THE younger the tissue, the lower the C/N ratio." This generalisation

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1 DISTRIBUTION OF CARBON/NITROGEN RATIO IN THE VARIOUS ORGANS OF THE WHEAT PLANT AT DIFFERENT PERIODS OF ITS LIFE HISTORY BY PHYLLIS A. HICKS, M.Sc. Fellow of the University of Wales. (With 3 figures in the text.) THE younger the tissue, the lower the C/N ratio." This generalisation was arrived at by a study of wheat plants during development. However, since an annual plant is metaphorically speaking all ages at once, meristem being active throughout life and constantly giving rise to new tissues of root, stem, and leaves, this rule should hold in the plant and its organs at any one time. Woo (6) concluded that the ratio of C/N varied in different plants and probably in different parts of a plant, while Kraus and Kraybill (3) qualified this by a fuller statement that in the stem nitrogen increases upward and carbon decreases upward; but gave no analytical results to support it. A fuller investigation was made by Harvey (l) and his results led him to two generalisations: (i) Those substances which normally decrease through the season are always most abundant in the "tip" portion and least abundant in the base. (ii) Conversely, those substances which tend to increase during the season are less abundant in the tip and more abundant in the base. These results he demonstrated by tables and graphs, but he took only three divisions tip, middle, and base, and his work was done on the apple shoot alone. No investigation was carried out on the whole plant and the period covered was merely a few consecutive months. Ribera(5), working before Woo's(6) time, found that lodging in wheat was decreased by cultural conditions that increase the dry weight, which, as Hedlund(2) has shown, is synonymous with increasing the carbohydrate value.

2 Variation of C/N Ratio during Life of Wheat 109 This result was verified practically at Rothamsted in 1926 when all oats on plots containing twice the normal quantity of nitrate lodged badly, while the same variety on "normal" plots side by side stood erect. Woo (6) considers that a high C/N value increases straw strength and decreases lodging by virtue of the amount of glucose present; or by the inducement of greater thickness of wall by increasing the development of mechanical tissue. It was thought that a study of the mature wheat plant, and particularly of the lower nodes, would show conclusively, the role of the C/N relation in determining straw strength. That the lower nodes are poor in nitrogen has been shown by LeClerc and Breazeale(J.) who found that nitrogen tends to recede from the dying tips of leaves into the living portions, and as these whole leaves die it is translocated into the stem. In order to show, the direction of the recession, whether upward or downward, they estimated the upper and the lower nodes and found that whether dead or alive, the upper nodes were considerably richer in this element than the lower nodes. During the previous work it was thought that a more detailed examination of the C/N relations inside the plant at certain definite stages would serve to show the metabolic processes most active at those stages in the various organs of the plant. Starling, as the most vigorous wheat among those tested and displaying the maximum growth period, was chosen. Estimations were made simultaneously with that investigation and the methods used were the same in every detail. The plant was studied in detail at one period during each of the three growth cycles; the Seedling cycle, the Vegetative cycle, and the Fruiting cycle. As an example of a seedling, a plant 10 days old was selected. No great detail was entered into, the chief object being the comparison of the plumule and the radicle. Fig. I shows the striking contrast between plumule and radicle at this age. The plumule, as yet in its early infancy, exhibits the low C/N ratio of 3^9 almost the embryonic ratio, being rich in nitrogen with medium carbon. The radicle, on the other hand, consisting of the three primary adventitious roots shows an extraordinarily high C/N ratio. This is interesting in the light of the fact that in all Gramineae these three "primary" roots develop quickly, reaching their maximum development at just about this period and then become senescent, while absorption is carried on almost entirely

3 no PHYLLIS A. HICKS by the rapidly developing adventitious roots. Their high ratio is due in part to low nitrogen less than half that of the plumule showing that no attempt at storage is made by the roots and that at this early period, translocation is rapidly upward. As an example of the plant at the height of the vegetative phase, a plant at the tenth leaf stage was chosen. This is the stage when carbon accumulation reaches its maximum. The leaves were removed and attention was confined to the stem and the last developed leaf, i.e. the tip of the plumule, as representing the chief seat of Fig, I, Seedling io days old. Distribution of Carbon and Nitrogen in percentages of dry weight, Phtmule. C %. N %, C/N 3-9, Radicte. C %. N %, C/N development at that time. This plant is represented, half natural size, in Fig. 2. Contrary to the views of Kraus and Kraybill(3) and the results of Harvey(1) for apple shoots, in the case of the wheat there was very little range in the carbon content of the stem. Within the limits of experimental error, the carbon is practically constant throughout the entire conducting system, or at most increases by less than I per cent, from the roots to the base of the tenth leaf. The leaf itself contains much less carbon, showing that the products of assimilation are rapidly removed to the stem.

4 Variation of CjN Ratio during Life of Wheat 111 LEAF (uppti hol ) (CftRBON INITBOOCN 3T.I21,7. L.T3V. CBRBON 30 ts"?;. NITROCtN STEM CftRBON 3 8-5"8 7, NiTROCEM i i ', 5TEM I CRRBON J8 5(,S'/, f-i.ddn) U]iTRoc fj 3-1 V. STEM (base) CRP.BOM 3 8 NITROCEN l-g X MESOCOTYL (CflRBON 37,72?. NITROGEN 2(, ). ROOTS CARSON 313?, NITROGEN 2-17'. Fig. 2. Distribution of Carbon and Nitrogen iu the ioth Leaf Stage, 5 months old.

5 112 PHYLLIS A. HICKS 21 Fig. 3. Distribution of C/N Ratio on Mature Plant at Ear Stage. i-ioth nat. size.

6 Variation of CjN Ratio during Life of Wheat 113 This seems to emphasise the fact that carbon accumulation is not alone responsible for fiower production; if it were so, at this period it should accumulate at some portion of the stem. The nitrogen, however, does show a marked increase up the stem as Kraus and Kraybill(3) predict. The tenth leaf is rich in nitrogen, particularly the tip which is unexpected since the actual meristematic tissue is at the base of the. It seems to point to a concerted upward translocation of this element probably from the root and from the older leaves at the base of the stem. The chief significance of the results lies in the descending values of the C/N ratio up the stem. The root bases show the high value of 17-2, the mesocotyl a ratio of 14-1, while this gradually decreases until a low value of 6-5 is reached in the tenth leaf tip. The youngest tissues have a low ratio characteristic of a plant 10 days old, the base of the stem that of a plant no days old. Hence, by knowing the C/N ratio of any portion, say a certain node, and by finding this value upon the curve of development (See This Journal, 27, p. 33, fig. 8) it is possible to predict the approximate "age" of that node. Conversely if we know the exact "age" of the node and are given the development curve, we may forecast roughly its C/N ratio. Perhaps the most complete picture is that of a small plant at fully ripe ear stage. The average plant at that stage had 12 leaves but this smaller plant had only 7 leaves on its main shoot; 6 on the 3 main tillers which resembled it exactly in size and development and 5 on each of the 4 younger tillers. Fig. 3 gives an accurate drawing of the main shoot and one secondary tiller one-tenth natural size. Only the C/N ratios are shown. For full results see Table I. The same general results as before are shown; the carbon being constant throughout the stem, showing no localised storage of food material, the carbon values being probably those of the wood vessel walls and the sclerenchymatous tissue alone. The lower nodes were not richer in carbon than any other part, no specialised thickening having occurred there. TABLE I. Distribution of Carbon and Nitrogen. Ear Stage. A. The Root: Carbon Nitrogen C/N Root axis Secondary roots Root hairs I

7 114 PHYLLIS A. HICKS B. The Main Stem: ist node ist internode... 2nd node 2nd internode... 3rd node 3rd internode... 4th node 4th internode... 5th node 5th internode... 6th node 6th internode... 7th node 7th internode... Axis of inflorescence C. The " Main" Leaves: ist leaf base... 2nd leaf base... 3rd leaf base... 4th leaf base... 5th leaf base... 6th leaf base... 7th leaf base... D. Tiller Stem : ist node 1st internode... 2nd node 2nd internode... 3rd node 3rd internode... 4th node 4th internode... 5th node 5th internode... E. Tiller Leaves: ist leaf base... 2nd leaf base... 3rd leaf base... 4th leaf base... 5th leaf base... TABLE I continued. Carbon Nitrogen ' r I-I9I 1-20 I-2I9 I-2I C/N I9'

8 Variation of C/N Ratio during Life of Wheat 115 F, The Ear: Flower youngest older oldest Stamens young old Ovaries unfertilised... fertilised Seed endosperm embryo,,. TABLE I continued. Carbon Nitrogen C/N I i-i The nitrogen shows the same consistent upward gradation there is no distinction in nitrogen content between node and internode the graphical representation would be an unbroken ascending line. However, it will be seen that the two lowest nodes of the main stem and the lowest node of the secondary shoot are giving rise to adventitious prop roots, and the whole region enclosed by them has become a regenerative centre. It is therefore assumed that a high nitrogen content, lowering the C/N ratio, is effective in initiating regenerative germination. Thus the supposition that a low nitrogen content makes for straw strength by increasing the ratio of C/N in the lower nodes finds no support from this investigation. The fact of the weakness of the straw in heavoy nitrated plots rather has its explanation in the theory that normally all the available nitrogen is used up to balance the accumulating carbon to produce vegetative growth. In the wheat there is no storage of available carbohydrate reserve in the vegetative tissues, the excess carbon going to the mechanical tissue. If the nitrogen is above the average, more of the carbon is used up for growth, and greater vegetative development occurs at the expense of the laying down of mechanical tissue, not at the nodes only, but throughout the entire stem. This is clearly demonstrated by the prematurely "woody" nature of nitrogen starved plants. The C/N ratio again shows the upward decrease in the stems and in the leaves. The flower paleae also show an increase in ratio with increase of age, while the lowest ratio in the whole plant is found in the embryo, which is potentially the most highly metabolic part present. A consideration of these figures leads to a modification of Harvey's (1) generalisations. It is true that nitrogen tends to decrease throughout the growing season, and is more abundant in the tips and less abundant in the base, yet carbon which fluctuates throughout the year shows very little marked gradation.

9 ii6 PHYLLIS A. HICKS It upholds the finding of LeCIerc and Breazeale(4) in showing the upward translocation of nitrogen from the dying to the living tissues, although I am rather inclined to interchange cause and effect and to suppose that the nitrogen is deliberately withdrawn from the leaves into the developing ovaries and embryos, and death thus follows as a result of the greatly increased C/N ratio (as in Kraus and Kraybill's Class IV) which is inhibitory of both growth and reproduction. The C/N ratio, however, shows a distinct upward decrease unless any particular portion of the plant is rejuvenated. Apart from this, a study of the composition of an annual plant at any time during its growing period upholds the statement " the younger the tissue, the lower the C/N ratio." BIBLIOGRAPHY. (1) HARVEY, E. M. A Study of Growth in Summer Shoots of the Apple with special consideration of the role of C and N. Oyegov Agric. Col. Experiynental Station Bulletin, (2) HEDLUND, T. t)ber die jmoglichkeit von der Ausbildung des Weizens in Herbst aiif die Winterfestigkeit der verschiedenen Sorten zu schiessen. Rev. Bot. Centratbl (3) KRAUS, E. J. and KRAYBILL, H. R. Vegetation and reproduction with special reference to the Tomato. Oyegon Agric. Coll. Experimental Station Bulletin, (4) LECLERC, J. A. and BREAZEALE, J. F. Plant Food removed from Growing Plants by rain or dew. U.S. Dept. Agyic. Yeay Book. (5) RiBERA. Uber die Ursache des Lagerns beini Weizen. Inteynat. Agyic. Techn. Rundschau, (6) Woo. M. L. Chemical Constituents of Amaranthus Retroflexiis. Bot.Gaz

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