CHANGES IN ASCORBIC ACID CONTENT OF THE SHOOT APEX DURING REPRODUCTIVE DIFFERENTIATION
IN MAIZE
J.J. ClliNOY, C.K. SllAll and H. K. Surn:AR
Botany Department, Gujarat University, Ahmedabad-9
Received on March 8, 1973 SUMMARY
Histochemical localisation of ascorbic acid (AA) contents in the organogenetic centres of shoot apices of maize during reproductive differentiation show that prior to morphogenetic differentiation, AA content increases in the growth centres. Differentiating lemma, palea, lodicules as well as stamens and carpels exhibit a higher AA content.
It is suggested that AA has a significant role to play in cell division, organogenesis and tissue differentiation.
INTRODUCTION
Ascorbic acid turnover has an important role to perform in plant growth, differentiation and development (Chinoy, 1962, 1966, 1968, 1969). The present investigation reports the changes in endo-genous AA content accompanying some morphogenetic and different-iation processes in the shoot apex of maize.
MATERIALS AND METHODS
Apical organs of maize grown under field conditions, were used for the study. Histochemical localization of AA in the vegetative and there-productive organs was carried out, using alcohol acetic acid-silver nitrate reagent at 3-5° Cas is described elsewhere (Dave et al., 1968; Chinoy, 1969). At all stages of apical differentiation, 10 fA thick sections were stained with acetic acid-silver nitrate reagent and the intensity of brown-black silver granules WCJS measured by a cyto-photo-electrometer
8 J. J. CHINOY, C. K. SHAH AND H. K. SUTilAR
Extinction values were multiplied by cell area (p,2
) to obtain
the total content of AA per cell. Extinction values were divided by cell area to obtain the concentration of AA per unit area of the cell.
RESULTS
The vegetative shoot apex of maize is a small rounded dome-shaped structure of undifferentiated meristematic cells below which the leaf primordia are produced in an acropetal succession. The vegeta-tive meristem gives rise to a leaf primordium by forming a sub-terminal
ridge of meristematic tissue perpendicular to the longitudinal axis. The inner buttress of a leaf primordium and its subsequent bulge records a high concentration of AA (Fig. 1.1)*. The shoot apex at this stage consists of (i) a mantle of a single cell layer, (ii) a self-perpetuating group of corpus of sub-apical initials, (iii) central meristem where rib meristem is formed and (iv) peripheral meristem forming a cylinder around the central meristem. Later, the shoot apex grows more in longitudinal direction so that it assumes an elongated conical struc-ture. Such an elongated shoot apex is shown in Fig. 1.2. The tip of the reproductive axis is raised up by the activity of an intercalary meristem.
Formation of an ear-or a tassel-meristem involves a change in size and shape of the axis-meristem and the initiation of longitudinal rows of branch primordia in acropetal sequence (Fig. 1.3 ). The shoot apical cells, thus, showed a slight reduction in cross-secticnal area when the plant became reproductive. Flower primordium showed cells with small cross-sectional area while the differentiating flower bud exhibited an increase in the cross-sectional area of cells (Table I).
*The nwnerical in the parenthesis preceding the point refers to the Fig. and that after the point to the number on the plate.
FIG. 1. Histochemical localisation of AA 1. Vegetative apex with leaf primordium. 2. Transforming apex-ear or tassel meristem with branch primordia. The transitional stage terminates at this point and the reproductive stage begins. 3. Transforming apex with a succession of branch primordia forming single ridges. 4. L. S. shoot apex showing the square ridges. The male inflorescence tassel bears several branches at the base. 5. Magnified single ridge of the branch primordium 6. Double ridge, each ridge represents a spikelet.
REPRODUCTIVE DIFFERENTIATION IN MAIZE 9
REPRODUCTIVE DIFFERENTIATION IN MAIZE 11
Table I. Mean cell area ( p.2), extinction value, AA per cell (extinction
value x cell area) and per unit area of the cell (extinction valuefcell area) during reproductive differentiation in maize
Cell Ext inc- Ext inc- Ext
inc-Stage area -tion tion vaule tion vaule/
value x cell cell
area area
I Vegetative apex 92.7 3.36 352.97 0.043
II Transforming apex 107.6 6.43 665.83 0.063
Ill Transforming apex :
Single ridge 92.25 5.80 535.05 0.063
IV Transforming apex :
Double ridge 66.73 6.35 421.32 0.096
V Floret meristem 78.6 8.11 640.36 0.102
VI Floret differentiation 97.32 5.42 570.26 0.059
AA concentration increases in the cells of the shoot apex at the onset of reproductive differentiation (Fig. 3.1). The total AA contents per cell and per unit cell area increase in cells of the apex during this process (Fig. 3.3 ). In young flower primordium, AA content/cell area went down but after the formation of flower bud, there was again an upsurge in AA content. At induction, a build-up of AA occurs in isolated pockets at a number of sites along the flanks of the apex where, later on branch primordia arise. Cells with high AA content have very large rounded nuclei which display a greater diffuseness than the nearby axial cells. Before the elongated primordium can differentiate a leaf, it develops several longitudinal rows of branch primordia, developing two growth centres instead of one and the ridgll assumes a flattened shape (Fig. 1.4). The cells of the peripheral meristem are small, heavily laden with silver granules and apparently have no vacuoles or only small ones. This is the zone in which the spikelet forming branch primordia are initiated. The high concentration of AA is
12 J, J. CE(INOY, C. K. SE(AE( AND I{. K. SUTHAR
ASCOA!IIC ACID
ASCORBIC ACID
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mainly confined to the nucleus especially in the central mother cell zone and peripheral zone of the shoot apex. In cells of the flower primordium as well as in young flower bud, a considerable amount of AA is present in the nucleus as well as in the cytoplasm. The appearan-ce of double ridges is due to considerable increase in the size of hypoder-mal dermatogen and sub-hypoderhypoder-mal cells and subsequently to periclinal divisions by both hypodermal and sub-hypodermal cells. Henceforth, the shoot apex enters the period of reproductive differentiation and the rate of formation of growth centres is greatly accelerated. The shoot apices at this stage and the new growth centres register higher conentra-tions of AA when compared to the growth centres of the previous stage. The branch meristem produces two spikelet primordia, each giving rise to two glumes. AA content increases in the newly formed growth centres of the shoot apex. This is maintained until florets are formed (Fig. 2.7, 2.8). Fig. 2.9 shows florets at different stages of development at the tip of a young spike. The primordia of florets record a very heavy AA concentration. The upper and lower floret-meristems are then derived from the spikelet meristem (Fig. 2.!\). The initiation of the palea, lemma, lodicules and stamens by the floret-meristems varies in extent and sequence. As the floral parts differentiate successively, high AA concentration is observed in the glume, lemma, palea and anther primordium, successively (Fig.2.10). Cells of the stamen and car-pel primordia register uniformly heavy concentration of AA (Fig. 2.11 ). But in a male flower, the filament and the associated bracts show neg-ligible AA while the anther-head shows high concentration (Fig. 2.12). The floret-meristem may then give rise to the pistil primordium and finally to the ovule primordium.
DISCUSSION
Studies from this laboratory on the role of AA at molecular and submolecular levels in relation to reproductive differentiation in
FIG. 3. Extinction values at different stages of development (3.1 ). Vegetative apex : 1. Tunica 2. Corpus 3. Central mother cell 4. Leaf primordia; Transforming apex, single ridge : 1. Tuncia 2. Corpus 3. Central mother cell; double ridge : 1. Older and 2. younger spikelet primordia. Floret meristem (3.2) 1. Glume older 2. Glume younger 3. Floret meristem older 4. Floret meristem youger. Differentiated floret : 1. Lemma 2. Palea 3. Stamen (Old floret) 4. Lemma 5. Palea 6. Stamen (Young floret) AA content per cell and per unit area of a cell (3.3) Stages :
1: Vegetative apex 2. Transforming apex 3. Single ridge 4. Double ridges
14 J. J, Cl{INOY, C. K, SF{Al{ AND 1{. K. SUTJ{AR
plants (Chinoy, 1962, 1968, 1969; Chinoy et a!. 1971) have shown that at the time of the onset of double ridges (floral induction) as well as through out the period of reproductive differentiation, AA content as well as its utilization are at higher levels as compared to those obtained during the periods of vegetative differentiation. Con-centrations of nucleic acids, proteins and carbohydrates as well as the activities of a number of enzymes ,are also higher during the entire period of reproductive differentiation. Along with this high metabolic activity, the rate of formation of new growth centres as well as the rate of cell division and elongation are greatly accelerated (Zoe. cit.). The rapid cell division is correlated with more rapid synthesis of DNA and the increased RNA content is associated with cell enlargement. Schopfer (1967) obtained an appreciable increase in the RNA content of Si1Wpsis alba as a result of AA treatment. The work of Price ( 1966) clearly demonstrates that there is a considerable increase in RNA synthesis in isolated nuclei of wheat resulting from AA treatment. In our laboratory also, Chinoy and Saxena (1971) have shown an enhanched biosynthesis of RNA by supplying exogenous AA to the germinating embryo axis of Cicer arietinum cv. Chafa.
High AA content in the organogenetic centres of shoot apices and its increasing trend during their differentiation into lemma, palea, lodicules, stamen as well as the carpel, prove that AA plays a significant role in cell division, organogenesis and tissue differenti-ation in maize. Work carried out on histochemical localizdifferenti-ation of DNA, RNA, histones and SH groups in vegetative and reproductive shoot apices of maize (unpublished), shows that there exists a close correlation between the AA content on the one hand and DNA, RNA and histone contents, on the other.
REFERENCES
Chinoy, J.J. (1962). Formation and utilization of ascorbic acid in the shoot apex of wheat as factors of growth and developmi:nt. Indian J. Plant
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- - - - (1966). Role of correlation between growth, metabolism and development in elucidating the mechanism of their heredity. J.
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REPRODUCTIVE DIFFERENTIATION IN MAIZE 15
- - - and Saxena, O.P. (1971). Inductive effect of ascorbic acid on RNA, amylase, protease and RNAse. 16th Intern!. Seed Testing Congr., Washington, preprint No. 13.
- - - Singh, Y.D. and Gurumurti, K. (1971). Some aspects of the physio-logical role of ascorbic acid in plants. 1 ndian Agriculturist, 15: 33-48. Chinoy, N.J. { 1969). On the specificity of the alcoholic, acidic silver nitrate reagent for the histochemical localization of ascorbic
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Price, C.E. (1966). Ascorbate stimulation of RNA synthesis. Nature, 212: 1481. Schopfer, P. (1966). Der Einflul3 Van phytochrom auf die stetionaren
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---(1967). Further investigation on the phytochrome-mediated accumul-ation of ascorbic acid in mustard seedling, (Sinapsis alba L.). Planta,
