THE NEW PHYTOLOGIST STUDIES OF THE PHYSIOLOGICAL IMPORT- ANCE OF THE MINERAL ELEMENTS IN PLANTS

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1 THE NEW PHYTOLOGIST VOL. XXXVII, No. i 28 February, 1938 STUDIES OF THE PHYSIOLOGICAL IMPORT- ANCE OF THE MINERAL ELEMENTS IN PLANTS THE VARIATION IN POTASSIUM CONTENT OF MAIZE LEAVES DURING THE DAY BY NORAH L. PENSTON Department of Botany, King's College, University of London (With 5 figures in the text) CONTENTS PAGE Introduction i Materials and methods Experimental results Plant samples Leaf samples 4 Discussion Changes in water, dry matter, residual ash and potassium content of the maize leaves Relation of the changes to the stage of development of the leaves. 10 The nature of the relationship between potassium and water, and potassium and dry matter Summary and conclusions References INTRODUCTION earlier paper of this series (Penston, 1935) describes the daily. changes in potassium content of potato leaves. It was shown that significant variations occur in the weight of potassium in any given leaf at different times of the day, and that they are related to changes in dry matter and water content. Potassium reaches its maximum at about 3.0 p.m. when dry-weight accumulation due to photosynthesis is also at its highest. Towards evening when the elaborated food materials are being translocated from the leaf to other parts of the plant, potassium also moves out of the leaf. Four quantities were determined; water, dry matter, potassium and PHVT. XXXVII. I '

2 2 NoRAH L. PENSION residual ash; and all, plotted graphically, show a more or less rhythmic rise and fau during the hours of daylight. The present paper is concerned primarily with a similar investigation of the daily changes occurring in maize leaves. This plant differs considerably, in many respects, from the potato, and so should provide a good check as to whether the earlier results are general or not; that is to say, whether the state of affairs found in the potato leaves can be considered as holding for all metabolizing leaves of plants grown under natural conditions. In addition to determining the dauy changes in the quantities mentioned, some attempt has been made to correlate them with the stage of development of the leaves concerned and of the rest of. the plant. For this purpose records have been taken, at regular intervals throughout the season, of the dry weights of all the aerial portion of the plant. MATERIALS AND METHODS The maize plants were grown on a plot of groimd at Sydenham in The soil was good, and there was no danger of mineral deficiency. The weather during the summer was on the whole very favourable to the growth of these plants, as the maximum dry weight will show; and throughout the season their development appeared to be quite uniform. The plant samples. Seven samples of the aerial parts of plants were collected between 18 June and the end of September. The number of plants gathered ranged from thirty-five on the first occasion to ten on the last, when, owing to the fact that the plants were 7 ft. high, very bulky, and the laboratory some miles away, no more could conveniently be handled. It was thought that as the growth of the plants had been so uniform, this would not be too small a number to take. The selection of plants was made at random, and the samples were collected at 2.30 p.m. on each occasion, to avoid errors due to fiuctuation of the dry weight as a result of photosynthesis. The fresh weight of the material was determined immediately, after which it was separated into leaf blades, leaf sheaths, stems, and fiowers and fruits when these were present. In the laboratory the separate portions were dried in an electrical oven at 100 C. for 48 hr., weighed, ground to powder and stored. The leaf samples. Three collections of leaves were made, one on 15 July and two on 9 August. Each of these experiments represents a series of samples of forty-five to fifty blades. All the leaves in any

3 Potassium content of maize leaves 3 one series of samples were of the same age, but leaves of different ages were taken in the three experiments. The samples, selected at random, were collected at hourly intervals from 10 a.m. to 5.0 p.m. summer time. The experiments could not be continued longer because of the difficulty of getting the samples back to the laboratory and into the dr5dng oven before nightfall. The fresh weight of the leaves was determined immediately after collection, then they were wrapped in cellophane and stored in a cool place until all were ready to be taken away. Dry-weight determinations were made in the usual way, the dried matter being then ground to powder and stored in airtight containers for subsequent determinations of ash and potassium. The analyses. In each case triplicate portions of 0-5 g. of dry material were ashed in porcelain crucibles at a dull red heat, weighed, then treated with HCl, dried again, dissolved in boiling distilled water and made up to 50 e.c. Potassium was determined by the cobalt hexanitrite method. Only one modification was introduced to the method previously described (James & Penston, 1933). The precipitate, instead of being centrifuged, was filtered off on to a small Jena scintered glass filter funnel, where it could be thoroughly washed vsath 70 % alcohol to remove the reagent before being redissolved in distilled water for the volumetric estimation with permanganate. EXPERIMENTAL RESULTS Plant samples. The stage of development at times of sampling can be summarized as follows: The seed was sown on 11 May and by I June (3 weeks later) the plants were just visible above ground. Four leaves were counted on the main stem on 18 June (5 weeks), nine leaves on 2 July (7 weeks), twelve leaves on 16 July (9 weeks), the total number of leaves on the main stem, eighteen, in the variety used, had all been formed, and the terminal tassels and some of the axillary female branches were beginning to develop on 2 August (11-12 weeks). Thus leaf formation on the main axis extended over a period of 11 weeks. They were not all fully developed, however, until the end of August (15 weeks). By the end of September the cobs were ripe and many of the leaves showed signs of senescence (20 weeks). In Table I are given the results of the dry-weight determinations of the separated aerial portions of the seven samples collected between 18 June and 31 September. They are expressed in grams per 100 plant parts. The total dry weights represent the summations of the weights of individual organs. The total fresh weight also given was, as

4 4 NoRAH L. PENSION pre-viously stated, determined directly on the samples when first gathered. Theflowering portions were weighed for the first time on 2 August, on this and subsequent dates the second figure in brackets under the total dry-weight column represents total dry weight less weight of reproductive portion. In this way it can be seen that loss in weight occurred in the vegetative regions during the last month when the cobs were ripening. By the end of September the fully ripened cobs had a weight equivalent to just over a quarter of the total weight of the aerial parts of the plant. TABLE I. Maize plant samples: dry weights of the aerial portions expressed in grams per 100 plant parts Date of sampling 18 June 2 July i6 July 2 Aug. 13 Aug..'Vug. 30 Sept. Dry weights in grams Leaf Leaf Flowers blades sheaths Stems etc. Total * * ' ( ) ' ( ) ( I) ( ) * Stems and sheaths were weighed together on these dates. Fresh weight total The dry-weight values are represented graphically as log curves plotted against time in Fig. i. The curves consist of three distinct parts: a steeply ascending region indicating the rapid growth of the main axis and its leaves; a middle, less steeply ascending period when the majority of these leaves were mature and increasing only very slightly in weight, while the laterals and flowers were probably responsible for most of the growth; and a final period when the curves begin to fall off, here the leaves were becoming senescent and only the fruits were increasing in weight. Leaf samples. Table II summarizes the results of the determinations of water content, dry matter, potassium and residual ash, in the three series of daily samples of leaves. These results have been expressed as grams weight per hundred leaves (calculated from the weights of the leaves collected in each sample at the times given), as this is the best method of showing and comparing the changes which occur in a leaf or other organ. The results are given in graphical form in Figs. 2, 3 and 4. The first series on 15 July (Fig. 2) represents the fifth leaf up from the base of the main stem; the actual age of this leaf was between 3 and 4 weeks. The second and third series on 9 August (Figs. 3 and 4)

5 Potassium content of maize leaves

6 6 NoRAH L. PENSTON represent the tenth and the eighth leaf up from the base of the main stem respectively, the former was 5-^ weeks, the latter 7 weeks old. These three leaves were, therefore, at different stages of development, the first still actively growing, the third more or less mature, and the second somewhere in between the two. A comparison of the dry weights at 11 o'clock in the morning, the minimum weight in each TABLE 11. Water, dry matter, residual ash and potassium, in maize leaves. Weights in grams per 100 leaves Time (G.M.T.) H,O D.M. Ash-K K Exp. I. 15 July Fifth leaf up from the base of the mam stem. Age 4 weeks: 9 a.m O 10,, ^ noon ^5^i I p.m > Exp. II, 9 August [i). Tenth leaf up from the base of the main stem. Age 5-6 weeks; 9.15 a.m II p m Exp. Ill, 9 August (ii). Eighth leaf up from the base of the main stem. Age 7 weeks: 9 a.m ,, ,, noon p.m ,, " 65 3,, case, -wdu emphasize this point (i) 37 g., (ii) 200 g., (iii) 300 g. The log curve of these weights, drawn in the comer of Fig. i, also shows how the rate of increase in dry weight is falling off -with age. The reason for collecting two sets of leaves on 9 August, was to get a comparison in the changes in dry matter, etc., in leaves of different ages under similar external conditions controlling transpiration and photos5mthetic activity. Allowing for the slight difference in time at which the samples were collected, it is interesting to find that the general trend of the variations is substantially the same in both sets of leaves.

7 Potassium content of maize leaves 50 t 1 ^ o 2 40 Dry mailer? 35.a 2-25! > Polassuiiii Residual asli G.M.T. 9 _L J 10 a.m ).m. 3 Fig ^ Variations in water, dry matter, potassium and residual ash content of maize leaves, expressed in grams weight per hundred leaves: (i) the fifth leaf up from the base of the main stem. L

8 8 NoRAH L. PENSTON DISCUSSION Changes in water, dry matter, residual ash and potassium content of the maize leaves Definite fluctuations in water content occur during the period of observation in all three experiments. It is seen in Figs. 2, 3 and 4, S 850 > S CO S' 225 a t> 200 a CO 'S' Water Dry matter Potassium Residual ash G.M.T. 9 -L J_ I I 10 a.m p.m. 4 Fig; 3. Variations in water, dry matter, potassium and residual ash content of maize leaves, expressed in grams weight per hundred leaves: (ii) the tenth leaf up from the base of the main stem. that the leaves contain least water at eleven in the morning. This is evidently the result of the interaction of factors such as high temperature and light intensity increasing transpiration, and soil factors limiting absorption. Water content of the leaves rises again between

9 Potassium content of maize leaves 9 II and 2 p.m. in the first experiment and between ii and i p.m. in the second and third, to be succeeded by a further small loss and subsequent gain later in the afternoon. Although water content thus increases after ii o'clock, it does not, in the period examined, equal the amount present prior to that hour. The dry weight and potassium IOO leaves1200 Water per CO B ao Weighi:s in Dry matter II Potassium G.M.T a.in. II 12 I 2 p.m. 3 4 Fig. 4. Variations in water, dry matter, potassium and residual ash content of maize leaves, expressed in grams weight per hundred leaves: (iii) the eighth leaf up from the base of the main stem. curves reveal similar changes to the curves for water content: a morning increase in weight followed by a loss at 11 o'clock, after which they rise again in the afternoon. Movement of potassium corresponds a

10 10 NoRAH L. PENSTON little more closely with water than does dry matter. The residual ash in the first and third leaves also shows considerable changes similar to the other substances, but in the second leaf these are not so well marked. Relation of the changes to the stage of development of the leaves An analysis of the fluctuations reveals one or two interesting points. For instance, the changes in weight between the mornmg maximum and ii a.m., and again between ii a.m. and the afternoon maximum can be compared («) as change in weight in grams and [h) as percentage change in weight based on the minimum value. In Table III this has been done for water, dry matter and potassium. TABLE III. Magnitude of the changes in weight in B.f>, dry matter and Kfrom the morning maximum to the minimum at ii a.m. {a.m.) and from ii a.m. to the afternoon maximum (p.m.) compared in the three leaves {a) as change in weight in grams, (h) as percentage of the minimum weight Water Dry matter Potassium Age of No. of leaf in exp. weeks I 3-4 II 5-6 III 7 'eriod a.m. p.m. a.m. p.m. a.m. p.m. (a) Wt. in g % (a) Wt. in g Bearing in mind that thefirstleaf was sampled under different external conditions from the other two, the comparison shows that the changes in weight of the three substances increase as the leaf nears maturity. In the case of dry matter for instance, the greatest change in the first leaf is about 15 g., and in the second leaf about 56 g. Clearly then, in a growing leaf the magnitude of the changes will bear some relation to the age and size of the leaf. When however, the nearly mature second leaf is compared with the mature third leaf, the magnitude of the changes is seen to be of the same order; which means that beyond a certain stage of development they will tend not to increase much further with age. Using the second method of comparison, namely expressing the maximum fluctuation as a percentage of the mininaum weights it is (&) ' % x (a) Wt. in g ((,) % O i'i

11 Potassium content of maize leaves found that the values for all three substances fall off with age of the leaf. The percentage increases are very large in the youngest leaf, for example dry weight about 40%, potassium about 70%, above the minimum weight, whereas in the mature leaf the corresponding values have fallen to 18 and 40 %. By this method of comparison therefore, it would appear that relative to size, the photosynthetic activity and capacity for absorption of water and salts is greater the younger the leaf, because it does not take into account the increase in dry weight due to the building up of permanent tissues, or of water and mineral elements which are retained by the increased number of living cells as the leaf grows older. Thus too much emphasis cannot be given to the fact that in all comparisons based on percentage values the age of the organ should always be taken into accoimt. The nature of the relationship between potassium and stage of development of the leaf can be brought out by another method of analysing the results. If the minimum daily potassium weight in any of the three leaves is taken as representing as nearly as possible the basic weight required by the tissues of that leaf at its particular stage of development, then the difference in the basic weights between the 3-^4 weeks and the 5-6 weeks old leaves represents the amount of potassium which has been absorbed during the two weeks' growth. The basic weights of these two leaves are 1-33 and 8-03 g. per 100 leaves, respectively (see Table II), the difference is therefore 6-7 g. and works out at an average daily increase during the two weeks of g. per 100 leaves. Examining the daily fluctuations in potassium in the light of this theoretical daily rate of increase, it will be seen that in the 3-4 weeks old leaf between, for example, 12 and i o'clock, or between 2 and 3 o'clock (see Table II), the change in i hr. is equal to the whole of the average increment for the day, whilst in the 5-6 weeks old leaf the increase from a quarter past twelve to a quarter past one is four times the average daily increment. Even in the mature 7 weeks old leaf which has reached a stage of development when daily increase as a result of growth has become very small, changes equivalent to an increase of 40 % above the minimum content can occur in the space of a few hours. These points indicate that mineral elements such as potassium, can be absorbed into the leaf during certain hours of the day, in quantities exceeding the requirements for growth of the leaf itself; and secondly, that the power to absorb under conditions of metabolic activity, e.g. photosynthesis, continues past the growing period when increase in structural material is takiag place. ii

12 12 NoRAH L. PENSION The nature of the relationship between potassium and water, and potassium and dry matter Potassium in the plant is almost entirely water soluble, and although the changes in both these substances during the day are considerable, the percentage value K/H3O is relatively constant. The values lie between o-59-o75% in the first leaf, o-8-i-2% m the second, and o-8-i-2i% in the third (Fig. 5)- When potassium is expressed as a percentage of the dry weight, the results indicate that there is a slight disposition for potassium to accumulate in excess of dry matter while the latter is being built up in the leaf, and to leave the leaf in greater relative quantity than dry materials when translocation is being carried on (Fig. 5, between 11 a.m. and i p.m. and between io-ii a.m.). The values of the percentage K/D.W. He between 3-5 and 4-9% whatever the age of the leaf. The close relationship between water and potassium suggests one of two things: either that water content is controlling the movement of potassium into and out of the leaf in such a way that the concentration in the cells is kept within certain limits, in which case osmotic pressure may be an important subsidiary controlling force; or that other forces which operate towards absorption and movement of ions, are themselves primarily controlled by water content. In the latter case, for instance, the decreasing water balance between 10 and II o'clock, by affecting stomatal opening and carbon dioxide intake, may tend to check photosjmthesis. Thoday (1910) found that turgid leaves of Helianthus annuus carried on photosynthesis approximately ten times more rapidly than did leaves which were wilted to drooping. With this falling off in photosynthesis, secondary changes would probably take place, in, for example, H-ion concentration, which would lead to hydrolysis of starch if it had been formed, and subsequent translocation of materials from the leaf, leading to the observed loss in dry weight. These changes in metabolic activity may then be the direct cause of the migration of potassium. In other words, while the cells of the leaf are well supplied with water they are able to carry on photosynthesis and to accumulate ions such as potassium, but with fall in water content, active anabolism tends to cease and leads to changes in the state of the cells such that they can no longer retain cations already absorbed.

13 Potassium content of maize leaves o o- (1) 15 July K%D.W. -O o- K%H,0 (2)9 Aug. (i) K%D.W (3) 9 AuR. (ii K%D.\V. K % H.,0 0-5 I G.M.T. 9 J I I I I L 10 a.m. II 12 2 p.m. 3 Fig. 5. Potassium content expressed as a percentage of dry weight and H.,O content in the three series of maize leaf samples.

14 14 NoRAH L. PENSTON SUMMARY AND CONCLUSIONS 1. Daytime changes in dry matter, water, residual ash and potassium content, in 3-4, 5-6 and 7 weeks old leaves of maize, are recorded. The results are expressed as grams weight per hundred leaves. 2. In all the leaves changes in these substances occur. They increase in the early morning, fall to a minimum value about 11 a.m., and rise again in the afternoon. 3. The general nature of the dauy changes in maize leaves is found to be similar to what has already been described for leaves of the potato plant. 4. The magnitude of the changes, expressed as weight in grams, increases with age while the leaf is still actively growing, up to a time near maturity after which it no longer increases in proportion to the basic weight of the leaf. When, however, the changes are expressed as a percentage of the minimum weight of the leaf, there is a continuous falling off with age. 5. The changes in potassium and other mineral elements are considerable, whatever the age of the leaf, and are closely related to the changes in dry matter and water content. It is concluded that absorption and movement of mineral elements in leaves during the day is dependent upon the rate of metabolic activity. ACKNOWLEDGEMENT The writer's thanks are due to Prof. Gates, in whose department this work was carried out, and to Dr and Mrs Catcheside for their kindness and help in the collection of material at Sydenham. REFERENCES JAMES, W. O. & PENSTON, N. L. (1933). Studies of the physiological importance of mineral elements in plants. IV. The quantitative distribution of potassium in the potato plant. Ann. Bot., Lond., 47, 279. PENSTON, N. L. (1935). Studies of the importance of mineral elements in plants. VIII. The variation in potassium content of potato leaves during the day. New Phvtol. 34, 296. TiioDAY, D. (1910). Experimental researches on vegetable assimilation and respiration. VI. Some experiments on assimilation in the open air. Proc. roy. Soc. B, 82, 4^1.

IN a previous paper (Wood & BarHen, 1939) an experiment was described wherein

IN a previous paper (Wood & BarHen, 1939) an experiment was described wherein 5 STUDIES ON THE SULPHUR METABOLISM OF PLANTS II. THE EFFECT OF NITROGEN SUPPLY ON THE AMOUNTS OF PROTEIN SULPHUR, SULPHATE SULPHUR AND ON THE VALUE OF THE RATIO OF PROTEIN NITROGEN TO PROTEIN SULPHUR