INFLUENCE OF POTASSIUM.MAGNESIIJM,A.NTAGONISM PLANT GROWTH K. L. KABU and E. W. TOOP Department ol Plant Science, (Jniversity o'f Alberta, Edmonton, Alberta. Received June 11,1970, accepted August 24,1970. ON TOM.A.TO ABSTR.ACT The influence of low and high levels of sub- antagonism, leaf content of magnesium restrate potassium on the uptake and distribu- mained constant at the expense of the stem. tion of magnesium was studied in tomato Greater quantities of magnestum were replants. Magnesium content of the stems and quired in the plant tissues to prevent defileaves increased with increases in substrate ciency under conditions of high substrate magnesium, regardless of the level of potas- potassium. However, under these conditions sium. However, high substrate potassium (12 essentially the same level of magnesium is meq/liter) significantly reduced magnesium needed in the indicator tissue of tomato plants uptake (content of aerial portions of the (fifth leaf from the top) as is normally beplants) at the high substrate magnesium level lieved adequate for optimal growth and yield. (3 meq/liter). Under this potassium-induced INTR.ODUCTION Magnesium deficiency has been reported on a wide range of crops under field conditions. In greenhouses it has been observed with considerable frequency on crops such as tomatoes, cucumbers and chrysanthemums. The symptoms of magnesium deficiency, as they occur on a variety of crops, have been described and illustrated by Chapman (2), Jacob (6) and Wallace (7). It is natural to expect magnesium deficiency to occur on soils low in exchangeable magnesium, but it is frequently observed even when soil magnesium is considered adequate. The latter situation is encountered more frequently under greenhouse than field conditions, and is believed to be induced by the relatively high levels of potassium usually maintained in greenhouse soils. This situation is often referred to as potassium-induced magnesium deficiency, and many references have been made to the aggravation of magnesium deficiency by potash fertilizing, especially in recent years ( 1, 5, 1 0, 1 1 ). Surprisingly, not much attention seems to have been paid to the extent to which potassium-induced magnesium deficiency can affect the growth and yield of crops susceptible to this malady. The object of this study was to investigate the influence of K-Mg antagonism on the growth and mineral composition of tomato plants, and to relate this information to the established levels of magnesium in tomato indicator leaves believed to be adequate for optimal growth and yield. MATERIALS AND METHODS Tomatoes, cultivar Michigan-Ohio Hybrid, were seeded directly into 72 Zo-cm plastic pots which were filled with perlite moistened with distilled water. Three seeds were planted per pot, but only one seedling was allowed to remain after germination. Nutrient solutions were applied as soon as emergence had occurred. The nitrate-type, standard nutrient solution as used at Long Ashton (3), including minor elements according to recommendations of Hoagland and Arnon (4), was prepared with variations in the potassium and magnesium levels as required by the various treatments. Thirty-six pots received potassium at 4 meq/liter of nutrient solution, and the remaining half (36 pots) at 72 meq/liter. These two levels of potassium are hereafter refeued to as the low and high potassium levels, respectively. Plants of each potassium treatment were divided into six subgroups, Can. J. Plant Sci. 50: 711-715 (Noy, 1970) 7rl
712 CANADIAN JOURNAL OF PLANT SCIENCE each receiving the following amounts of magnesium, respectively: O.I25, O.Z5O,,, 1.50 and 3.0 meq/liter. 'fhe 12 treatments were randomized, with six replications. Nutrient solutions were applied and replaced at weekly intervals, with interim applications of distilled water when needed. The plants were harvested 11 weeks after emergence, at which time the first truss was setting fruit, a stage of growth when the requirement for mineral nutrients is presumably high. The harvested plants were washed and then dried in a forcedair oven at 7O C. The dry weights of roots, stems and leaves were recorded separately. The samples were ground in a Wiley mill to pass a 60-mesh screen. Each of the ground samples was thoroughly mixed, a weighed quantity ashed and an extract prepared by the method outlined by Ward and Johnston (9). The extracts were then anatyzed for magnesium and potassium concentrations on a Perkin-Elmer 303 atomic absorption spectrophotometer. RESUI,TS,dND DISCUSSION Dry weight of plants The average dry weight of plants obtained from the various treatments is given in Table i. Under the low potassium level, no significant increases in growth (dry weight) were obtained by increasing the magnesium concentration in the nutrient solution beyond meq/liter. At the high potassium level, a comparable growth response was obtained only when the substrate level of magnesium reached -1.5 meq/liter of nutrient solution. This indicates a requirement for higher substrate magnesium when substrate potassium is increased. Heavy potassium fertilization, therefore, can cause magnesium deficiency of a magnitude sufficient to restrict growth as measured on a dry weight basis. Magnesium concentration in the plant The concentration of magnesium as a percentage of dry weight is summarized in Table 2. Irrespective of the kind of tissue involved and the level of potassium fertilization, the magnesium concentration showed an increase with the increase in the substrate magnesium level. The high potassium level caused a reduction in stem tissue magnesium with substrate nutrient levels of 1.50 and meq/liter. The situation in the leaves was quite different, since there was apparently no significant influence of substrate potassium level on magnesium content. Table 1. Drv weight of tomato plants as affected by magnesiur.n and potassium substrate levels Magnesium concentration in nutrient solution meq/liter 0. 250 Dr1' weight per plant (g) 4 neq K+/liter 12 meq K+/liter 4.36 a 8.54 b 15.06 c 15.87 c 76.06 c 16.07 c 5.74 a 7-44a 6.92 a 12.67 bc 15.20 c 15.50 c o-c Means not followed by the same letter are significantly different (P : 0.05 ) according to Duncan's multiple range test.
KABU AND TOOP-K-MG ANTAGONISM ON TOMATO 713 'fable 2. Nlagnesium content of tomato plants as aflected b)'magnesium and potassium substrate levels \il agr.resium concelltration in nutrient solution meq/liter 0.250 u.j/.') Sten'rs 4 meq K+ 12 meq /liter K+T liter 0.02 ab 0.01 a 0.03 abc 0.02 ab 0.06 cd 0.05 bc 0.09 e 0.08 de 0.29 h 0.r7 J i 0.24 g Magnesium content (c2 of dry weight) [-eaves 4 meq K+ 12 meq / liter K+/liter 0.05 o 0.06 ab 0.09 bc 0.15 d 4.37 c 0.69 f 0.06 c.b 0.08 b 0.13 cd 0.16 d 0.43 e 0.7r 1 Whole plant (stems and leaves) 4 meq K+ 12 rneq /liter K+/liter 0.01a 0.05 ab O.08 cd 0.13 e 0.34.f 0.62 h 0.05 ob 0.06 bc 0.70 de 0.73 e 0.33 / 0.53 g o-i Means not followed by the same letter within each group (i.e. stems, lear.es, whole plant) are significantly difierent (P : 0.05) according to Duncan's mulriple range tcsr, The magnesium content of the total aerial part of the plant at the lower levels of substrate magnesium appeared to be reduced in association with low-level potassium feeding, but the differences were not statistically significant. At the highest levels of substrate magnesium, there was a reversal of this trend. In spite of the comparable uptake of magnesium at lower substrate levels under both potassium levels, the plants showed magnesium deficiency symptoms and reduced plant growth under substrate conditions of low magnesium and high potassium. It appears, therefore, that the uptake of magnesium is not hindered and that the magnesium apparently becomes unavailable within the plant, having accumulated in the leaves. At the highest substrate concentration of magnesium the uptake was significantly reduced by the high level of potassium. However, magnesium levels in the leaf remained constant at each level of substrate magnesium, regardless of the stress imposed on uptake by potassium substrate levels. Ward and Milter (10) tried to develop potassium-induced magnesium deficiency symptoms in tomatoes. They used 3 meq Mg/litet and 12 meq K/liter of solution. No symptoms were produced even after 73 days. Results indicated high potassium content in leaves but no depression in magnesium levels. The authors suggest that the amounts of potassium were not large enough to prevent absorption of magnesium. The present investigations indicate that a depression of magnesium in their plants could have gone undetected, since only the leaves were afialyzed. Under the similar nutritional conditions of this study, a decrease of over 5OVo in stem magnesium contributed to an overall decrease of about 15% in the magnesium content of the entire aerial portions of the plant. These decreases are statistically significant. Potassium concentration in the plant An increase in the concentration of potassium in the substrate significantly increased the potassium content in the plant, irrespective of the substrate level of magnesium (Table 3). Under both levels of potassium there was a trend toward decreasing potassium content in the plant with an increase in the magnesium concentration in the substrate. Significant decreases occurred only with the low rate of potassium nutrition in association with 0.250 and meq/liter Mg. Beyond the latter
714 Table 3. CANADIAN JOURNAL OI' PLAN'T SCIENCE Potassiun content of tomato plants as altected b)'mzrgnesium and potassium substrate Ievels N{ agnesiu n'r concentrittioll in nutrierrt solution meq/liter 0.250 3.00 Pot:rssium content in :rerial portion of plant (percent of dry rn,eight) 4 meq K-/liter L2 meq K+/liter 6.05 c +.90 b 3.84 o 3.76 a.1. lo I 3.75 a 7.06d 7.45d l.tt o 7.r8 d 6.98 d 6.85 d o-d.means not followed by the samc letter are signillcantl:' difierent (P : 0.05 ) according to Duncan's multiple ratrgc substrate level of magnesium no further reduction took place. It can be inferred from this that magnesium hinders the uptake of potassium, but this antagonistic effect is significant only in an environment of relatively low concentrations of both potassium and magnesium. This is opposite to the conditions under which potassium exerts an antagonistic effect on magnesium, that is, at higher concentrations of the two elements. Foliar analysis as a diagnostic tool to study magnesium requirernent Plant tissue analysis as a means of diagnosing the nutritional needs of agricultural crops has become a well-recognized practice. Many investigators have published values for optimal nutrition of selected crops. Ward and Miller (10) found incipient magnesium deficiency in tomato plants, cultivar Michigan-Ohio Hybrid, to be associated with a magnesium level of O.3O% of dry weight, but they recommended a level of 0.45% for healthy, normal growth of greenhouse tomatoes. To study the effect of K-Mg ion interaction on the critical nutrient level, indicator leaves were analyzed separately from the rest of the plant. The indicator tissue consisted of the fifth leaf from the growing tip, as recommended by Ward (8) and Ward and Miller (10). The magnesium content in the indicator tissue from plants subjected to the various treatments is recorded in Table 4. The magnesium levels associated with maximum growth (dry weight) were 0.33Vo and O.38Vo for plants under the low TeLble 4. Influence of substrate potassium leve[:l"lt Nlagnesiu m concentratior.r in nutrient solution meq,/liter 0. 250 m:rgnesiurn content of indic:rtor tissue* Nlagnesium content of nervly matttring leaves (percent of dry rveight) 4 n-req K+/liter 12 meq K+/liter 0.20 a 0.28 h 0.33 cd. 0.35 d 0.47 e 0.57 fg o.26 b 0.29 bc 0.37 d 0.38 d 0.51 ej 0.60 g *Fifth leaf from the growing titrl a-g Means not follow-retl by ihe'iame lelter are signiticantly diffcrcnt (P : 0.05 ) accorcling to Duncan'-. multiple range test.
KABU AND TOOP-K-MG ANTAGONISM ON TOMATO 7t5 and high level of potassium, respectively. Both levels were in close proximity to the findings of ward and Miller (10). Foliar analysis, therefore, appears to be a reliable tool for diagnosing magnesium nutritional needs, even in cases where K-Mg ion interactions are involved. ACKNOWLEDGEMENT Funds for this study were provided by the Alberta Agriculturel Research Trust. REFERENCES 1. BRANSON, R. L., SCIARONI, R. H. and RIBLE, J. M. 1968' Magnesium deficiencv in cut-flower chrysanthemums. California Agriculture. August: 13-14. 2. CHAPMAN, H. D. 1966. Diagnostic criteria for plants and soils. Univ. Calif., Div. Agr. Sci., pp. 225-263. 3. HEWITT, E. J. 1966. Sand and water culture methods used in the study of plant nutrition. Commonwealth Agr. Bur'. Tech. Commun. 22. 2nd ed. 4. HOAGLAND, D. R. and ARNON, D. I. 1950. The water culture methods for growing plants without soil. Calif. Agr. Exp. Sta. Circ. 347. 5. HOHLT, H. E. and MAYNARD, D. N. 1966. Magnesium nutrition of spinach. Proc. Amer. Soc. Hort. Sci. 89: 478-482. 6. JACOB, A. 1958. Magnesium-the fifth major plant nutrient. Staples Press, London. 7. WALLACE, T. 1951. The diagnosis of mineral deficiencies in plants by visual symptoms. H.M. Stationery Office, London. 8. WARD, G. M. 1963. The application of tissue analysis to greenhouse tomato nutrition. Proc. Amer. Soc. Hort. Sci. 83: 695-699. 9. WARD, G. M. and JOHNSTON, F. B. 1962. Chemical methods of plant analysis. Can. Dep. Agr. Publ. 1064. 10. WARD, G. M. and MILLER, M. J. 1969. Magnesium deficiency in greenhouse tomatoes. Can. J. Plant Sci. 49: 53-59. 11. WELTE, E. and WERNER, W. 1963. Potassium-magnesium antagonism in soils and crods. J. Sci. Food Aer. tr4: 180-186.