Biological Basis for the Breeding

 of Better Food Yams

 L. M. Degras
 Station d'Amelioration des Plantes
 Institute National de la Recherche Agronomique (INRA)
 97179 Petit-Bourg, Guadeloupe


 The food yams can now be considered to be no more diffi-
cult to improve than many other plants. Their system of
building up and accumulating useful reserves, their system of
ecological adaptation as well as their system of genetic varia-
tions offers a sufficient range of diversity to be within the
reach of selection pressures. Their sexual behaviour is progress-


ively reset among the important species where it was deficient.
Evolutive evidence and the sapogenine species experience give
wide prospects for attempts of intraspecific introgression. Last
but not least, through in vitro technology, new horizons are
rising for the modern venture of food yam improvement.


 Efforts to improve edible yams (Dioscorea sp.p.) are not very
 ancient. This fact, added to the low number and the wide disper-
 sal of concerned research workers, account for the doubtful feel-
 ings often expressed against this improvement. However, it ap-
 pears not more hazardous than, among others, the banana or the
 sugarcane ones. The weak international interest in yam exchange
 is a major difference with these crops and, in turn, accounts wide-
 ly for their relegation aside of agronomic priorities, while
 biological hindrances are generally put forth. Our thesis is that
 basic knowledge exists now, which warrants actual achievements
 to pluridisciplinary programs aiming at yam improvement
 through breeding.
 Biological processes in yams which are at the breeder's disposal
 can be outlined from three levels: systems of reserve biosynthesis
 and assimilates accumulation, systems of ecological adaptation,
 and systems of genetic variation. Considering the genus
 Dioscorea as a whole, the diversity of biological paths brought to
 our knowledge offers an uppermost but interesting challenge to
 the breeding prospects.


 Systems of Biosynthesis and Storage of Assimilates
 Photosynthetic Apparatus
 Even over a small country, the photosynthetic apparatus of the
 yams can establish itself quite at variation among species and
 cultivars. The cataphylles phase duration, the phyllotaxy and the
 vine architecture, the lamina individual area, are concerned.
 Burkhill (1960) mentioned the wide phyllotaxic spectrum of
 the genus, going from alternate to seven-verticillate leaves.
 How such situations could be practically significant can be seen
 through the low sensitivity to no-staking of the more or less deep-
 ly lamina indented species (D. dumetorum, D. trifida), accord-
 ing to the deeper penetration of light in their canopy.
 Highly different efficiencies of transportation and sink setting
 of assimilates are to be expected from the different weight ratios
 of aerial part to tuber, this not only among species but also
 cultivars. However, care must be taken when the plantation time
 is changed: its shift from March to June has reversed the rank of
 ratios of two D. alata cultivars (Degras et al., 1977).

 Characters of the Products Stored in Tubers
 There is no need to detail this basic point that a hundred
 Dioscorea species which store starch (85-99% of tuber d.m.) are
 edible with a useful content of protein and vitamins, even if for a

VOL. XX-PROCEEDINGS of the CARIBBEAN FOOD CROPS SOCIETY


limited number a detoxification could be required. It is worth
mentioning that:
 1. The tuber can be eaten crude in some species like D. fan-
 dra of Malgasy (Madagascar);
 2. In the tuber's enzyme activities noticeable variations have-
 been found between and within species (Diopoh, and
 Kamenan, 1981, etc.);
 3. In starch viscosity a large variation is known (Coursey,
 1980) as well as in flavour at cooking (Osinowo, 1977); and
 4. Starch digestibility may be as high (D. dumetorum) as the
 Cassava one (Szylit et al., 1980), while unknown
 substances seem to carry some digestibility problems in D.
 alata cultivars (Martin, 1980).

Morphology and Physiology of the Tuber Growth

 Though a fair number of cultivars need a good soil depth for
their tuber, it is not uncommon to access easily to the starch sink.
Letting aside the aerial production of D. bulbifera (and at a lower
level of some D. alata), rather superficial underground produc-
tions are known with D. alata (i.e. 'Lupias' or 'Belep' types of
New Caledonia), D. cayenensis (cv. 'Krengle' of Ivory Coast), or
D. trifida (cv. 'Moengo 5' of Guyana) etc.
 Quite different balances between the size of individual tubers
and their number by plant can be managed through cultivar
variation, enforced by plantation date, vegetative seed size,
chemical treatment, plantation densities, staking, fertilization
and so on.
 The well known inter-species variation for harvesting time rests
upon a large intra-species one. A recent datum: among a popula-
tion of about 400 hybrids pertaining to three progenies of D.
cayenensis-rotundata obtained from sexual seeds of IITA in
Guadeloupe, maturity extended last year from seven to eleven
months after planting.
 The general inverse correlation between duration of vegetative
growth and duration of dormancy over the annual cycle, does not
impede some independent combination, as have resulted from
selection pressure in D. trifida progenies (Arnolin, unpublished
data).
 A differential sensitivity of cultivars of D. alata, under cool
(18 C) storage of their tuber, is expressed by the extension of the
onset of germination of 79 cvs from seven to thirteen months
after harvest.


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