L., I IDRC-080e ARCHIV MACINTedings of the Fourth Symposium of the 23101 Lational Society for Tropical Root Crops Held at CIAT, Cali, Colombia, 1-7 August 1976 Edited by James Cock, Reginald Maclntyre, and Michael Graham (' N The International Society for Tropical Root Crops in collaboration with Centro Internacional de Agricultura Tropical International Development Research Centre United States Agency for International Development IDRC-080e PROCEEDINGS of the FOURTH SYMPOSIUM of the INTERNATIONAL SOCIETY FOR TROPICAL ROOT CROPS held at CIAT, Cali, Colombia, 1-7 August 1976 Edited by James Cock, Reginald MacIntyre, and Michael Graham '041 20 IV1977 The International Society for Tropical Root Crop in collaboration with Centro Internacional de Agricultura Tropical International Development Research Centre United States Agency for International Development @ 1977 International Development Research Centre Postal Address: Box 8500, Ottawa, Canada K1G 31-19 Head Office: 60 Queen Street, Ottawa Cock, J. MacIntyre, R. Graham, M. International Society for Tropical Root Crops CIAT IDRC USAID IDRC-080e Proceedings of the Fourth Symposium of the International Society for Tropical Root Crops held at CIAT, Cali, Colombia, 1-7 August 1976, Ottawa, IDRC, 1977. 277 pp. / IDRC pub CRDI /. Proceedings of a symposium on / root crop / / plant production / in the / tropical zone /includes / list of participants /, / bibliography/s, and / statistical data /. UDC: 633.4(213) ISBN: 0-88936-115-0 Microfiche Edition $1 CONTENTS Foreword5 Society Council, 1976-79 6 Welcoming addresses 7 Participants 11 Section 1:Origin, dispersal, and evolution 19 Papers by:Léon 20; Plucknett 36; Sadik 40; Martin 44; Mendoza 50; Kobayashi and Miyazaki 53; Degras 58; and Warid et al. 62 Summary of discussions65 Section 2:Basic productivity69 Papers by:Loomis and Rapoport 70; Holmes and Wilson 84; Ferguson and Gumbs 89; Dharmaputra and de Bruijn 94; Nitis and Suarna 98; Obigbesan et al. 104; Ngongi et al. 107; Howeler et al. 113; Rendle and Kang 117; Mohan Kumar et al. 122; Edwards et al. 124; Wahab 131; Umanah 137; Montaldo and Montilla 142; Montilla et al. 143; Wilson et al. 146; Tanaka and Sekioka 150; and Sykes 151 Summary of discussions152 Section 3:Preharvest and postharvest losses155 Papers by:Lozano and Terry 156; Bock et al. 160; Mukiibi 163; Mukiibi 169; Terry 170; Ninan et al. 173; Leu 175; Terry 179; Obigbesan and Matuluko 185; Bellotti and van Schoonhoven 188; Nyiira 193; Yaseen and Bennett 197; Pillai 202; Thompson et al. 203; and Albuquerque 207 Summary of discussions208 Section 4:Utilization211 Papers by:Christiansen and Thompson 212; McCann 215; Chandra and De Boer 221; Valdes Sanchez 226; Phillips 228; Oke 232; Delange et al. 237; Hew and Hutagalung 242; Khajarern and Khajarern 246; Varghese et al. 250; Hutagalung and Tan 255; Gomez et al. 262; Gregory et al. 267; Nartey 270; Nakayama et al. 274; and Jeffers 275 Summary of discussions277 3 58 TROPICAL ROOT CROPS SYMPOSIUM Vegetative and Sexual Management in Food Yam Improvement L. Degrasi Yam models are described, giving several required characteristics for clonal cultiva- tion of Dioscorea alata, D. cayenensis, and D. trifida. Intraclonal selection is possible because of tuber internal heterogeneity, clonal population diversity, and time dispersed production and germination of individual plant bulbil. A kind of "somatic drift" is observed. Several aspects of determinism of tuberization and flowering are cited, and day/night ratio, light intensity, soil mineral balance, internal vegetative phases, and genetic factors are recognized in flowering and in sex-ratio determinism. Basic management using axillary structure growth-substance treatment for vegetative, tuberization, flowering, and sex control is discussed. In addition, the genetic analysis of characteristics, and different cultivation systems are examined. Yam improvement has normally been basedexisting widelyinthespeciesare:good on the introduction of clonal cultivars selecteddorrnancy, resistance to Penicillium oxalicum, from traditional populations. Use of the sexualgermination in dry conditions, and good yield system for food yams has been attempted; how-without staking); D. cayenensis ssp. rotundata ever, it is among pharmaceutic yams that the (1) fair tuber maturity long before foliage sexual system has been extensively used anddecay, (2) good tuber regrowth after com- studied. mercial harvest,(3)high cooking quality Improvement of the food yam through its(whitish flesh), (4) year-round tuber develop- sexual system began in 1966 with D. trifida inment, (5) medium storage duration of com- Guadeloupe (Degras 1969), and itis nowmercial harvest, (6) prickless roots, (7) high spreading through an inter-Caribbean selectionearly yield; D. trifida (1) tubers available on behalf of the ISTRC-CFCS yam studyyear-round because of: (a) fresh production all group. The largest food yam breeding poten-season, (b) food storage duration, (2) high tial now comes from IITA where, since 1970,cooking quality with sweet taste, (3) tuber Sadik and Okereke (1975) have developedgrouping near soil surface, (4) high yield with the sexual utilization of D. cayenensis ssp. 10% seed sized tubers, (5) resistant to viruses, rotundata, and tested a large amount of seedsPenicillium oxalicum, nematodes, and mealy throughout the tropics. bugs, (6) spheroidal tuber shape, (7) resistant This does not, however, mean that clonalor tolerant to drought. selection should be stopped, rather it still may have an important role to play. Intraclonal Selection Variation within a clone exists.Heads, Yam Models in Current Clonal middles, and tails from the same tuber differ Cultivation in earliness of germination, yield, and number At the present time, all cultivated yams areof stems or tubers produced. In addition, clonally propagated. The characteristics of D.normal bud regulation is suppressed when the alata, D. cayenensis, and D. trifida in theslice size is greater than 5 g (Degras and French West Indies are as follows: D alata Mathurin 1975). It is important that differ- (1) high cooking quality (white flesh), (2) ences have been repeated over first and second long storage without loss of weight, (3) re-generations of two D. trifida clones obtained sistant to anthracnosis and viruses, (4) resistantfrom different parts of a tuber of cultivar to water stress, (5) high yield with mediumINRA 25: flowering time differences at the and regular size tubers (other characteristicssecond generation were in accordance with be- haviour in the first generation, differences in 1Plant Breeder, Institut national de la recherchetime to maturity were up to 1 month. agronomique (INRA), Centre des Antilles et de We do not know the level of genetic homo- la Guyane, Domaine Duclos, Prise d'eau, Petitgeneity of traditional cultivars, but some may Bourg, France. include mutational variations. Off-types are DEGRAS: YAM MANAGEMENT 59 Table 1. Agronomic variability from D. cayenenis cultivarKrenglé intraclonal tuber selection. Tuber shape Plant size Growth Tubers Mean tuber weightb (seed tuber) (200 days) durationa Yieldb harvestedb Cylindricala 100 100 100 100 100 Cylindro-conical 110 111 91 103 89 Conical 100 104 88 108 82 94 Ovoidal 113 104 82 88 Spheroidal 103 100 73 102 67 aStatistical significance has been found for shape x seed tuber weightinteraction in cycle duration. bSignificative differences p= 005. aCylindrical data are taken as basis = 100 for each trait. normally easily recognizable when cultivatedthe sexual phase is very depressed in annual with cultivars: cultivar individuality is a fact.cultivation. A kind of competition exists be- Nevertheless, in the well known cultivar D.tween vegetative accumulation and sexual de- cayenensis ssp. rotundata "Krenglé," in centralvelopment for the following reasons. Ivory Coast, a number of tuber forms could be Tuberization and flowering seem to be separated. When cultivated separately theyfavoured by short days for a number of species. have distinct performances (Table 1). This has been shown for bulbils and for tuber Another case of intraclonal variation thatgrowth. D. alata and D. trifida normally flower may be explored arises from different times toin the autumn, but Henry(1967)obtained maturation of the bulbils of the plant. In D.earlier flowering for a number of seedlings bulbifera and D. alata, the difference mayunder short days. reach 1 month. High light intensity is necessary for flowering These examples show thata form ofand tuberization: Henry(1967)observed a "somatic drift" of clonal expression of a yam lower level of flowering of D. trifida seedlings genotype could proceed from conscious suc-when shaded. For most species, staking, which cessive selection of extreme phenotypic varia-permits more light to enter the canopy, gives tions. It is obvious that in current cultivation,higher tuber yields. The difference is striking such variations do not cause marked changesfor a number of dry-forest climax species like of the clonal population. But we think thatD. cayenensis, whereas it is less important for through controlled cultivation (planting timerain-forest climax species like D. alata and D. and density) and controlled dormancy dura-trifida. tion we could expect to alter the phenotypic The physiological response to fertilization balance of clonal properties. The selection ofsuggests a common process for initiation of the type of planting material from tubers of atuberization and flowering. We observed that clone could at the same time determine