«Andean roots and tubers: Ahipa, arracacha, maca and yacon M. Hermann and J. Heller, editors Promoting the conservation and use of underutilized and ...»
varietal names for arracacha: ñut’u racacha (= small arracacha), toctoccha and walla (= cordillera arracacha). Herrera (1942) says that four types of arracacha are distinguished in southern Peru: r’umur’accacha (= cassava arracacha), arros-r‘accacha, huaisampilla and morada. A good source for Peruvian varietal names of arracacha is Arbizu and Robles (1986). Among the many names listed in this germplasm catalogue, chaucha amarilla (= early yellow) and chaucha blanca (= early white) are perhaps the most remarkable since they indicate the existence of arracacha varieties with shorter crop duration. Bristol (1988) lists several varietal names in the Kamsá language of the Sibundoy (southern Colombia).
96 Arracacha (Arracacia xanthorrhiza Bancroft) 4 Biology and agronomy
4.1 Life form Arracacha is a perennial plant. In the absence of vigorously competing weeds, arracacha will survive in a state of minimal growth for many years even without human intervention. This is of significance to the germplasm collector since the plant, in contrast to the tuber crops potato, oca and ulluco, may still be found where people have abandoned its cultivation but provide a niche for its survival such as in backyard gardens or fruit groves. Cultivation practices seek to maintain planting stocks vegetative; however, arracacha flowers occasionally. Certain conditions, especially dehydration, may induce very strong flowering responses that exhaust the plant and lead to its death (Section 4.3.1). This phenomenon presumably has led some authors to believe that arracacha is a biennial plant (for example, León 1964);
Generally, however, the development of generative structures does not intervene with its continued vegetative growth.
4.2 Plant architecture, morphology and development
4.2.1 The vegetative plant Figure 3 shows a mature vegetative arracacha plant from highland Ecuador, 20 months after the vegetative propagule was planted in sandy soil. Although this plant shows an undesirably large aboveground plant mass (which also results from the abnormally long crop duration of 20 months), the picture serves to illustrate arracacha’s unique architecture, and its implications for plant development and propagation. The plant has four distinctive fractions: the storage roots, the central rootstock, the aerial stems and the leaves (for terms in Spanish and other languages, see Table 3).
The conical to cylindrical storage root (Fig. 3A) constitutes the principal economic product. A storage root can reach a weight of around 1 kg, but, more typically, individual roots weigh between 100 and 300 g. The root is proximally constricted and connected to the rootstock through ‘necks’ which break easily at harvest. Like the cassava or yac6n (Polymnia sonchifolia) storage root, it does not regenerate shoots.
The storage root can therefore not be used as a propagule.
The central rootstock (Fig. 3B) is a highly swollen and compressed stem structure.
In the cultivar shown, the rootstock is comparatively large. In other cultivars, the rootstock may be less prominent. Although genetically determined to some extent, shape and size of the rootstock depend greatly on the propagule used and cultivation practices (especially hilling).
The aerial stems or offshoots (Fig. 3C are very peculiar structures unique to arracacha and several authors have struggled to find an appropriate term for them (e.g.
Hodge 1954). For the sake of convenience, we will call them ‘cormels’, although these are normally more compressed structures as, for example, in Gladiolus species. However, the arracacha cormels are also derived from stem tissue and are composed of internodes, nodes and scars left by shed leaves. The cormels therefore have a segmented structure.
Promoting the conservation and use of underutilized and neglected crops. 21. 97 Fig. 3. Mature 20-month-old arracacha plant grown in the equatorial Andes (Ecuador) at 2400 m altitude (accession ECU1161, planted in July 1994, harvested in March 1996, density 15 000 plants/ha, propagated from entire cormels). A: storage roots; B: rootstock; C: stems or cormels;
D: leaves. In this specimen, roots, rootstock, cormels and leaves accounted for 38, 17, 41 and 4% of a total dry matter of 702 g (fresh weight 3674 g).
Arracacha (Arracacia xanthorrhiza Bancroft) Table 3. Terms applied to arracacha plant parts
Apical sections of the cormels with the basal part of the petioles serve as propagules;
these develop over the crop’s duration by growth and enlargement into the rootstock.
Each cormel carries on its apical nodes 3-5 petioled leaves which are of comparatively short duration (Fig. 3D). The leaf consists of a long petiole with a weakly developed basal sheath and the characteristically bipinnate blade. The leaves are 30-60 cm long; leaf size and dissection vary considerably within a plant and with the conditions of cultivation (Fig. 4).
The storage root formation of arracacha has been painstakingly studied by Roth (1977) in her microscopic investigation of a yellow-rooted cultivar in Venezuela (Fig. 5).
Promoting the conservation and use of underutilized and neglected crops. 21. 99 Fig. 4. Plasticity of leaf shape and size of cultivated arracacha. Leaves in the upper row show the influence of nutrient supply on two accessions (left pair: MH800; right pair MH548): A: field-grown plants with abundant nutrient supply; B: starving greenhouse plants. In the lower row, a succession of leaves along a generative shoot is shown (accession ECU1224). Scales: 20 cm.
Arracacha (Arracacia xanthorrhiza Bancroft) Storage roots can be differentiated from filamentous roots by the naked eye about 2-3 months after planting. Initially, the filamentous root enlarges mainly owing to the growth of phloem parenchyma (Fig. 5A). When the root diameter has reached 2-3 cm, the xylem (the inner part of the root enclosed by the vascular ring) begins to develop starchy parenchymatic tissue which makes up most of the root at maturity Starchy parenchyma in the (outer) cortex contain numerous longitudinal oil ducts, which are lined with secretory cells (Fig. 5B, C). In open-cut roots, the ducts exsude a yellow, resinous oil with a typical umbelliferous odour. In wild Arracacia species, these oil ducts are more numerous and cause the astringent taste. Owing to root enlargement the rhizodermis experiences enormous dilation and a special mechanism of cork formation (‘Etagenkork’), which is typical rather of monocotyledons, has evolved to allow for the continuous renewal of the root skin. Rhizodermic cork is formed in tangential layers separated by several layers of periclinally deviding cells. As new layers are formed from the underlying phloem tissue, the outer (= older) layers scale off (Fig. 5D, E, F).
4.2.2 The generative plant The generative structures of arracacha and the factors inducing their emergence are poorly studied. Arracacha inflorescences and fruits are also conspicuously absent from most herbarium specimens. In most arracacha-growing regions, the plant rarely flowers. As will be seen later (Section 4.3.1), this is the result of cultivation practices and prevailing climatic conditions. Given appropriate flowering stimuli, however, all arracacha genotypes studied so far can reproduce sexually (Hermann, unpublished results).
Each well-developed cormel has the capacity to develop one generative shoot bearing several inflorescences. Therefore, flowering responses in a given arracacha plant range gradually from one generative shoot to as many generative shoots as there are cormels (see Fig. 6). Spontaneously flowering plants in the Andes seldom have more than 2-3 generative shoots. Figure 7 presents schematically a vigorous generative shoot (actually a ramified shoot system) of about 1.20 m height (the only generative shoot spontaneously developed on a greenhouse plant). It carried 38 umbels, none of which was found to be fruiting. Fruiting specimens have a more restricted shoot growth and a lower number of umbels. The umbels appear to be terminal. Sheathed nodes below each umbel continue shoot growth and the formation of new umbels until a certain size (and carrying capacity ?) is reached.
Because of this growth pattern, the proximal umbels flower first and the peripheral ones last. The flowering period of any given shoot is between 1 and 2 months.
Leaf size and thus the photosynthetic capacity are largely restricted along the generative shoots (Fig. 4). Farmers remove these sinks as soon as they become visible, knowing that their undisturbed development can reduce root yields. Usually the ‘generative’ cormel develops a basal vegetative shoot which ensures the survival of vigorously flowering plants (those with generative shoots developing from all cormels).
The arracacha inflorescence is a compound umbel as shown in the schematic drawing (Fig. 8). This figure also gives the terms used to describe the inflorescence Promoting the conservation and use of underutilized and neglected crops. 21.
Fig. 5. Storage root development in arracacha. A: cross-section of a root with 4 mm diameter;
B: mature oil ducts surrounded by secretory cells; C: developing oil duct within starchy parenchyma;
D, E, F: progressive stages of 'Etagenkork' formation in rhizodermis (Source: Roth 1977).
102 Arracacha (Arrcacia xanthorrhiza Bancroft) of Arracacia species. The umbel has 8-14 rays, which carry the 10-25-flowered umbellets. The outer rays are stronger and carry more flowers than the inner ones.
Hermaphrodite or perfect flowers, that is flowers with stamens and functional gynoecia, are found mostly on the outer umbellets; there are typically 3-5 perfect flowers per umbellet. The number of perfect flowers per umbellet decreases toward the centre of the umbel and the inner umbellets consist almost always of male or staminate flowers only. However, there are arracacha accessions, such as CA5026 (Peru, Yauyos), which deviate from this common pattern in that almost all flowers are perfect irrespective of the umbellet position. In such cases, the number of flowers per umbellet is lower and ranges from 5 to 20. Clearly, such genotypes have potential as seed parents as the possible seed yield per umbel and plant is greatly increased.
Across a wide range of genotypes, perfect flowers account typically for 15-25% of all flowers, except in a few accessions such as the aforementioned CA5026 with 85-90% perfect flowers. A distinctive feature of the arracacha umbel within genus Arracacia is the absence or reduction of the involucre to one lanceolate bractlet (for terminology see Fig. 8).
Arracacha flowers are actinomorphic (= radially symmetric) and with 2-5 mm length comparatively small (see Fig. 9). The epigynous, perfect flower has five petals, five stamens and two carpels, each with only one ovule; hence only one seed develops
per carpel, and two seeds per flower and fruit. The styles emerge from an epigynous disk which functions as a nectary. The styles are basally enlarged to form the more or less conical stylopodium (Fig. 9). The stylopodium varies little between genotypes of cultivated arracacha, but its variable shape between wild Arracacia species is of diagnostic value (see Section 6.2.1). The formation of aberrant perfect flowers with 3 or 4 styles is characteristic of certain arracacha genotypes.
The male or staminate flower is similar to the perfect flower as far as petals, stamens and the basal disk is concerned, but it lacks functional female organs. In most genotypes, sepals are normally absent from both perfect and male flowers (asepalous flowers; see Fig. 9A-D, F). There are, however, genotypes with sepalous flowers, although these seem to be rare (Fig. 9E, G, H). The petals of immature flowers are green but they always turn maroon at anthesis. Other flower parts, such as petals, anthers, filaments, carpels and styles, are either free of the purple-maroon pigment or have it at varying intensities and combinations. Pollen from green anthers is white and pink in the more common maroon anthers. In the Ecuadorean accession JJV06, anthers with either pink or white pollen can occasionally be found within the same flower. It is obvious that the pigmentation of the various parts of the arracacha flower would lend itself for use in germplasm characterization.
The first flowers of an umbel to commence anthesis are the perfect flowers of the outer umbellets. The development of such a flower is illustrated in Figure 9A-D. In the embryonic flower (Fig. 9A) the styles barely protrude from the perianth and their stigmatic surfaces lean against each other. Carpels and styles expand greatly until anthesis. Recent pollination experiments (Hermann, unpublished results) have shown that the stigma becomes receptive when the styles separate and display the stigmata as seen in Fig. 9B, C. At this stage, the stamens are still curled underneath the petals.
Three to four days later, the stamens straighten, expand and project through the perianth, dehisce and release pollen (Fig. 9D). Anther dehiscence concurs with the peak of nectar production from nectaries on the epigynous disk. Anthers and petals now become caducous. During the entire flower development the petals remain curled.
Flowering progresses gradually from the periphery of the umbellet to the central staminate flowers and from outer to inner umbellets. Total flowering time of an umbel is about 15 days.
Generative characters of the arracacha plant show a certain variation that offers potential for the morphological differentiation of genotypes, a circumstance which is overlooked in arracacha germplasm characterization.
Fig. 9. Flower morphology and phenology of cultivated Arracacia xanthorrhiza. A: hermaphrodite flower a week before displaying stigmata (front petals and stamens removed); B: receptive hermaphrodite flower; C: receptive hermaphrodite flower with 2 front petals removed;
D: hermaphrodite flower toward end of anthesis; note nectar production, dehiscent anthers and caducous petals; E: sepalous hermaphrodite flower, petals and stamens removed (length 4 mm);
F: male asepalous flowers (2 petals removed); G, H: male sepalous flowers with dehiscent anthers, stamens and sepals partially removed. Length of hermaphrodite flowers from carpel base to stamen tips in B, C, D, E and A is 4-4.5 mm and 2 mm, respectively. Male flowers are 2-4 mm wide.