Acknowledgements
T. L. Davenport
5.3 Shoot Development
Flushes of vegetative extension growth of mango stems terminate with for- mation of determinate panicles. Several weeks to a few months after separa- tion of the last fl ower or fruit from these panicles are required for the central axis of the panicle or rachis to dry and mechanically separate from the sup- porting stem, depending on the longevity of attached fruit. Five to ten lateral vegetative shoots typically develop from axillary buds located at the termi- nal intercalation positioned in a compact whorl surrounding the panicle scar of each stem (see Fig. 1 of Reece et al., 1949). These lateral shoots become the branch points of stems. These branching shoots form 10–15 leaves before the apical buds return to a resting state to establish them as individual stems. Ini- tiation of these lateral vegetative shoots may occur 2–3 months after desicca- tion of panicles which fail to set fruit. Fruit-bearing stems do not initiate new lateral shoots until several months after separation of fruit and rachis from the stem (Kulkarni and Rameshwar, 1989). Such delayed vegetative growth can reduce the potential for new shoots to fl ower during the next fl owering sea- son (Singh and Khan, 1939; L.B. Singh, 1960, 1972; Monselise and Gold- schmidt, 1982). The apical bud of stems is at rest for most of the year in mature trees. Stems on centennial trees typically produce only one vegetative fl ush during the year (N. Golez, personal communication, the Philippines, 1989).
The apical resting bud of each newly established lateral stem (intercalary unit) is surrounded by a compact whorl of 10–12 leaves with short inter- nodes (intercalation) (Fig. 5.1). Protective bud scales are green but may be brown at the tips due to desiccation (Sen and Mallik, 1941; Mustard and Lynch, 1946; Singh, 1958b; Ravishankar et al., 1979). Resting buds possess a number of pre-formed nodes, each of which contains a leaf bract or leaf pri- mordium and a lateral meristem (Fig. 5.2; see Figs 7–12 in Chaikiattiyos et al., 1994). The outermost, proximally located dried leaf bracts (bud scales) pro- tect the more distal interior leaf bracts, leaf primordia and lateral meristems from mechanical damage and desiccation. Leaf bracts are vestigial non- developed leaves. Scales abort upon evocation of new shoots. Proximally located bracts in apical buds fail to further develop beyond some enlarge- ment and also abort with elongation of shoots.
Reproductive Physiology 101
If apical buds are initiated during vegetatively inductive conditions, bracts develop as small leaves and the leaf primordia develop as the full- sized leaves of vegetative shoots. Additional leaves result from nodes formed by renewed activity of the apical meristem. The number of leaves (nodes) is dependent upon the mean temperature during initiation, and increases as temperatures rise (Whiley et al., 1989). The lateral meristems of the apical bud develop as axillary buds at the base of petioles in the elongating vegeta- tive shoot, each bearing protective bracts, leaf primordia and lateral mer- istems (Fig. 5.3; see Fig. 1 of Reece et al., 1949).
In contrast, if shoot growth is initiated under fl oral inductive conditions, the leaf bracts and primordia fail to fully develop, but the lateral meristems Fig. 5.1. Apical bud of resting mango stem.
Leaf primordia (bracts) Apical dome (includes meristem) Lateral meristematic
primordia
Fig. 5.2. Stylized cross-section of apical bud showing positions of apical meristem, lateral meristems and leaf primordia.
T.L. Davenport 102
begin to elongate and branch at each node forming secondary, tertiary and quaternary lateral meristems. Each branch point in the lateral infl orescence from the panicle axis to the fl oral pedicels bears a fl oral bract (i.e. partially developed vestigial leaf primordium) (Fig. 5.4). The distal half of the panicle structure is derived from newly formed nodes laid down by cell divisions in the apical meristem prior to returning to a resting state. Mixed shoots, bear- ing both leaves and infl orescences at each node, result from development of both the primary leaf primordia and the lateral meristems, which form the infl orescences in the same nodes as leaves.
Vegetative shoot induction, thus, involves stimulating development of leaf primordia from resting buds while repressing development of lateral meristems. Leaf primordia then follow a predetermined cascade of genetic signals resulting in leaf development at each node. Because all shoots emerge from resting buds, a vegetatively induced event does not involve simply inhibition of fl owering. The putative inductive signal directing differentia- tion of leaf primordia onto leaves upon initiation is termed a vegetative pro- moter (VP) rather than a fl oral inhibitor.
Shoots bearing only infl orescences (generative shoots) result from induc- tive development of lateral meristems and suppression of leaf primordial development. A predetermined cascade of fl owering gene signals is activated in lateral meristems resulting in lateral cymose infl orescences terminating with fl owers. A distinct fl origenic promoter (FP) may be responsible for spe- cifi c activation of the lateral meristems of mango. Mixed shoot induction results in combined development of leaf primordia and lateral meristems.
Vegetative shoots
Vegetative shoots bear only leaves (Fig. 5.5). The anatomy of mango vege- tative shoot development has been described (Singh, 1958b; Chaikiattiyos
Stem
Fig. 5.3. Axillary bud of resting mango stem. Leaf petioles (arrows).
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et al., 1994). Vegetative shoots may arise either from axillary buds, if no apical bud exists due to fl owering in the previous fl ush, or from the apical bud when present. The latter is considered extension growth or addition of an intercalary unit on the existing stem, but the developmental events during shoot formation from either apical or lateral buds are basically the same.
Cells in the leaf primordia of initiating buds begin to form individual leaves in the proximal portion of the vegetative shoot. Soon thereafter, the apical meristem activates to form more nodes bearing leaf primordia and lateral meristems. These newly formed leaf primordia develop as the distal portion of the vegetative shoot if environmental conditions remain vegetatively inductive (Núñez-Elisea et al., 1996). Newly elongating vegetative shoots are green in most cultivars but may be bronze or red in others. Fully expanded
1°
Pedicel
1°
Pedicel
1° Bract 2° Bract
3° Bract 2° Bract 1° Bract
2°
Pedicel 1°
Pedicel 1
1°
Pedicel
1° Bract 4°
3° 2°
5°
º 1°
Axis
Axis
Fig. 5.4. Diagram and photos of mango infl orescence depicting the panicle axis and primary (1°), secondary (2°) and succeeding levels of pedicel and cymose fl oral archi- tecture. Vestigial leaf promorida (fl oral bracts) are depicted at the base of each level of pedicel architecture.
T.L. Davenport 104
leaves are a shade of red, depending upon cultivar and cultural conditions and are thin and limp from lack of lignifi cation. The apical buds of vegetative shoots generally become quiescent before completion of the limp, red-leaf stage (Núñez-Elisea and Davenport, 1995). Internodes are compressed at the apex, and leaf development is arrested thereby forming a bud with protec- tive outer scales, inner leaf primordia, lateral meristems and the apical mer- istem. Fully expanded leaves become light green and stiff as they become lignifi ed and suberized. Vegetative shoots are mature when leaves become dark green, which occurs when they are c.2 or 3 months old.
Reproductive shoots
Two types of reproductive shoots typically occur in mango. Generative shoots display only fl owers and have fl oral bracts or non-developed leaves at the base of each lateral infl orescence (Fig. 5.5). Terminal infl orescences, i.e. panicles or thyrsoids (Weberling, 1989), develop from dormant apical buds. The anatomy of panicle development has been described (Juliano and Cuevas, 1932; Musahib-ud-din, 1946; Mustard and Lynch, 1946; Singh,
VEGETATIVE GENERATIVE MIXED CHIMERIC V/F TRANSITION F/V TRANSITION
GENERATIVE CHIMERIC V/F TRANSITION VEGETATIVE MIXED F/V TRANSITION
Fig. 5.5. Stylized diagrams and photomontage of shoot types found in mango. Transition shoots shift from vegetative to fl oral (V/F) or fl oral to vegetative (F/V). Arrow ( ) represents individual leaves; fl oral diagram ( ) represents lateral infl orescences.
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1958b; L.B. Singh, 1960; Sturrock, 1966; Ravishankar et al., 1979; Scholefi eld, 1982; Scholefi eld et al., 1986). The complexes of primary to quaternary branch- ing lateral structures of the infl orescence each terminate with three cymose fl owers. The terminal fl ower opens fi rst, followed by two subtending lateral fl owers. These complexes form the lateral infl orescence structures emerging from the central axis of the panicle. The central axis extension also terminates in a similar fashion. Morphological stages of fl oral buds and panicle develop- ment were described by Shu (1981) and Oosthuyse (1991a). Reece et al. (1949) described the development of infl orescences initiated in lateral buds when the terminal bud is missing. There are more nodes in dormant apical buds and their bracts are more developed than in axillary buds; however, fl oral evocation is indistinguishable.
Generative shoot development in apical buds initially involves swelling of the lateral meristems and their bud scales. Each axillary meristem devel- ops as an infl orescence on a primary peduncle. The apical meristem then forms new lateral meristems and leaf primordia for the distal portion of pan- icle development if fl oral inductive conditions persist (Núñez-Elisea et al., 1996). Panicles may be open or compact, depending upon internode elonga- tion, which is cultivar dependent (L.B. Singh, 1960), but the architecture gen- erally conforms to that in Fig. 5.5. Mixed shoots develop under weak fl oral inductive conditions (i.e. in the low-latitude tropics). Both leaves and pri- mary pedunculate infl orescences develop from the same nodes (Fig. 5.5).
Leaf primordia and lateral meristems develop as leaf and fl oral structures, respectively.