THE origin and early growth of seedling pineapple plants have

Transcription

1 [305] DEVELOPMENTAL STUDIES OF THE PINE- APPLE ANANAS COMOSUS (L) MERR. I. ORIGIN AND GROWTH OF LFAVES AND INFLORESCENCE1 BY KENNETH R. KERNS, J. L. COLLINS AND HAROLD KIM Pineapple Experiment Station, University of Hawaii, Honolulu, Hawaii (With Plate IV and 6 figures in the text) THE origin and early growth of seedling pineapple plants have been discussed by Miles Thomas & Holmes (1930). As is the rule for monocotyledons a single leaf is first formed followed in succession by the origin of younger leaves higher up on the axis of growth. In the present report we are concerned with the origin of new leaves in vegetative reproduction and the phenomena involved at the apex of the plant during the transition from leaf production to the formation of an inflorescence. In a mature plant the axis or main stem of the plant is terminated by the inflorescence which develops into the pineapple fruit. With the cessation of vegetative growth at the apex due to the formation of the inflorescence, axillary vegetative growth takes place giving rise to a number of lateral shoots which are the principal means of propagation of the pineapple. The observations reported here were made on paraffin and freehand sections from the apical meristem of plants of the Cayenne variety. LEAF DEVELOPMENT The apical meristem of a plant producing only leaf growth is characteristically small in comparison with the cross-sectional area of the main stem at the apex of which it is centrally located. In shape, the meristem area is circular and slightly convex. The first visible evidence on this apex of the beginning of a new leaf is a small bulge or ridge composed of a number of cells thrown up 1 Published -with the approval of the Director as Technical Paper No. 88 of the Pineapple Experiment Station, University of Hawaii.

2 3o6 K. R. KERNS, J. L. COLLINS AND H. KIM at one edge of the circular meristem area. This protuberance of the meristem cells increases in size and elongates into an embryonic leaf (Text-fig, i). After this primordial leaf has advanced in growth for a Text-fig. I. Longitudinal median section through the growing point of a plant producing only leaves. The meristem area is shown, x 45. Text-fig. 2. Cross-section just above the meristem area and showing the. relative size and position of the three youngest leaves, x 90. short time the next younger leaf primordium becomes evident some distance away around the circumference of the circular growing point. During the normal growth of the plant it appears that the time interval between the origin of successive leaves is relatively uniform.

3 Developmental Studies of the Pineapple 307 as is also the angular distance between successive leaf primordia. The constancy of the relative sizes of the three young leaves is an evidence of this fact. The several youngest leaves at the growing point appear to have a constant relative cross-sectional area such that each is Text-fig. 3. Section similar to Text-fig, i but from a plant beginning to form an inflorescence. The larger meristem area is clearly shown. X45. Text-fig. 4. Cross-section of a style showing the three-lobed shape and the stylar canal. X72. about one-third the size of the next older leaf (Text-fig. 2). It is this time and space relationship which determines the type of phyllotaxy of stem and fruit. This subject of the phyllotaxy will be discussed in a later section of this paper. Growth takes place at the base of the leaf at its point of attachment to the main stem, the leaf tip, therefore, being the oldest part of the leaf.

4 3o8 K. R. KERNS, J. L. COLLINS AND H. KIM THE ORIGIN AND DEVELOPMENT OF THE INFLORESCENCE During a part of the growth season of 1934 and 1935, twenty-five plants were removed each week from a normally growing field. The growing point of each plant was sectioned both freehand and by use of the microtome and examined under a microscope in order to study the morphological changes taking place when the apical meristem ceased to produce foliage leaves and began to form an inflorescence. The numerical results of these observations are recorded in Table I A. The first evidence of this change in type of growth was seen in the marked increase in the diameter of the meristem area. This is shown clearly by comparing Text-figs, i and 3. This change takes place rapidly and uniformly in plants of comparable age and health. In the series of twenty-five plants removed 17 December 1934, all had only the typical leaf producing meristem, but in Series 3, removed i week later, twenty-three of the twenty-five plants possessed the wide meristem area indicative of the beginning of the inflorescence. The growing point meristem reaches its greatest diameter when the first rows of flowers or "eyes" are formed. Measurements of the width of the apical meristematic area {a of PI. IV B) of each of the twenty-five plants removed weekly in 1936 showed a marked inverse correlation between the rows of eyes formed and the average width of the apical meristem. This relation apparently hmits the number of florets (eyes) which can be formed on a single fruit, for when the reduction in meristem diameter reaches certain hmits, leaf primordia replace flower primordia. Following the enlargement of the meristem area, flower-bud primordia are produced as bulges at the margin of the meristem similar to the origin of leaf primordia. The first organ to appear on the margin of the meristem with increased diameter is the bract which subtends the pineapple flower. This is followed in rapid sequence by primordial ridges of the sepals, petals, stamens and pistil. In Text-fig. 5, successive stages in the development of a single flower are shown in semi-diagrammatic drawings. In this set of drawings, A shows the primordia of the bract which will later subtend a single flower. B shows two of the three sepal primordia which appear adjacent to and above the bract. In C two of the three petal primordia are shown starting adj acent to the sepals, and in D stamen primordia make their appearance. In E we see the beginning of the carpel structure, further developments of which are shown in F, G andh.

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6 310 K. R. KERNS, J. L. COLLINS AND H. KIM TABLE IB. Correlation between rows of eyes formed, peduncle height and width of the growing point meristem Peduncle height mm. None None Rows of eyes originated None Wide growing point 1 row 2 rows > 5. 6, 7. 8, 9, 10, 11,, and all AU Average width of growing point mm Average stage 0: oldest eye No eyes.,,. Basal eye tissue Forming sepals I >>, petals I 1,, carpels, ovules G Text-fig. 5. Diagrams showing the development of the pineapple flower and fruitlet. A, the floral bract primordia; B, sepal primordia; C, petal primordia; D, stamen primordia; F, carpel primordia; F, enlargement and closing up of the carpel; and H, mature flower, and a non-median section through another. The period of time from the beginnings of the first flower until the complete formation of the last flower of the inflorescence is approximately 3 weeks. The number of flowers formed varies with the size and vigour of the plant. Healthy and normal-sized plants develop about 150flowerson the first inflorescence produced by a single plant. The flower parts originate in sets of three: three sepals, three petals, six stamens (two groups of three) and three carpels. The members of each set grow simultaneously and the different sets appear in succession. During growth, the sepals and petals extend upwards

7 Developmental Studies of the Pineapple 311 at an acute angle with the receptacle and thus produce, in form, a double tent enclosing the essentialflowerorgans. All theflower parts appear to originate at almost the same level and grow away from this point. The central portion of the growing point around which theseflower parts are grouped does not appear to share in this growth but remains at the original level. The growth of the surrounding parts thus forms a cup or cavity. The outer flower parts which are older appear to grow at a faster rate than the inner parts, thus causing the walls of the cavity to converge at the top. The growth and development of the carpels then fill this cavity, becoming folded in a characteristic fashion. The apex of each carpel grows upwards between the stamens and there fuses into a single unit which becomes the style of the pistil having the three stigma lobes as its apex. Extending through the centre of the style is a three-lobed canal down which the pollen tubes grow to effect fertilization. Text-flg. 4, a cross-section of the style, shows this stylar canal. During growth the edges of the three carpel primordial leaves are folded towards each other and come to occupy the cup cavity at the centre of the point of origin of the flower parts as mentioned above. The edges of these carpel primordia not only come together during growth but are forced to curl back upon themselves, causing the edges to be turned in opposite directions. When formation of the carpels is completed, this twin nature of the placental area is not so evident. In PI. IVA, which is a tangential section of a young fruit, we can see the indistinct line where the walls of the carpels have come together. The position of the ovules is evidence that the edge of the carpel leaf had turned in towards the centre of the leaf during development of the carpels. During carpel development the sides where two carpels meet sometimes fail to close up completely and leave a small, narrow, elliptical opening extending downwards between the carpels, from the bottom of the blossom cavity of the cup, permitting air to enter these spaces. As the fruit develops to maturity the cells of the carpel walls thus exposed to air become dark coloured and hardened. Black spots of this origin are illustrated in PL IV A. In this figure it is possible to see the point where black spots originate in the failure of the carpel leaves to form a perfect contact. This blemish is much more prevalent in fruits of the Queen and Ruby varieties than it is in Cayenne. Since the transition from a leaf producing meristem to a flowerbud producing meristem is abrupt, and the appearance of flower-part

8 312 K. R. KERNS, J. L, COLLINS AND H. KIM primordia follows immediately after the increase in diameter of the growing point, it becomes obvious that the leaves, fifteen to twentyfive in number, which are later found on the peduncle, were all initiated before the beginning of the inflorescence although there was no peduncle evident at that time. Within about 5-6 days after this change at the growing point, evidence can be seen that the peduncle elongation has started by a pronounced elevation of the central portion or apex of the stem of the plant (Text-fig. 6). As the development of the inflorescence proceeds, the elongation of the peduncle also proceeds and has apparently reached its full length by the time flowers commence to open. Text-fig. 6. Median longitudinal section of the growing point showing the beginning of the peduncle growth by the elevation of the central area, x J. Whether the altered appearance of peduncle leaves and the short leaves at the base of the fruit (involucral leaves) is due to arrested development of foliage leaves or to a change in mode of development is a question to which no answer is apparent. The last peduncle leaves to be produced before flower-bud formation starts develop a characteristic back fold or kink at the tip which is apparent as late asflowering.this kink is clearly shown in PL IV B. When the inflorescence is completed the meristem area at the growing point returns to a leaf-producing condition but less abruptly than the opposite change. Following the return to a small leaf type of growing point the group of short leaves known as the crown or top is produced, which terminates the axis of that particular plant. Coincident with completion of the flower formation the involucral and peduncle leaves assume a pink and later a reddish colour. When this pink coloration can be found in these leaves it can be taken as the indication that flower formation has been completed, and that crown development is taking place.

9 Developmental Studies of the Pineapple 313 The nature of the stimulus which causes the change in meristem type is unknown. It has been shown, however, that the plant or young shoot can be treated at any time during its growth with certain unsaturated hydrocarbons, and that these cause the foliar type of meristem to change abruptly to the flower-bud producing type; the plant then proceeds to form a fruit which in size is largely proportional to the size and age of the plant when treated. From the study of fruit development it has been possible to determine the approximate length of time required for several different stages of fruit development to be completed. The periods of time given in Table II below are the averages from a number of plants observed during two successive years. TABLE II. The time required for completion of different stages of the development of a pineapple inflorescence and fruit Average time duration Periods of fruit development days (i From planting to beginning of inflorescence (2) From beginning to end of inflorescence formation (3) From end of inflorescence formation to first open flower 43 (4) Period of flowering (5) From last open flower to ripe fruit (6) Total period of fruit development (7) From planting to mature fruit THE ORIGIN AND GROV^^TH OF SLIPS Slips are branches of the main stem which develop in the axils of the peduncle leaves. They usually have a small, vestigial, fruit-like structure at their base through which the slips are attached to the peduncle. At a very early stage of leaf development it is possible to see an island of meristem cells from which slip buds on the peduncle and shoot buds on the main stem are developed. Fxamination of the peduncle at the stage when theflower-budformation is just completed has shown slip buds to be present on the lower or older portion of the peduncle. The slip buds then originate some time previous to the appearance of the red coloration of the leaves in the centre of the plant. PHYLLOTAXIS OF THE PINEAPPLE The pineapple exhibits the spiral ty?^ f phyllotaxy in regard to both the leaves and flowers. Miles Thomas & Holmes (1930) from a study of angular distance between leaf primordia of seedling pineapple plants stated that the

10 314 K. R. KERNS, J. L, COLLINS AND H. KIM phyllotaxy was 11/18. In this system of expressing the phyllotaxy of a plant the numerator of the fraction indicates the number of spiral turns about the axis and the denominator the number of leaves in this spiral until one is vertically above another and older leaf in its attachment to the stem. We, however, have not been able to confirm these authors' findings in the phyllotaxy of the more nearly mature plant. In the pineapple the central point of each leaf on the main axis is marked by the presence of a small dormant axillary bud. After leaves have been removed from a plant these small buds remain on the stem and serve as markers for the midpoint of each leaf. The leaves were removed from a number of normal plants and a vertical line inscribed on the axis beginning at one of these dormant buds near the butt end of the main stem. It was noticed that this line with great regularity bisected the 14th and 27th buds. This indicated an interval of thirteen buds before another appeared vertically above bud number one. Tracing the buds around from Nos. I to 14 there were five complete turns about the axis. These findings therefore establish the leaf phyllotaxy as 5/13. TABLE III. Data from which the leaf phyllotaxy of the pineapple plant was determined Position of the 14th bud (leaf) with respect to the ist Vertically above Slightly to the Slightly to the right in right left in left spiral plants spiral plant No. of plants 10 i 1 Position of 27th bud 27th above 14th 27th to the right 27th to the left No. of plants The deviation to the right or left in right and left spiral plants of the 14th and 27th buds from the vertical line is possibly due to a slight torsion of the axis during growth. This explanation for these deviations appears more plausible from the fact that in each case the deviation was in the same direction as the leaf spiral. Preliminary observations of the leaf arrangement on the pineapple plant stem were made by Sideris (1926) in the course of studies of the fibrovascular system of the plant. While he did not state what type of phyllotaxy was present it was clear from his analysis of the fibrovascular system of the stem and successive leaves that the 5/13 t5rpe was present. Priestley et al. (1935) have studied in detail the phyuotaxy of a monocotyledon in relation to the internal arrangement of the fibro-

11 Developmental Studies of the Pineapple 315 vascular system and the origin of new leaves at the meristem area. They discuss the possibility of a transition from a simple type of phyuotaxy to a more complex one, as being the logical result of an increase in the size of the meristem area. We have shown in the pineapple that such a change is the first visible evidence of the formation of the inflorescence, which we show has a phyuotaxy one degree more complex than the phyuotaxy of the stem bearing only leaves. The phyuotaxy of the inflorescence has been found by Dr M. B. Linford of this Station (unpubhshed data) to be of the 8/21 type and is thus different from that of the leaves on the same plant. Plants of the Cayenne variety were found to have either right- or left-hand spirals and the fruits were of the same type as the main stem of the plant. Whether the shoots produced by a plant have right- or left-hand spirals appears to be a matter of chance, depending on whether the second leaf primordium develops on the right or the left side of the initial primordium. The investigation concerning phyuotaxy has been carried on using the Cayenne variety. An examination of pineapples growing in the variety garden has shown that the large fruited varieties. Queen, Taboga, Pernambuco and Wild Kailua, have the same phyuotaxy as Cayenne. The smauer fruited pineapples. Ananas microstachys Lindm. and A. microcephalus (Bak.) Bertoni, have a fruit phyuotaxis of 5/13 and a phyllotaxis of 3/8 for the plant. The F^ hybrids produced by crossing A. comosus L. (Merr.) variety Cayenne with A. microstachys show a segregation of fruits, some being hke Cayenne and some like A. microstachys. The phyuotaxy of the pineapple is not, however, a fixed genetic character but appears to be dependent upon the size or area of the meristem at the growing point. We have already shown that the meristem area increases in size as the first visible steps in formation of the inflorescence. Evidence supporting this conclusion is to be found in the fruits produced by plants forced toflower prematurely. It has been known for a long time that pineapples could be forced toflowerby smudging with smoke. Rodriquez (1932) in searching for a constituent of smoke which was responsible for this result found that ethylene gas was effective. Later the senior author found that acetylene was equauy efficient in forcing flowering. Using the latter gas, Cayenne plants of different ages have been forced to flower and form fruits. It was found that smau plants up to 2 months of age when treated

12 3i6 K. R. KERNS, J. L. COLLINS AND H. KIM produced fruits having the 5/13 fruit phyllotaxy, but that plants treated when 4 months of age or older produced fruits having the normal 8/21 phyllotaxy. We have called attention to the increase in the size of the meristem area preceding the development of the inflorescence and a reduction at the completion of flower formation and beginning of the crown development. These two transitional zones are marked in the mature inflorescence and fruit by a series of small short leaves, the involucral bracts at the base of the fruit and sterile bracts at the top between the fruit and the crown. These leaves mark the transition from a 5/13 phyllotaxy to the 8/21 phyllotaxy of the fruit and back again to the 5/13. The first change, we beheve, is quite abrupt and rapid but the second one much less so. These two periods may be looked upon as critical ones in the development of fruits. Abnormalities of fruit and crown development such as double monsters, fasciations and multiple crowns may be the results of interference with the regular transition from one to the other of these regular systems of pbyllotaxy. Multiple crowns may be the result of tbe failure of the meristem area to return completely to the 5/13 type following the 8/21 type of the fruit. The normal phyllotaxy of the plant and fruit appears to rest upon a co-ordinated space relationship in the meristem area. It is quite probable that chemical and physiological factors play an important role in bringing about these size changes in the meristem, but we are not at this time attempting to visualize their place in these phenomena. SUMMARY 1. The pineapple plant is a monocotyledon with a terminal inflorescence. The first visible evidence that an inflorescence is about to be formed is tbe increase in the size of the meristem area at the apex of the plant axis. 2. The average period of time for the plant to complete inflorescence formation and growth and the several divisions of these processes have been determined. 3. The inflorescence begins development before the peduncle starts elongation. 4. The phyllotaxy of the large fruited varieties is 5/13 for the plant, and of the fruit, 8/21. Small fruited varieties have a 3/8 plant phyllotaxis and 5/13 for the fruit. 5. The time when the phyllotaxy changes from that of the leaf-

13 THE NEW PHYTOLOGIST VOL. XXXV, PLATE IV A KERNS, COLLINS & KIM DEVELOPMENT OF PINEAPPLE

14 Developmental Studies of the Pineapple 317 producing growing point to the inflorescence growing point is considered a critical one for the production of fruit and crown abnormalities. REFERENCES MILES THOMAS, E. N. & HOLMES, L. E. (1930). The development and structure of the seedling and young plant of the pineapple (Ananas sativus). New Phytol. 29, PRIESTLEY, J. H., SCOTT, L. I. & GILLETT, E. C. (1935). The development of the shoot in Alstroemeria and the unit of shoot growth in monocotyledons. Ann. Bot. 49, Q, ANTONIO, G. (1932). Influence of smoke and ethylene on the fruiting of the pineapple [Ananas sativus Schult.). /. Dep. Agric. P. R. 16, No. I. SiDERis, C. P. (1926). Root growth and behaviour. Fifth Ann. Short Course in Pineapple Production Rep. pp EXPLANATION OF PLATE IV A. Tangential section of a young pineapple fruit showing the carpels, ovules and intercarpellary fissure, the latter indicated by arrows, x i. B. The stem growing point at the beginning of inflorescence development showing: a, the meristem area; b, the first flower bud; c, the floral bract subtending the first flower; and d, the last involucral leaf showing the characteristic fold at its tip. x 45.

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