s y m p t o m s o f e i t h e r c a l c i u m or magnesium deficiency were observed on sunflower. Effects of solution potassium concentration on the absorption, distribution and utilization of c a l c i u m and m a g n e s i u m are discussed in a s u b s e q u e n t p a p e r ( S p e a r et al., 1978b).

Root weight ratios While t h e r e s p o n s e s o f r o o t s a n d s h o o t s t o e x t e r n a l p o t a s s i u m c o n c e n t r a t i o n w e r e q u a l i t a t i v e l y similar, t h e e f f e c t s o f p o t a s s i u m o n s h o o t yields w e r e larger, so t h a t s u b s t a n t i a l c h a n g e s in r o o t w e i g h t r a t i o o c c u r r e d o v e r t h e range o f c o n c e n t r a t i o n s s t u d i e d ( T a b l e II). T h u s in cassava, r o o t w e i g h t ratios in t h e l o w e s t t h r e e p o t a s s i u m t r e a t m e n t s r a n k e d 0.5 p M > 2 p M • 6 p M , irrespective o f w h e t h e r t h e r a t i o s w e r e c a l c u l a t e d o n a fresh w e i g h t basis ( T a b l e II), o r o n a d r y w e i g h t basis ( d a t a n o t p r e s e n t e d ) . A t c o n c e n t r a t i o n s g r e a t e r t h a n

352

TABLE II R o o t weight ratios of cassava, sunflower, and maize grown at eight c o n s t a n t solution potassium c o n c e n t r a t i o n s Potassium concentration in solution (#M)

0.5

Species

R o o t weight ratio* (g fresh roots/g fresh roots + shoots)

Cassava -- m e a n -- range

0.42 0.31 to 0.49 0.29 0.30

Sunflower Maize

2

0.31 0.21 to 0.38 0.26 0.29

6

0.26 0.20 to 0.33 0.22 0.21

32

0.27 0.23 to 0.34 0.23 0.23

122

0.29 0.26 to 0.33 0.24 0.22

525

0.29 0.24 to 0.39 0.23 0.21

2064

0.28 0.23 to 0.36 0.24 0.20

8024

0.27 0.24 to 0.30 0.25 0.20

*Values not joined by the same horizontal line differ significantly(P ffi0.05) by Duncan's Multiple Range Test.

6 pM K, there was little effect of potassium concentration on ~ o t weight ratio. The root weight ratios for sunflower and maize were generally lower than those for cassava, sunflower showing less effect of potassium concentration on this ratio than either cassava or maize.

Dry matter percentage The dry matter percentage ( g dry weight/100 g fresh weight) was found to vary widely among species, plant parts and potassium treatments. These data are not presented but may be calculated from Table III as the ratio of the potassium concentrations on a fresh to those on a dry weight basis, multiplied by 0.39. The mean dry matter percentages over all cultivars and plant parts of cassava were substantially higher than the corresponding values for sunflower and maize. Thus, the mean dry matter percentage in the whole shoots of cassava grown at 6 ~M K was 14.3, compared with values of 5.9 and 6.6 for sunflower and maize, respectively. At all solution potassium concentrations, the dry matter percentages in cassava leaves increased with increasing leaf age. At 6 ~M K, the mean dry matter percentage in cassava increased from 13.5 in the immature leaves to 21.1 in the oldest leaves. However, in sunflower leaf dry matter percentages decreased with increasing leaf age, with the only exception the oldest leaves in the 0.5 ~M K treatment. Dry matter percentages in maize leaves increased with increasing leaf age at low solution potassium concentrations, but tended to decrease with increasing leaf age at solution potassium concentrations of 32 pM and above. In general, the dry matter percentages of mature leaves (leaf 2 and older) and stems decreased with increasing solution potassium concentration, where-

353

as the dry matter percentage of roots tended to increase. In maize, there was also a substantial decrease in the dry matter percentage of the immature and youngest fully expanded leaves with increasing solution potassium concentration. This trend was not apparent in cassava and sunflower.

Tissue potassium concentrations Large differences in tissue potassium concentration were observed among tissues, among species and among potassium treatments. Tissue potassium concentrations are presented on a fresh weight basis as well as the more usual dry weight basis {Table III) because of the difficulties in comparing concentrations expressed on a dry weight basis among tissues of widely differing dry matter percentage (Asher and Ozanne, 1967; Asher and Loneragan, 1967). In the lowest potassium treatment {0.5 ~M) the mean concentration of potassium in fresh cassava shoots (35 pmol/g fresh weight) was substantially higher than for sunflower or maize (25 and 22 ~mol/g fresh weight, respectively}. Even larger differences were observed in potassium concentrations in fresh root material {16, 3 and 7 ~mol/g fresh weight, respectively, for cassava, sunflower and maize). On a dry weight basis the relative differences among the three species were smaller because of compensating differences in the dry matter percentages of the respective shoots and roots. These observations, together with less severe deficiency symptoms and the generally smaller reductions in relative yield in cassava in the 0.5 pM K treatment, suggest that as a species cassava was less susceptible to very low external potassium concentrations than sunflower or maize. At 0.5 pM K there was substantial variation among cassava cultivars in the potassium concentration in shoots {27 to 42 pmol]g fresh weight} and roots (14 to 18 ~mol]g fresh weight}, but in no case were the concentrations as low as in sunflower or maize. In addition, the degree of yield reduction among cultivars was not closely related to tissue potassium concentration in this treatment (r = 0.45 n.s.}. Thus, in M Aus 7, which had the smallest reduction in relative yield in the 0.5 ~M K treatment, and in Ceiba, which had the largest yield reduction, the shoot tissue concentrations were quite similar at 32 and 35 pmol]g fresh weight, respectively. Increasing the solution potassium concentration from. 0.5 to 6 pM caused substantial increases in the concentration of potassium in all tissues (Table III}. Thus in cassava, tissue potassium concentrations in whole shoots and roots expressed on a fresh weight basis increased by factors of 2.9 and 3.3, respectively, whereas the corresponding increases in sunflower were by factors of 5.2 and 7.0, and those in maize by factors of 3.3 and 3.4, respectively. When the same tissue concentrations are expressed on a dry weight basis the relative increases from 0.5 to 6 ~M K were greater in the shoots for all species and in the roots of cassava because of simultaneous decreases in dry matter percentages of these tissues. However, the relative increases in potassium concentrations in the roots of sunflower and maize were less on a dry weight

95 95 97 78 80 66 73 24

120 106 104 104 81 62 107 104 21

119 121 121 115 106 101 78 103 53

145 145 157 145 130 113 126 59

96 95 92 95 95 89 128 115 37

131 129 130 141 140 132 102 124 66

161 166 189 176 169 136 160 66

174 166 177 179 163 143 137 145 40

124 125 126 136 141 130 99 120 68

145 163 165 180 180 167 169 70

150 138 136 128 139 116 134 136 70

134 135 145 160 166 151 121 143 81

*Specific s h o o t p a r t s a r r a n g e d in o z d e r o f i n c r e a s i n g m a t u r i t y . * * V a l u e s are m e a n s of 12 cultivars. ~ ' Y o u n g e s t fully e x p a n d e d leaf.

77 56 33 27 23 31 33 10

98 67 62 37 29 18 31 40 9

I01 79 65 52 55 56 36 55 28

160 171 194 215 214 152 165 82

158 180 170 146 139 109 136 138 59

139 141 159 184 184 170 139 160 83

43 26 16 13 9 21 22 7

2064

Maize I m m a t u r e leaves YFELt L 2 3 4 L 5 6 7 L 8 9 10 Stem Whole s h o o t Roots

525

91 57 33 20 12 I0 15 25 3

122

Sunflower I m m a t u r e leaves YFEL~ L 2 3 L 4 5 L 6 7 R e m a i n i n g leaves Stem Whole shoot Roots

32

60 44 36 33 37 36 25 35 16

6

Cassava** I m m a t u r e leaves YFEL t L 2 3 4 L 5 6 7 L 8 9 10 R e m a i n i n g leaves Stem Whole shoot Roots

2

(a) P o t a s s i u m c o n c e n t r a t i o n in f r e s h p l a n t tissue (/~mol/g f r e s h w e i g h t )

0.5

Species a n d plant part*

(~M)

Potassium concentration in s o l u t i o n

137 161 203 215 250 165 167 72

172 179 184 163 171 145 138 148 67

151 160 194 223 221 234 182 197 85

8024

2

6

32

122

525

2064

8024

1.45 0.82 0.46 0.31 0.17 1.41 0.89 0.61

2.89 1.81 1.08 0.58 0.40 0.26 1.14 1.04 0.59

1.44 0.96 0.67 0.56 0.55 0.46 0.70 0.68 0.80

2.55 1.77 1.00 0.80 0.71 2.43 1.74 0.85

3.27 2.46 2.12 1.41 1.07 0.78 2.67 2.15 1.01

2.53 1.83 1.35 1.03 0.93 0.78 1.06 1.21 1.24

3.38 3.01 2.95 2.51 2.90 6.03 4.30 1.77

4.41 4.18 4.60 4.38 3.96 3.38 10.33 6.91 3.52

3.44 3.16 3.04 2.73 2.33 1.87 3.25 2.81 2.86

5.22 5.25 5.52 5.81 5.50 9.53 7.35 4.08

3.30 3.39 3.63 3.90 4.26 4.24 12.17 7.22 4.98

3.55 3.20 3.31 3.03 2.79 2.28 3.47 3.15 3.19

5.35 5.33 6.22 6.49 6.02 12.86 9.08 4.82

6.17 6.26 7.48 8.37 8.17 8.67 13.72 10.23 6.85

3.66 3.42 3.56 3.33 3.15 2.47 4.16 3.47 3.89

5.68 6.33 6.33 6.97 7.87 14.28 10.01 4.69

5.63 5.49 6.66 7.22 7.20 6.15 12.84 9.17 8.71

3.63 3.35 3.37 3.50 3.48 2.72 4.26 3.62 3.75

5.94 6.33 7.43 8.78 9.61 13.73 10.41 5.70

5.22 6.11 7.26 7.31 7.99 7.45 13.46 9.85 7.97

3.57 3.34 3.51 4.03 3.96 2.98 4.49 3.89 3.93

5.59 6.30 7.59 8.97 10.73 15.39 11.62 5.25

5.59 6.30 7.16 7.25 8.44 8.97 13.65 10.01 8.61

3.62 3.55 4.23 4.94 4.57 3.48 5.96 4.68 3.91

(b) P o t a s s i u m c o n c e n t r a t i o n in d r y p l a n t tissue ( g / 1 0 0 g d r y w e i g h t )

0.5

C o n c e n t r a t i o n s o f p o t a s s i u m in specific p l a n t p a r t s of cassava, s u n f l o w e r , a n d m a i z e e x p r e s s e d o n (a) f r e s h w e i g h t basis a n d (b) d r y w e i g h t basis

T A B L E III ol

355

basis because of increases in corresponding dry matter percentages over the same range of solution concentrations. When solution potassium concentrations were increased beyond 122/~M there were substantial increases in the concentration of potassium in the roots of all three species, but only in cassava was there also a substantial increase in the concentration of potassium in the shoots (Table III). It is evident that in all species and cultivars, fresh shoot tissues contained substantially higher concentrations of potassium than the corresponding root tissues, indicating a clear concentration gradient from the root (low) to the shoot (high) (Table IIIa). At low solution potassium concentrations, and particularly at 0.5/~M K, there was a marked gradient in the concentration of potassium in the fresh leaf tissue from older (low) to younger leaves (high). The magnitude of the gradient within the shoots was greatest in sunflower in which the potassium concentration in immature leaves was 9.1 times greater than in the oldest leaves, and least in cassava, in which the potassium concentration in the immature leaves was only 1.6 times greater than in the oldest leaves. These observations suggest that potassium was less readily distributed to the younger growing tissue in cassava than in the other two species. Hence the normal pattern of high mobility of potassium in the phloem (Epstein, 1972) may not be fully applicable to cassava. At the higher solution potassium concentrations there was a tendency for potassium to accumulate in the older leaves of cassava and maize, but in sunflower this tendency was not apparent even at 8024 gM K (Table IIIa). In all three species the concentration of potassium in the fresh stem tissue at most solution potassium concentrations was intermediate between that in the younger and older leaf positions. In a number of instances these patterns of concentration in the fresh tissue are not clearly evident when concentrations are expressed on a dry weight basis because of differences in dry matter percentages among the tissues. For example, with cassava at 8024/~M K, the potassium concentration increased from 151/~mol/g fresh weight in immature leaves to 234 pmol/g fresh weight in leaves older than leaf 10 (Table IIIa), but the dry matter percentage also increased substantially (16.3 to 26.2 per cent) and as a consequence the potassium concentration, when expressed on a dry weight basis, decreased from 3.62 per cent in the immature leaves to 3.48 per cent in leaves older than leaf 10. Similarly with maize in the 0.5/~M K treatment, the concentration of potassium in leaves 8 to 10 and roots were rather similar in the fresh tissue (9 and 7 pmol/g fresh weight, respectively), but these tissues differed markedly in dry matter percentage (20.7 and 4.5 per cent, respectively). When the potassium concentrations are expressed on a dry weight basis (Table IIIb) it appears that the potassium concentration in the roots (0.61 per cent) was much higher than in leaves 8 to 10 (0.17 per cent).

356

DISCUSSION

Solution potassium concentration required for maximum growth The results of the current study strongly support recent reports that plants can make vigorous growth at comparatively low solution potassium concentrations. In studies in which no ammonium has been added to the nutrient solution, plants have generally achieved maximal growth at solution concentrations of 25 pM K or less. Thus Wild et al. (1974) showed that four temperate pasture species achieved maximal growth at solution potassium concentrations of 1 to 3 gM, and Williams (1961) found that barley plants required about 25 pM K for healthy growth. Results of the present study in which cassava, sunflower and maize reached maximum growth within the the range 2 to 6 pM K in the absence of ammonium are in agreement with these earlier studies. In view of the reported strong inhibition of potassium uptake by ammonium (Tromp, 1962), somewhat higher optimal solution potassium concentrations may be required in the presence of that ion. Thus, Asher and Ozanne (1967) found that the optimum solution potassium concentration for a group of 14 temperate pasture species grown with 300 pM NH4 ÷ ranged from 24 to 95 ~M. More recently, Fageria (1974, 1976) reported that groundnuts and rice both required about 250 pM K for maximal growth in the presence of 454 pM NH4 ÷. The relatively low potassium concentrations needed for healthy plant growth in the present study and other studies cited above (viz. less than 250 pM K) contrast markedly with the high potassium concentrations (greater than 4000 pM) commonly used in non-renewed or intermittently renewed solution culture and sand culture systems (Hewitt, 1966).

Tissue potassium concentrations required for maximum growth In the study of Asher and Ozanne (1967) it was found that the lowest tissue potassium concentrations corresponding with maximum yield ranged from 112 to 197 ~mol/g fresh weight for shoots and from 54 to 126 gmol/g fresh weight for roots. In the current study the tissue potassium concentrations required by maize for healthy growth (68 and 24 pmol/g fresh weight for shoots and roots, respectively) and by sunflower roots (21 ~zmol/g fresh weight) were much lower than these values. However, concentrations required by sunflower shoots (104 pmol/g fresh weight) and by cassava (84 to 125 and 40 to 72 gmol/g fresh weight for shoots and roots, respectively) were in close agreement with those reported by Asher and Ozanne (1967). The results of these two studies therefore suggest the existence of only rather limited variation amongst species and cultivars in the minimum concentrations of potassium needed in the fresh shoot and root material for healthy growth. For tissues of the same dry matter percentage there is a linear relationship between nutrient concentration in the fresh plant material and the corre-

357

sponding concentrations on an oven dry basis. However, for any particular nutrient concentration in the fresh plant material there is an inverse relationship between dry matter percentage and the nutrient concentration expressed on a dry weight basis. The theoretical form of the relationship relating tissue potassium concentration in the fresh tissue and the dry matter percentage to the potassium concentration on a dry weight basis is shown in Fig. 1. Also shown is the distribution of data points from the present study over the surface relating these three parameters. From Fig. I it is apparent that potassium concentrations expressed on a dry weight basis are very sensitive to variation in dry matter percentage at low dry matter percentages (lessthan 10 per cent), but are relatively insensitive to such variations at high dry matter percentages (greater than 20 per cent). As indicated previously, differences in dry matter percentage among tissues can be large enough to preclude the use of concentration data expressed on a dry weight basis to indicate the distribution of potassium concentration among "-~.

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Fig. 1. Relationship between tissue potassium concentration expressed on a fresh weight and on a dry weight basis, and dry matter percentage in shoots and roots of cassava (4,A) (mean of 12 cultivars), sunflower (re,o) and maize (o,o).

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fresh tissues at the time of harvest. Furthermore, large differences in dry matter percentage can confuse comparisons of tissue concentrations among species. Thus, the minimum potassium concentration in the shoots of cassava free of potassium deficiency symptoms ranged from 1.04 to 3.23 per cent on a dry weight basis, compared to 6.91 and 4.30 per cent for sunflower and maize, respectively. However, when these values are expressed on a fresh weight basis, the differences are much reduced (47 to 121 pmol/g fresh weight for cassava, 104 and 73 ~mol/g fresh weight for sunflower and maize, respectively) because of the large differences in dry matter percentage between the species. Adaptation o f cassava cultivars to low potassium supply The demonstration of substantial differences in the ability of individual cultivars to grow at low substrate potassium concentrations could be of significance in relation to work on genetic improvement of the crop. The major softs throughout the tropics on which cassava is grown are moderately to strongly acidic oxisols and ultisols (Tan and Bertrand, 1972), many of which are likely to be low in both total and available potassium. Thus the ability to grow well at low soil solution potassium concentrations would appear to be a desirable characteristic. However, the data presented in this paper refer to growth of young plants for a relatively short period of time (4 weeks) under glasshouse solution culture conditions. Longer term studies are needed to establish how closely the ability of cassava cultivars to produce acceptable yields under potassium deficient field conditions is correlated with performance in such short term experiments. A number of authors have referred to the apparent adaptation of cassava to growth on softs of low fertility (Jones, 1959; Rogers and Appan, 1971; de Geus, 1973; Cock, 1974; Gondwe, 1974). However, few quantitative studies are available concerning the relative performance of cassava and other crops under such conditions. In the present study, cassava, sunflower, and maize all produced maximum dry matter yield at similar solution potassium concentrations. However, observations during the course of the experiment suggest that, at suboptimal potassium concentrations, cassava may have regulated its growth rate more successfully in relation to potassium supply than either sunflower or maize. At the lowest solution potassium concentration (0.5 ~M) cassava continued to grow slowly with less reduction in relative yield from the maximum, less obvious development of deficiency symptoms and with a smaller gradient in tissue potassium concentration from older to younger leaves. Adaptation of other species to low fertility conditions has been shown to be associated with slower growth rates and lower yield response than would be usual for plants adapted to higher fertility conditions {Gerloff, 1963; Rorison, 1968; Grundon, 1972; White, 1972; Christie and Moorby, 1975). However, species adaptation to limiting nutrient supply conditions need not

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only reside in slower growth rates and smaller yield reductions under deficiency conditions. The ability of a species to respond quickly to variations in nutrient supply would appear to be another mechanism of some importance to the survival and growth of that species. Visual observations suggest that cassava may possess the ability to adjust rapidly its rates of growth downward to match decreases in the external supply of nutrients available to the plant. Another attribute which may afford some plant species the potential to grow better than other species in low soil fertility conditions is an ability to increase the size of the root system relative to the size of the tops under conditions of nutrient stress (Asher and Ozanne, 1967). The observed substantial increase in root weight ratio in potassium deficient plants grown at the lowest solution potassium concentration (Table II) is in agreement with the results of previous studies (Asher and Ozanne, 1967; Freeman, 1967; Lastuvka and Minar, 1970; Wild et al., 1974). Cassava generally had higher root weight ratios than both sunflower and maize over the wide range of solution potassium concentrations studied (0.5 to 8024 pM). In addition, cassava increased the relative size of its root system to a greater extent than either sunflower or maize at low solution potassium concentrations. Thus, as the solution concentration decreased from 6 to 0.5 pM K, the average root weight ratio of cassava cultivars increased by 58 per cent (range of 31 to 90 per cent), whereas for sunflower and maize the increases were 32 and 43 per cent, respectively. Both the higher root weight ratio of cassava and its ability to increase the root weight ratio at limiting solution potassium concentrations have important implications in the uptake of potassium. In a subsequent paper (Spear et al., 1978a) it will be shown that cassava had lower uptake rates per unit root weight than both sunflower and maize. However, the generally higher root weight ratios of cassava and its ability to increase the root weight ratio at limiting potassium concentrations (0.5 to 6 gM) would partially offset these lower uptake rates per unit root weight. ACKNOWLEDGEMENTS The senior author gratefully acknowledges financial support during the course of this study from a Commonwealth Postgraduate Research Award and the 1976 Farter Memorial Research Scholarship. The technical assistance of B. Kenyon, G. Kerven, J. Mercer and G. Waiters is gratefully acknowledged. The glasshouse and flowing culture equipment used in this study were provided as a result of grants from the Australian Research Grants Committee, the Rural Credits Development Fund and the Australian Wheat Industry Research Council. REFERENCES Araullo, E.V., Nestel, B. and Campbell, M., 1974. Cassava Processing and Storage. I.D.R.C., Ottawa, 125 pp.

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Asher, C.J. and Loneragan, J.F., 1967. Response of plants to phosphate concentration in solution culture. I. Growth and phosphorus content. Soil Sci., 103: 225--233. Asher, C.J. and Ozanne, P.G., 1967. Growth and potassium content of plants in solution cultures maintained at constant potassium concentrations. Soil Sci., 103: 155--161. Christie, E.K. and Moorby, J., 1975. Physiological responses of semi-arid grasses. I. The influence of phosphorus supply on growth and phosphorus absorption. Aust. J. Agric. Res., 16: 423--436. Claassen, M.E. and Wilcox, G.E., 1974. Comparative reduction of calcium and magnesium composition o f corn tissue by NH4-N and K fertilization. Agron. J., 66: 521--522. Cock, J.H., 1974. Agronomic potential for cassava production. In: E.V. ArauUo, B. Nestel and M. Campbell (Editors), Cassava Processing and Storage. I.D.R.C., Ottawa, pp. 21--26. Cours, G., 1953. Amelioration des plants: le manioc. Bull. Rech. Agron. Madagascar, 2: 78--88. Coursey, D.G. and Haynes, P.H., 1970. Root crops and their potential as food in the tropics. World Crops, 22: 261--265. Cox, W.J. and Reisenauer, H.M., 1973. Growth and ion uptake by wheat supplied nitrogen as nitrate, or ammonium or both. Plant Soil, 38: 363--380. De Vries, C.A., Ferwerda, J.D. and Flach, M., 1967. Choice of food crops in relation to actual and potential production in the tropics. Neth. J. Agric. Sci., 15: 241--248. Edwards, D.G. and Asher, C.J., 1974. The significance of solution flow rate in flowing culture experiments. Plant Soil, 41: 161--175. Epstein, E., 1972. Mineral Nutrition of Plants: Principles and Perspectives. John Wiley and Sons, New York, 412 pp. Fageria, N.K., 1974. Uptake of potassium and its influence on growth and magnesium uptake b y groundnut (Arachis hypogaea L.) plants. Biol. Plant., 16: 210--213. Fageria, N.K., 1976. Influence of potassium concentration on growth and potassium uptake by rice plants. Plant Soil, 44: 567--573. Forno, D.A., Asher, C.J. and Edwards, D.G., 1976. Mist propagation of cassava tip cuttings for nutritional studies: Effects of substrate calcium concentrations, temperature and shading. Trop. Agric. (Trinidad), 53: 47--55. Freeman, G.G., 1970. Studies on the potassium nutrition of plants. III. Effects of potassium concentration on growth and mineral composition of vegetable seedlings in solution culture. J. Sci. F o o d Agric., 21: 121--126. Gerloff, G.C., 1963. Comparative mineral nutrition of plants. Ann. Rev. Plant Physiol., 14: 107--124. Geus, J.G. de, 1973. Fertilizer Guide for Tropical and Subtropical Farming. Centre d'Etude de l'Azote, Zurich, pp. 206--212. Gondwe, A.D.T., 1974. Studies on the hydrocyanic acid contents of some local varieties of cassava (Manihot esculenta Crantz) and some traditional cassava food products. E. Aft. Agric. For. J., 40: 161--167. Grundon, N.J., 1972. Mineral nutrition o f some Queensland heath plants. J. Ecol., 60: 171--181. Hewitt, E.J., 1966. Sand and Water Culture Techniques in the Study of Plant Nutrition. Comm. Bur. Hort. and Plant. Crops Tech. Comm. No. 22. Howell, D.J., 1974. Symptoms of nutrient deficiency in cassava (Manihot esculenta Crantz). Thesis, Univ. Guelph, Ont., Canada. Johnson, C.M. and Ulrich, A., 1959. Analytical methods for use in plant analysis. Calif. Agric. Exp. Stn Bull. No. 766. Jones, W.O., 1959. Manioc in Africa. Stanford University Press, Stanford, California. Krochmal, A. and Samuels, G., 1968. Deficiency symptoms in nutrient p o t experiments with cassava. Ceiba, 14: 9--16. Lastuvka, Z. and Minar, J., 1970. The relation between nutrient solution concentration and growth and ion absorption of peas (Pisum sativum L.). I. Growth of peas. Plant Soil, 32: 189--197.

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Nestel, B., 1973. Current utilization and future potential for cassava, In: B. Nestel and R. MacIntyre (Editors), Chronic Cassava Toxicity. I.D.R.C., Ottawa, pp. 11--26. Parker, C.R., 1970. Water analysis b y atomic absorption. Varian Techtron Pty. Ltd., Springvale, Victoria. Rogers, D.J. and Appan, S.G., 1971. What's so great about cassava? World Farming, 13: 14, 16--22. Rogers, D.J. and Appan, S.G., 1972. Cassava (Manihot esculenta Crantz), the plant, world production and its importance in world food supply. In: C.H. Hendershott et al. (Editors)~ A Literature Review and Research Recommendations on Cassava. Univ. Georgia, Athens, Georgia, pp. 1--14. Rorison, I.H., 1968. The response to phosphorus of some ecologically distinct plant species I. Growth rates and phosphorus absorption. New Phytol., 67: 913--923. Spear, S.N., Asher, C.J. and Edwards, D.G., 1978a. Response of cassava, sunflower, and maize to potassium concentration in solution. II. Potassium absorption and its relation to growth. Field Crops Res., 1: 363--373. Spear, S.N., Edwards, D.G. and Asher, C.J., 1978b. Response of cassava, sunflower, and maize to potassium concentration in solution. III. Interactions between potassium, calcium and magnesium. Field Crops Res., 1 : 375--389. Tan, K.H. and Bertrand, A.R., 1972. Cultivation and fertilization of cassava. In: C.H. Hendershott et al. (Editors), A Literature Review and Research Recommendations on Cassava. Univ. Georgia, Athens, Georgia, pp. 37--75. Tromp, J., 1962. Interactions in the absorption of ammonium, potassium and sodium by wheat roots. Acta Bot. Neerl., 11: 147--192. Wallace, T., 1951. The Diagnosis of Mineral Deficiencies in Plants. H.M.S.O., London. White, R.E., 1972. Studies on mineral ion absorption b y plants. I. The absorption and utilization of phosphate by Stylosanthes humilis, Phaseolus atropurpureus and Desmodium uncinatum. Plant Soil, 36: 427---447. Wild, A., Skarlou, V., Clement, C.R. and Snaydon, R.W., 1974. Comparison of potassium uptake by four plant species grown in sand and in flowing solution culture. J. Appl. Ecol., 11: 801--812. Williams, D.E., 1961. The absorption of potassium as influenced by its concentration in the nutrient medium. Plant Soil, 15: 387--399.