SEASONAL VARIATION IN RESPONSE TO GRAVITY IN LUPINUS POLYPHYLLUS SEEDLINGS: DISTRIBUTION OF STATOLITH STARCH

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1 [ 292 ] SEASONAL VARIATION IN RESPONSE TO GRAVITY IN LUPINUS POLYPHYLLUS SEEDLINGS: DISTRIBUTION OF STATOLITH STARCH BY E. D. BRAIN [Received ii April 1954) (With 2 figures in the text) In the previous paper (Brain, 1952) it was noted that the decrease in response to gravity which occurred in the winter months could be correlated with the slower development of the seedling. In this paper a survey is given of the distribution of statolith starch in Lupinus polyphyllus seedlings at different times of year. All the seedlings examined for the purpose have been at the ' B' stage of development, that is, from when the cotyledons open until growth of the hypocotyl ceases. It was at this stage that the difi^erence in response to gravity was evident (Brain, 1952). Seedlings grown in the dark as well as those grown in normal conditions of daylight have been used. Investigations of (i) the extent of statenchyma in the hypocotyl and cotyledons, (2) the number and size of the statoliths in each statocyte in the upper part of the hypocotyl, have been made and form the basis for comparison of the statolith apparatus in seedlings at the same stage of development at different times of year. METHODS Waterer's Lupinus polyphyllus var. 'Sunshine' was used. Seedlings were grown in pots of soil under a glass cloche as described previously (Brain, 1952). Before fixing the seedlings they were laid horizontal in their pots for at least 20 min., and fixed in the same position in 95% alcohol. After 24 hr. they were transferred to 70% alcohol. Sections were cut by hand and examined in potassium iodide and glycerine. Statoliths, when present, are found grouped on the statocyte walls which were lower when the plants were horizontal. Slides were drawn, using a drawing eyepiece, and the number and size of statoliths estimated in transverse sections. Note. Statolith starch is starch which migrates to the lower side of the cell when the equilibrium of the plant is changed. Cells containing these starch grains are termed Statocytes, specialized tissue containing statocytes Statenchyma, and the individual grains Statoliths. RESULTS In Lupinus polyphyllus seedlings the statolith apparatus is found in the endodermal starch sheath. Its ontogeny is similar to that of L. albus, described previously (Brain, 1939), and of other epigeal seedlings described by Hawker (1932). At its optimum development statocytes occur in the endodermis all down the hypocotyl from the cotyledonary node to the collet and in the endodermal sheath of the cotyledonary petiole bundles up to the

2 Response to gravity in Lupinus polyphyllus seedlings 293 base of the cotyledons (Fig. i). Six or more statoliths, measuring 4-9/x in diameter can be seen in each statocyte in transverse section. As the seedling grows older fewer and smaller statoliths are found in the lower part of the hypocotyl. Seedlings examined have been classified into the following groups according to: {a) The extent of statenchyma in the hypocotyl: (i) from the cotyledonary node to the collet; (2) from the cotyledonary node halfway down to the collet; (3) at the apex or in the cotyledonary petioles, {b-d), {b-c') and {a-c) in Fig. i. Fig. I. Transverse sections through the seedling, showing distribution of statenchyma in spring and summer, at the levels marked a', a, b, c and d. Statenchyma is marked in heavy line and vascular tissue is cross hatched. (b) The number of statoliths in each statocyte seen in transverse section at the upper part of the hypocotyl: (i) more than 4, (2) less than 4. (c) The size of individual statoliths in statocytes at the upper part of the hypocotyl: (i) diameter 4-9^, (2) diameter 2-5/J, or less. In this way, by comparing the numbers found in each group as percentages of the series examined, the following results can be stated. Plants grown in the dark show a greater extent of statenchyma in spring and summer months than in January and November. In February and March there are 90-9 %, with four or more statoliths visible in transverse sections of the statocytes and % having statoliths of 4-9 p. in diameter. Otherwise the statoliths are small in dark-grown plants. When grown in the light the extent of the statenchyma is greater in the summer and markedly less in January. Less difference is shown in the number of statoliths in each cell, % of the seedlings

3 294 E. D. BRAIN grown in April to June, as compared w^ith 6o-o % grown in January having more than four visible in transverse sections, in each statocyte. Slightly more difference is shown in the size of the statoliths. In April to June 78-59% of the plants have statoliths of 4-9;u. in diameter compared with 46-66% in January. These figures are recorded in Table i and Fig. 2 gives examples of the statocytes in January and April when statolith apparatus is at its minimum and maximum condition. Table i. Percentage of plants Date No. of plants {a) (i) b-d Extent of statenchyma (2) b-c' (3) a-c None (6) Number of statoliths (i) More than 4 per cell (2) Less than 4 per cell (c) Size of statoliths (i) Diameter 4-9 /i (2) Diameter 2-5 fi or less Grown in dark: Jan. Feb. 1 Mar.f Apr.) June^ Oct ) Nov. Grown in light: Jan. Feb.) Mar.f Apr. 1 May y June ) Aug. 1 Sept. Oct ) Nov. II io-o o-o o-o o'O Fig. 2. Examples of statocytes from transverse sections of the hypocotyl below the cotyledonary node, X285, showing statoliths on the lower side walls, a, grown in January in the dark. 6, grown in January in the light, c, grown in April in the dark, d, grown in April in the light.

4 Response to gravity in Lupinus polyphyllus seedlings 295 DISCUSSION A seasonal variation in the quantity of statoliths at the ' B' stage of development has been found in seedlings whether they are grown entirely in the dark or in daylight, so that the increased efficiency in the summer months cannot be ascribed to the effect of longer daylight. As indicated previously (Brain, 1952) it is the rate of development which is affected by the time of year, and when the seedling develops more slowly or the epicotyl remains dormant the statoliths disappear from the hypocotyl and the response to gravity declines more rapidly than in the spring and summer. Hawker considered that the internal factors of statolith efficiency and redistribution of grovrth hormones may be limiting factors which govern the course of geotropic sensitivity in the plant (Hawker, 1933). In this case the activity of the epicotyl bud appears to be the factor in determining the growth and geotropic sensitivity of the seedling, both efficiency of the statolith apparatus and response to gravity being correlated with the seasonal rhythm of development in the young plant. SUMMARY 1. The distribution of statolith starch has been examined in Lupinus polyphyllus seedlings at the later ' B' stage of their development and a comparison made between those grown in daylight and entirely in the dark at different seasons of the year. 2. Seasonal variation has been found in: {a) the extent of statenchyma in the hypocotyl ; (b) the number of statoliths in each statocyte; (c) the size of the individual statoliths. The seedlings grown in the spring and summer in both light and dark show a greater quantity of statenchyma and more and larger statoliths than those grown in the winter. 3. The results recorded demonstrate the correlation which appears between the efficiency of the statolith apparatus and the variation in geotropic sensitivity at different times of year. REFERENCES BRAIN, E. D. (1939). Studies in the effect of prolonged rotation of plants on a horizontal klinostat. II. Anatomical structure. New Phytol. 38, 247. BRAIN, E. D. (1952). Seasonal variation in the response to gravity in Lupinus polyphyllus seedlings. New Phytol. 51, HAWKER, L. E. (1932). A quantitative study of the geotropism of seedlings with special reference to the nature and development of their statolith apparatus. Ann. Bot., Lond., 46, 132. HAWKER, L. E. (1933). The effect of temperature on the geotropism of seedlings of Lathyrus odoratus. Ann. Bot., Lond., 47, 513.

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