Studies on agronomy and crop physiology of Plectranthus edulis

160 Pages·2008·3.88 MB·English

by  Mulugeta Taye Ababora

Studies on agronomy and
crop physiology of
Plectranthus edulis (Vatke) Agnew
Promotor: Prof. dr. ir. P.C. Struik
Hoogleraar Gewasfysiologie,
Wageningen Universiteit

Co-promotor: Dr. ir. W.J.M. Lommen
Universitair docent bij de leerstoelgroep Gewas- en Onkruidecologie,
Wageningen Universiteit

Promotiecommissie:
Prof. dr. L.H.W. van der Plas (Wageningen Universiteit)
Prof. dr. O. van Kooten (Wageningen Universiteit)
Dr. ir. A.J. Haverkort (Plant Research International, Wageningen)
Dr. ir. J.S. Siemonsma (Plant Resources of Tropical Africa, Wageningen)

Dit onderzoek is uitgevoerd binnen de C.T. de Wit onderzoekschool: Production
Ecology and Resource Conservation.
Studies on agronomy and
crop physiology of
Plectranthus edulis (Vatke) Agnew

Mulugeta Taye Ababora

Proefschrift
ter verkrijging van de graad van doctor
op gezag van de rector magnificus
van Wageningen Universiteit
Prof. dr. M.J. Kropff
in het openbaar te verdedigen
op woensdag 4 juni 2008
des namiddags te half twee in de Aula
Mulugeta Taye Ababora (2008)
Studies on agronomy and crop physiology of Plectranthus edulis (Vatke) Agnew

Mulugeta Taye Ababora – [S.l.: s.n.]. Ill.
PhD thesis Wageningen University. – With ref. –
With summaries in English and Dutch.
ISBN: 978-90-8504-915-9
Abstract

Mulugeta Taye Ababora, 2008. Studies on agronomy and crop physiology of
Plectranthus edulis (Vatke) Agnew. PhD thesis, Wageningen University,
Wageningen, The Netherlands. With summaries in English and Dutch, 148 pp.

Plectranthus edulis (Vatke) Agnew (Lamiaceae) is an ancient Ethiopian tuber crop grown in mid and
high altitude areas in the north, south and south-west of Ethiopia. Cultivation dates back from c. 3000
BC, but in recent years its acreage and production have declined. Renewed interest to conserve the
crop and increase its production is limited by absence of accurate information on growth, development
and cultural practices of P. edulis. This project aimed at providing the basic knowledge needed to direct
further applied research.
A standard production technique was developed after interviewing farmers in Chencha and
Wolaita in southern Ethiopia, and was used in later experiments. The standard planting material
chosen were de-sprouted tuber pieces, prepared from a medium (12–15 cm) sized mother tuber broken
into three pieces. Three pieces were planted per hole, at a hole spacing of 75 × 90 cm. Shoot tipping
(pinching; the removal of the apices with 1−2 leaf pairs) was carried out when the crop was 10–15 cm
high.
The general structure of the crop was similar to that of Irish potato. Plant components were: the
seed tuber pieces, sprouts, main stems, branches, leaves, inflorescences, fruits, seeds, roots, stolons
and tubers. The crop had a long growing period. In two growth studies, maximum fresh tuber yields
were attained c. 34 weeks after planting (WAP). Above-ground development was characterised by a
late emergence (c. 4 weeks), a slow development of the canopy after emergence until full ground
cover was attained (c. 20 weeks), a very short period during which ground cover was full (c. 2 weeks)
and a relative fast decline in ground cover thereafter (6−8 weeks). Primary and secondary branches
constituted the major part of the canopy. The first stolons were formed c. 10−12 WAP on below-
ground nodes of main stems and primary branches. Tubers were first recorded at 18 WAP as a
swelling on the tip of the stolon and sometimes as a swelling of the middle part of stolons. Tubers
attained a maximum length of 20−25 cm, and a maximum diameter of c. 2 cm. Aerial stolons were
initiated 12−16 weeks later than below-ground stolons and could be up to 2.5 m long.
The increase in tuber fresh weight with time was realized by an increase in both number of
tubers and in average weight per tuber over the entire tuber formation period. Fresh tuber yields at 34
WAP were 45−49 Mg ha−1. Yield levels in other sets of experiments in which the harvest date was
chosen arbitrarily were c. 21 Mg ha−1 (29.7 WAP) and c. 30 Mg ha−1 (34.7 WAP). Experimental yields
were very high compared to those reported by farmers.
Nevertheless, in growth studies, the average daily dry matter production of the crop over the
whole growing period was only 4.2−4.6 g m−2 day−1. The dry matter production was limited by a poor
radiation interception by the canopy – only one third of the incident radiation was intercepted − and a
low radiation use efficiency (RUE) – on average only 1.59 g MJ−1 photosynthetically active radiation
(PAR). RUE gradually increased after emergence to about 2.7 g MJ−1 PAR when tuber formation was
still in an early stage (24−26 WAP), but then declined because of a stagnation or decline in total crop
dry weight, that lasted several weeks. Dry matter production decreased in that period because the
decrease in canopy dry matter – especially stem dry matter − was not yet compensated for by the
increase in tuber dry matter. This was attributed partly to a still limited capacity of the tubers to
convert and / or store assimilates in this stage. Later this changed and total dry weight and RUE
increased again. Harvest index was 81−99% at the moment when tuber yield was maximum.
Shoot tipping significantly increased ground cover and delayed canopy senescence. Tipping
also had a positive – though not always significant – effect on tuber yield. Tipping enhanced early
stolon formation, but did not consistently affect the number of stolons later in the growing season
Because differences among tipped treatments were not large, limiting the tipping frequency to one will
help to save time, labour and money.
Across experiments in which the number and size of the tuber pieces planted per hole were
varied, the tuber fresh weight increased when the number of main stems per m2 increased up to 2.5−3
main stems per m2. This sufficiently high stem number could usually be achieved by planting
sufficient seed tuber material (equalling at least one medium-sized mother tuber per hole) and
breaking it into two or three pieces. This confers with the farmers practice. Over all treatments, an
increase in fresh tuber yield was never realized by merely increasing the individual tuber weight, but
either by combined effects on number of tubers and individual tuber fresh weight or by an effect on
number of tubers alone.
A further increase in radiation interception by advancing and improving canopy development
could likely be achieved by planting larger seed pieces, pre-sprouting the seed tuber pieces and using a
higher plant density. However, the below ground development should be geared to that. At present the
late initiation and formation of tubers already seems to limit production, and this should be improved
when an enhanced canopy cover should result also in higher tuber yield.
On short term notice, however, the major constraints to concentrate on will be the shortage of
seed tubers and the poor storability of the progeny tubers. Shortage of seed tubers was mentioned by
the interviewed P. edulis farmers as a major constraint and the principle reason for the decline in
production of P. edulis. The present practice by farmers of storing tubers in situ in the ground was
shown to reduce tuber fresh weights by 36−59% and the number of tubers by 18−48% in 6 weeks.

Keywords: Development, morphology, plant density, potato, radiation interception, radiation use
efficiency, seed size, seed tuber, spacing, stolon, tipping, tuber
Preface

This thesis is about the indigenous orphan crop Plectranthus edulis. It was written
based on survey data collected from the southern region of Ethiopia and experiments
that were carried out in two localities in south Ethiopia. The thesis has attempted to
understand the production practices, major production problems, and also the
physiology and agronomy of the crop. It is hoped that the results obtained during the
research will serve as a base in an effort to improve the tuber production of this crop.
During my research work several organizations and individuals contributed to the
realization of this thesis.
I am very grateful to Crop and Weed Ecology of the Plant Sciences Group,
Wageningen University, for giving me the opportunity to work on my PhD on this
crop. I am also thankful to the Ethiopian Science and Technology Commission and the
Norwegian Agency for Development Cooperation (NORAD) for their financial
assistance.
I am heavily indebted to Professor dr Paul C. Struik, first for considering my
application as a PhD candidate, and then for his unreserved assistance to realize my
work. Paul, I thank you for all the effort you made to realize the completion of my
work. I am really grateful for all activities including checking, analysing, developing
ideas and editing the papers. Paul, I hope you have enjoyed the time you had in
Ethiopia and particularly in Wolaita where the farmers tried to explain how the crop is
grown.
I am also very much indebted to Dr Willemien J.M. Lommen for her support,
checking, editing, compiling, and for all assistance she provided to the completion of
my thesis. Willemien, I hope you have also enjoyed the travel you had to the Areka
Research Station and the Bouditi farmers both located in Wolaita, and the Awassa and
Wondogenet research stations.
I also want to thank the staffs of Crop and Weed Ecology, Plant Production
Systems, PROSEA / PROTA and Tupea for creating a conducive environment for the
successful completion of my work. I really have enjoyed sharing experiences,
particularly during the discussion group seminars and other scientific presentations we
had with the CWE and PPS groups. I am very thankful to all research, associated PhD
research and technical staffs. During my stay several staff members have encouraged
me. I want to make special thanks to Prof. Dr Holger Meinke, Prof. Dr Huub Spiertz,
Dr Jan Vos, Dr Tjeerd-Jan Stomph, Dr Lammert Bastiaans, Dr Wopke van der Werf,
Dr Xinyou Yin, Dr Gerhard Buck-Sorlin and Dr Jochem Evers. I can tell that I do not
really forget the special encouraging words on the corridor by Gon van Laar. It always
gave me pleasure.
I am also very grateful to Dr Claudius van de Vijver of the PE&RC office for his
advice and assistance, and also very grateful to the Central Student Administration.
I am also most grateful to so many who made my life comfortable during my two
visits at Wageningen. I am very grateful to the administration staffs. I want to make
special thanks to Jenny Elwood, the secretary of CWE, who has been so kind and
helpful in arranging the air tickets, visas, accommodation, and office space.
I want to thank Birhanu Temesgen, Melkamu Derseh and Ababu Yirgah for their
encouragement at Wageningen. I want also to make special thanks to Workneh
Tessema and Peter Gildemacher.
Despite the quiet mood in the UN room (Den Nuij, 1.017B), the occasional talk,
low tone discussions and sometimes laughter with my colleagues especially over the
lunch break always made life more bearable, thanks David Kamau, Huynh Tin, Wen
Jiang, Gisella Cruz and Benno Burema. Benno, I have really enjoyed your short stay
with us particularly for cheering us with your rose flowers.
Back to Ethiopia, I thank the administrative offices of the Awassa College of
Agriculture and Hawassa University. I am also particularly thankful to the
Departments of Horticulture and Plant Sciences staffs for their cooperation. Without
their cooperation I know I would not have been successful. I thank you all. I want to
make special thanks to Admasu Tsegaye, Tesfaye Teshome, Tesfaye Abebe,
Awdenegest Moges, Ayele Taye, Firdu Azerefegn, Bizuayehu Tesfaye Yibrah Beyene,
Kefyalew Beryihun and Amsalu Gobena. I also thank the Wondogenet Forestry
College and Research Institute Dean’s office and administration for providing me an
experimental area for my research, and I am particularly indebted to Genene Asfaw for
facilitating land for my research.
I also would like to thank the International Livestock Research Institute (ILRI),
the Ethiopian Health and Nutrition Institute (EHNRI), the Ministry of Agriculture of
Wolaita and Chencha, the Soil laboratory and the Biodiversity Research Institute. I
want to make special thanks to the Biodiversity Research Institute for providing me
with planting material to start my research.
I am also very grateful to my field assistants Tsegaye Fantahun, Mirab Zewdie,
G/Michael Ayele, and Mesfin Castro.
All my relatives, family, and friends have been supportive throughout. Although
I cannot mention all by name, your support and contribution has really humbled me
and I thank you all. I really wished my father Taye Ababora, my mother Zenebech
Deneke, and my beloved brother Derege Taye were alive. I miss you.
Lastly, I thank my dear wife Roman Asfaw for her love, support and strength. To our
children Beakal and Emanuel: thank you for your long patience and for being so
understanding to cope with situations that were sometimes beyond my handling. I
dedicate this thesis to Romi and you. Finally, I thank our Almighty God who has seen
me this far.

Mulugeta Taye
Wageningen, March 2008