Site temperature and the presence of the genet affected the three independent components of inflorescence architecture in both clone-sets of edible Musa (AAB) in this experiment, broadly supporting the hypothesis.
4.1. Fruit per Bunch, Fb, per Hand, Fh, and Hands per Bunch, Hb
Increasing fruit per bunch can contribute to increased seed production in individual inflorescences of wild
Musa spp. and to increased bunch weight in the edible bananas. In the data reported here,
Fb was affected by site temperature, clone-set, and the genet. There was a clear difference between clone-sets (
Figure 1C) in absolute terms, but for both clone sets the increase in
Fb with warming site temperatures was of a similar proportion, about 100% across the range (
Figure 1C). This would suggest a similar response of each clone-set to temperature, at least in relative terms. We found that the responses of
Fh and
Hb to site temperature differed, and so while at the level of
Fb, the response of the two clone sets was similar, they arrived at that ‘similarity’ by different pathways.
Morphologically, each node on the peduncle consists of a bract subtending the lateral ‘cushion’ meristem from which the flowers arise, and fruit are attached. The number of flowers and subsequently fruit per hand are determined during the early development of the hand, with a single cincinnus taking up to seven plastochrons to form (Fahn, 1953). The flower initials that arise within the cushion meristem first appear about three nodes distant from the apical inflorescence meristem (White, 1928; Fahn, 1953; Ram et al., 1962; Moncur, 1988; Kirchoff, 2017). At this point, the peduncle has already begun to expand radially. The cushion meristem, subtended by the bract, is arc-shaped on the expanding peduncle and its final length can physically limit Fh.
In the developing inflorescence, the basal width of the bract provides the outer limit of the cushion meristem as the radial growth of the base of the bract is completed before the growth of the cushion meristem it subtends (Fahn, 1953). The cushion may not develop to the full extent of the bract base (Kirchoff, 2017) and the number of flower initials that form then depends on the length of the cushion and the space occupied by each flower meristem.
Flower meristems appear within the cushion, usually in a sequence best described as a cincinnus (Fahn, 1953; Kirchoff, 2017). If temperature affects the rate at which flower meristems are produced, but not the size of the bract base then, because the latter limits the length of the cushion meristem, it is unlikely that an association between site temperature and
Fh will be established. Consistent with this interpretation, the association between site temperature and
Fh was weak in the data illustrated in
Figure 2B except for lower
Fh in the French clone-set at Ndihira, the coolest site.
Fh varies from hand to hand within an inflorescence (Alexandrowicz, 1955; De Langhe, 1961; Swennen and Vuylsteke, 1987) but associated data for the width of the base of the subtending bract are not available. Measurements of peduncle size and bract basal width would be informative in identifying the factor(s) reducing Fh in the cultivars of the French clone-set at Ndihira.
Hb is a function of the rate of lateral node production by the inflorescence meristem and an irreversible change from fruit-forming to non-fruit-forming flower types along the developmental sequence of the peduncle. In Musa there are several flower types: fruit-forming hermaphrodite and female flowers and non-fruit-forming transitional, neuter, and male flowers. Not all species or cultivars have all types, but all species and cultivars have the change from fruit-forming to non-fruit-forming flowers. The change in flower type separates the basal section with its fruit-forming hands from the non-fruit-forming distal section of the inflorescence (White, 1928).
If Hb is determined by a permanent switch in gene expression within the whorls (W3 and W4) of developing flowers, then accumulated reserves may function as an independent factor affecting the timing of this change. This interpretation is consistent with the observation that the number of flowers per hand does not normally change with the change in flower type, from one hand to another, during inflorescence development (Alexandrowicz, 1955; Swennen and Vuylsteke, 1987).
Compared with
Fh,
Hb was quite sensitive to site temperature (
Figure 2A). While
Hb was greatest at the warmest site and least at the coolest site for both clone-sets, there was a difference between clone-sets in response to temperature. Cultivars of the False Horn clone set showed an almost linear decline in
Hb as temperature fell, whereas with the French clone-set the decline in
Hb with decreasing temperature was slight at first, then becoming increasingly greater as temperature fell.
The rate at which hands of female flowers are produced at the inflorescence meristem and the time from inflorescence initiation to when the switch in gene expression occurs in whorls 3 and 4 of developing flowers, are the two processes that determine Hb. Temperature of the meristem strongly affects rate of node production with slower rates at cooler temperatures (Savvides et al., 2016). The change in flower type is sensitive to the quantity of reserves accumulated before the inflorescence was initiated (Turner and Gibbs, 2018). Thus, the reserves affect Hb indirectly through their accumulation before the inflorescence formed and their subsequent effect on gene expression in the flowers when they begin to develop.
4.2. Plant Reserves and Inflorescence Architecture
As the plant moved from the plant (C1) to ratoon crop cycles (C2 to C4), the genet developed with reserves present in the rhizome and stems of earlier generations becoming available for use by the current generation. These reserves complement the carbohydrates available from the functional leaves of the current generation with the expectation that the inflorescences of ratoons would be larger (more
Hb and
Fh) than those of the plant crop. This feature is usually observed (Robinson and Nel, 1990). However, the magnitude of the change may differ according to genomic group and whether
Fh and/or
Hb is affected (
Table 5).
From the data of Turner and Hunt (1984),
Fh increased proportionately much more than
Hb as the genet developed. Cultivars of the AAA genomic group (mostly Cavendish genotypes) changed the most, increasing by 60% (
Table 5). These data are consistent with the reserves accumulated by previous generations, that now form part of the genet, being used to support development of the inflorescence of the current generation.
The development of the genet in the French and False Horn clone-sets decreased
Hb at all four sites (
Figure 3) in contrast to what may be expected based on the observations of Turner and Hunt (1984) across a range of genotypes (
Table 5). On the other hand,
Fh increased from plant to ratoon crops at all sites except for the False Horn clone-set at Ndihira, the coolest site.
Turner and Gibbs (2018) used the data of Turner and Hunt (1987) from a plant crop to argue the case for
Hb being affected by carbohydrate reserves accumulated by the plant before the inflorescence was formed. Removal of all leaves, or half of each leaf, during the mid-vegetative growth phase reduced
Hb, formed in the following floral phase, by 40% to 50%. Defoliation in the floral phase, when hands are forming, did not change
Hb (
Figure 5). A lesser proportional effect (20% to 30%) of defoliation was recorded for
Fh, but there was a phase shift as well. In contrast to
Hb,
Fh was reduced when plants were defoliated in their floral phase beginning -13 to -8 leaves before flowering, but not in the mid-vegetative phase (
Figure 5). In the presence of the genet, defoliation had a lower proportional impact on
Fh and
Hb, with reductions of 7% to 8%. However,
Hb was reduced only when plants were defoliated in the mid-vegetative phase, not the floral phase, indicating
Hb depends on reserves accumulated before the inflorescence is formed. With the genet present, defoliation in either the vegetative or floral phases reduced
Fh, although the effect was small. Reserves influence
Fh more than current photosynthates when the genet is present, compared with its absence.
If these relationships hold for the two Musa AAB clone-sets grown in North Kivu, then a reduction of Hb in the ratoon crop cycles would imply that reserves accumulated before the inflorescence was present were not sufficient to delay the change in flower type and maintain or increase Hb. The reserves had either not been accumulated or had already been used for another purpose.
The reduced
Hb in the ratoon crop cycles (
Figure 4) may have been caused by the allocation of resources to the development of sib-suckers rather than
Hb. The reason for the difference in the behaviour of
Hb between plant crop and ratoons in this experiment and that of Turner and Hunt (1984) may lie in the different agronomic strategies used for desuckering (pruning). Turner and Hunt (1984) used the single sucker follower system with surplus suckers removed three times per year, equivalent to 4 to 5 times per life cycle, meaning that suckers were removed when they were small. On the other hand, in the experiment conducted by Sikyolo
et al. (2013) the suckers remained on the parent plant until flowering, when all were counted and removed except the one retained for the next generation. This delay in desuckering meant that many suckers were more developed before their removal. Suckers remaining attached to the parent plant benefit from this attachment considerably in terms of growth (Eckstein and Robinson, 1999) and are likely to capture more resources, reducing the capacity of the ratoon sucker selected for the next generation to accumulate reserves in the mid-vegetative phase.
Fh behaved very differently with genet development, compared with Hb. It increased considerably. We attribute this to more reserves being allocated to the expanding tissues behind the inflorescence meristem allowing extended cushion meristems to develop across all site temperatures and in all ratoon crop cycles. The reserves accumulated before floral initiation that affect Hb, do not affect Fh, and so the reserves that contribute to increased Fh in ratoon crops must have another source. This may be in the genet, which is absent from the plant crop.
Hb and Fh, both of which contribute to inflorescence architecture in Musa are independent components affected differently by environmental and internal plant factors. These different mechanisms would allow changes in Hb and Fh in wild species that may affect success in pollination by visiting birds and animals. In cultivation, they offer the options for managing the balance between Hb and Fh either genetically or by management practices.
4.3. Peduncle Total Length, Pr, and the Proportion that Was Female, Pf
The timing of termination of the inflorescence meristem differs between the two clone-sets. The mature male peduncles of cultivars of the False Horn clone set contain about fifty nodes (De Langhe, 1961), while those of the French clone-set contain about 100. Since the plastochron of node production in the floral phase is about 5 times faster than the phyllochron (Turner, 1981) and about 10 to 11 leaves emerge from a shoot in the floral phase, which ends at flowering, then in the False Horn clone-set all nodes on the male peduncle present at fruit maturity are already present at flowering. In this clone-set, the inflorescence meristem will have terminated and growth of the male peduncle after flowering, will be entirely due to elongation of the internodes.
For the French clone-set, termination of the inflorescence meristem occurs after fruit maturity, which is from 4 to 8 months after flowering, depending on the site (Sikyolo et al., 2013). These differences between clone-sets in the onset of inflorescence meristem termination are important for interpreting the different responses of
Pr to site temperature (
Figure 3A).
A reduction in
Pr was expected as site temperature decreased because of the effect of temperature on the rate of node production, but the similarity of
Pr across sites in the False Horn clone-set suggests negligible effect of site temperature in this clone-set on either node number or the internode length. The slightly longer reproductive peduncle at Ndihira, the coolest site (
Figure 3A), is consistent with measurements of Sikyolo
et al. (2013) and with a later termination of the inflorescence meristem. This is more likely than an increase in internode length which for female peduncles of the False Horn clone set was 12 cm at Ndihira, compared with 20 cm at the warmer sites.
In the French clone-set the length of the reproductive peduncle and its reduction as site temperature fell cannot be attributed to the termination of the inflorescence meristem as it is still functional at fruit maturity. Rather, it indicated the effect of site temperature on the elongation of internodes of the male peduncle.
The female proportion of the reproductive peduncle, Pf, was governed by Hb and the internode length within the female peduncle. At Ndihira, internode length was 11-12 cm, and the same for each clone set, compared with 16 cm at the three warmer sites for French and 20 cm for False Horn. Lower Hb and reduced internode length both contributed to a reduced length of female peduncle at Ndihira and a substantial reduction in Pf, compared with the three warmer sites.
For a single inflorescence of a wild species of Musa, Pf is a proxy for the balance between reproductive effort allocated to seed and pollen production. The data for the two clone-sets examined here suggest a proportional shift from production of female flowers at warm sites towards a high proportion of the peduncle supporting male flowers at cooler sites.
In terms of the number of female flowers per generation, the high number of suckers produced by plantains at high elevations (Sikyolo et al., 2013; Sivirihauma et al., 2016; Turner et al., 2020) more than compensated for the reduced Pf of individual inflorescences, the 5-fold increase in sucker numbers from Mavivi to Ndihira more than balancing the reduction in Fb. However, this compensation assumes all suckers develop sufficiently to flower and sib suckers within a generation do not reduce Fb, both of which are unlikely (Swennen et al., 1984). Nonetheless, the production of more suckers at high elevations could be a strategy of the plant to increase female flowers for pollination per generation but spread across more individuals within that generation.