Growth, water relation and hydraulic characteristics along a

317 Pages·2006·1.96 MB·English

Ecology and Development Series No. 46, 2006

Editor-in-Chief:
Paul L.G.Vlek

Editors:
Manfred Denich
Christopher Martius
Charles Rodgers

Taye Kufa Obso

Ecophysiological diversity of wild Arabica coffee
populations in Ethiopia: Growth, water relations and
hydraulic characteristics along a climatic gradient

Cuvillier Verlag Göttingen
I would like to dedicate this dissertation to my late family members:
Mom Ayelech Belihu, sister Guchi Kufa and brother Gebe Kufa, who can not share
my fulfilled dreams and joy today. May God give them peace and serenity.
ABSTRACT

Coffee (Coffea arabica L.) is a shade-adapted plant occurring in the undergrowth of humid
montane rainforests of southwestern Ethiopia, where it has its center of origin and diversity.
However, the remnant rainforests with the occurrence of wild coffee populations are under
continuous threat largely due to a high deforestation rate and recurrent drought, and demand
urgent actions for their conservation. This study was conducted with the aim to investigate the
ecophysiological diversity in growth architecture, hydraulic characteristics and water relations
of wild coffee populations in southeastern and southwestern Ethiopia. In-situ-investigations
were made within four rainforests along a rainfall gradient between 2,100 and 950 mm per year,
following the order of Berhane-Kontir>Yayu>Bonga>Harenna, with the driest site, Harenna,
situated in the southeast. Moreover, coffee accessions from these wild coffee populations were
established at the Jimma Research Center. In an ex-situ experiment, the one-year-old coffee
seedlings were evaluated under contrasting daylight and drought stress regimes over a period of
16 days. For this period and during the following weeks, diurnal soil and leaf water potentials,
root and shoot hydraulic conductance and saturated hydraulic resistance components were
recorded along with root and shoot parameters, extent of drought damage and recovery rates. In
addition, foliar chlorophyll, leaf composition and stomatal characteristics were determined. A
high-pressure flow meter was employed to measure hydraulic flows in root and shoot segments
of mature trees and coffee seedlings.
The Harenna and Berhane-Kontir populations had open crowns, while Yayu and
Bonga showed intermediate to compact canopies. The wild coffee trees displayed highly
significant variations in the diameter and length of coarse lateral roots, but not in the proportion
of root classes. Hydraulic conductances of stems and branches differed highly significantly
between populations (Harenna>Berhane-Kontir>Yayu>Bonga), both for whole branches as well
as length specific. The juvenile primary branches showed the lowest hydraulic conductance
within the crowns. The removal of growth parts significantly reduced hydraulic resistance
components in the primary branches of the coffee trees and shoot of the coffee seedlings.
The ex-situ results show highly significant differences among the coffee accessions in
seedling emergence and subsequent growth stages. In addition, the relationships between
growth and hydraulic characteristics, and soil and plant composition were different for shade
and irrigation levels. The analysis of variance also depicts significantly (P<0.01) higher total
biomass for unshaded and drought-stressed seedlings as compared to those in shaded and well-
watered plots. Again, significantly lowest and highest total biomass were obtained from the
Berhane-Kontir and Harenna coffee accessions, respectively. Moreover, the Harenna accessions
were also noted to have higher values for such variables as average leaf area, main stem
diameter, leaf chlorophyll, leaf nitrogen and stomatal density. The Berhane-Kontir accessions
had highly significant lowest root partitioning compared with Harenna, which had the highest
root share. In consistence with the in-situ results, the Harenna accessions exhibited higher root
and shoot hydraulic conductances as compared to the other sites. However, they failed to
withstand persisting drought-stress situations, while the accessions from the moist southwestern
regions were still performing well. The Harenna accessions, however, were better adjusted to
drought stress through such features as high root to shoot ratio, deep root system, thick leaves,
and their leaves showed greater changes in leaf water potentials and a high accumulation of
potassium ions. In contrast, the Berhane-Kontir and Bonga accessions had shallow roots and
maintained a better water storage in the dehydrated potting medium. As a whole, moderate
shading was noted to extend the occurrence of drought stress effects by a two-fold time period
over those seedlings subjected to soil drying rapidly in full sun.
This work outlines very different strategies for coping with drought stress in seedlings
of wild coffee accessions, whereby accessions from the southeastern and southwestern areas
followed opportunistic and conservative ways of water use in drought stress conditions,
respectively. The results provide the first detailed ecophysiological analysis and demonstrate
inter- and intra-regional variability among wild coffee populations in morpho-physiological
characteristics, water relations and hydraulic conditions. This variability underlines the
importance of the four studied natural coffee forests as gene pools for future breeding programs,
and the need for multi-site in-situ conservation strategy. The study provides research
recommendations on the management and use of coffee forests and wild coffee populations in
Ethiopia.
Ökophysiologische Diversität von Wildkaffee-Populationen in
Äthiopien: Wachstum, Wasserhaushalt und hydraulische
Eigenschaften entlang eines Klimagradienten

KURZFASSUNG
Der Ursprung und das genetische Zentrum von Kaffee (Coffea arabica L.) liegen in Südwest-
Äthiopien, wo die Art im schattigen Unterwuchs der Bergregenwälder vorkommt. Starker
Nutzungsdruck und zunehmende Trockenheit gefährden die noch verbliebenen
Bergregenwälder und machen Maßnahmen zu deren Schutz erforderlich. Die vorliegende Studie
untersucht die ökophysiologische Diversität von Wildkaffee-Populationen im Südosten und
Südwesten Äthiopiens mit Blick auf die Wuchsform, die hydraulische Konstruktion und den
Wasserhaushalt der Bäume. Die in-situ-Untersuchungen konzentrierten sich auf vier
Bergregenwälder mit Niederschlägen zwischen 2.100 und 950 mm/Jahr (Berhane-
Kontir>Yayu>Bonga>Harenna), wobei die drei feuchteren Standorte im Südwesten Äthiopiens
liegen, Harenna dagegen im Südosten. Die von allen vier Standorten gewonnenen Samen (Ak-
zessionen) wuchsen in der Forschungsstation Jimma zu einjährigen Setzlingen und wurden dann
unterschiedlichen Licht- und Wasserverhältnissen ausgesetzt (ex-situ-Experiment).Während und
nach einer 16-tägigen Trockenperiode wurden täglich die Boden- und Blattwasserpotenziale, die
hydraulische Leitfähigkeit von Wurzel und Spross, sowie Schäden und anschließende
Regeneration der Pflanzen ermittelt. Weiterhin wurden Blattchlorophyll, Blattzusammensetzung
und stomatäre Charakteristik bestimmt. Zur Bestimmung der hydraulischen Widerstände und
Leitfähigkeiten in Spross und Wurzeln der Kaffeepflanzen wurden der druckabhängige
Durchfluss durch die entsprechenden Segmente mittels eines Hochdruckflussmessgeräts
(HPFM) ermittelt.
Die Populationen in Harenna und Berhane-Kontir hatten einen offenen Kronenaufbau,
die Bäume in Yayu und Bonga dagegen hatten eher kompakte Kronen. Deutliche Schwan-
kungen traten bei Durchmesser und Länge der lateralen Wurzeln auf, nicht aber in der Zu-
sammensetzung der Wurzelklassen. Die hydraulischen Leitfähigkeiten von Stamm und Ästen
unterschieden sich hochsignifikant zwischen den einzelnen Standorten (Harenna>Berhane-
Kontir>Yayu>Bonga), sowohl bezogen auf die Gesamtlänge als auch längenspezifisch. Glei-
ches galt für die hydraulische Leitfähigkeit der Äste. Innerhalb einzelner Baumkronen zeigten
die jüngsten Äste jeweils die niedrigste hydraulische Leitfähigkeit. Ein Abschneiden der
Wachstumszonen reduzierte die Widerstandskomponenten in primären Ästen ausgewachsener
Bäume und im Spross von Setzlingen jeweils signifikant.
Die Ergebnisse des ex-situ-Experiments zeigten bei Austrieb und
Wachstumsentwicklung hochsignifikante Unterschiede zwischen den einzelnen Kaffee-
Akzessionen. Die Licht- und Beregnungsvarianten wiesen Unterschiede bei Wachstum und
hydraulischen Eigenschaften, sowie der chemischen Zusammensetzung von Boden und
Pflanzen auf. Die unbeschattete Variante hatte gegenüber der beschatteten eine höhere
Gesamtbiomasse, ebenso die schlecht wasserversorgte gegenüber der gut wasserversorgten
Variante (jeweils P<0,01). Die signifikant niedrigste Gesamtbiomasse trat bei den Berhane-
Kontir-Akzessionen, die höchste bei den Harenna-Akzessionen auf. Darüber hinaus hatten die
Harenna-Akzessionen höhere Werte von Blattfläche, Stammdurchmesser, Blatt-Chlorophyll,
Blatt-Stickstoff und Stomatadichte. Im Vergleich zwischen Berhane-Kontir und Harenna-
Akzessionen zeigten letztere einen signifikant höheren Wurzelanteil und hatten auch insgesamt
den höchsten Wurzelanteil. In Übereinstimmung mit den in-situ-Ergebnissen zeigten die
Harenna-Akzessionen die höchsten hydraulischen Leitfähigkeiten von Wurzel und Spross,
allerdings auch die stärksten Schäden bei anhaltender Trockenheit, jeweils verglichen mit den
Akzessionen der drei übrigen Standorte. Die Harenna-Akzessionen zeigten andererseits die
stärkste Tendenz zur Stressvermeidung, etwa durch hohes Wurzel/Sproß-Verhältnis, tiefes
Wurzelsystem, dickere Blätter; in ihren Blättern traten stärkere Schwankungen des
Wasserpotenzials und hohe Akkumulation von Kalium-Ionen auf. Dagegen hatten speziell die
Akzessionen aus Berhane-Kontir und Bonga ein flaches Wurzelsystem und der Boden blieb bei
Trockenheit feuchter. Trockenstress-Symptome traten bei Setzlingen der Beschattungsvariante
etwa zwei Wochen nach der unbeschatteten Variante auf.
Die Ergebnisse dieser Arbeit zeigen sehr unterschiedliche Strategien der Kaffee-
Wildpopulationen im Umgang mit Trockenstress. Die Akzessionen aus dem Südwesten
Äthiopiens zeigten einen wesentlich konservativeren Umgang mit Wasser verglichen mit dem
opportunistischen Umgang der Akzessionen aus Harenna. Die dargestellten Ergebnisse bilden
die erste detaillierte ökophysiologische Analyse von Wildkaffee-Populationen und zeigen inter-
und intraregionale Unterschiede der Morpho-Physiologie, des Wasserhaushalts und der
hydraulischen Bedingungen auf. Diese Unterschiede belegen die Bedeutung der untersuchten
vier Kaffeewälder als Genpools für zukünftige Züchtungsprogramme und die Notwendigkeit
einer Strategie zur in situ-Erhaltung. Die Studie schließt mit der Ableitung von Empfehlungen
für die entsprechende Nutzung der Waldressourcen in Äthiopien.
TABLE OF CONTENTS
1 GENERAL INTRODUCTION...............................................................................1
1.1 Background and problem statement............................................................1
1.2 Hypothesis of the study...............................................................................4
1.3 Scope and objective of the study.................................................................4
1.4 Thesis structure............................................................................................5
2 STATE-OF-THE-ART............................................................................................6
2.1 Morphological and physiological features of Arabica coffee.....................6
2.1.1 Morphology of Arabica coffee..................................................................6
2.1.2 Physiology of Arabica coffee....................................................................8
2.2 Ecological requirements of coffee...............................................................9
2.2.1 Temperature............................................................................................10
2.2.2 Water ..................................................................................................10
2.2.3 Light and wind........................................................................................12
2.2.4 Soil factors..............................................................................................13
2.3 Plant-water relations..................................................................................14
2.3.1 Plant response to drought stress..............................................................16
2.3.2 Soil moisture and transpiration...............................................................18
2.3.3 Photosynthesis and water-use efficiency................................................22
2.3.4 Biomass assimilation and partitioning....................................................23
2.3.5 Hydraulic characteristics.........................................................................25
3 THE STUDY AREA AND METHODOLOGY...................................................28
3.1 Study area..................................................................................................28
3.1.1 Field study...............................................................................................28
3.1.2 Nursery study..........................................................................................33
3.2 Methodology..............................................................................................34
3.2.1 Field experiment.....................................................................................34
3.2.2 Nursery experiment.................................................................................36
3.2.3 Soil analysis............................................................................................37
3.2.4 Data analysis...........................................................................................38
3.3 Ecology of the study montane rainforests.................................................38
3.3.1 Climate ..................................................................................................38
3.3.2 Plant association......................................................................................39
3.3.3 Soil ..................................................................................................42
3.3.4 Discussion...............................................................................................50
4 GROWTH ARCHICTURE OF THE WILD ARABICA COFFEE TREES........55
4.1 Introduction...............................................................................................55
4.2 Material and methods................................................................................57
4.3 Statistical analysis.....................................................................................59
4.4 Results.......................................................................................................59
4.4.1 Shoot characteristics...............................................................................59
4.4.2 Leaf characteristics.................................................................................65
4.4.3 Yield and yield components....................................................................67
4.4.4 Branch growth.........................................................................................71
4.4.5 Root growth............................................................................................73
4.4.6 Growth rate of wild coffee trees.............................................................79
4.5 Discussion..................................................................................................80
5 WATER RELATIONS AND HYDRAULIC CHARACTERISTICS OF
THE WILD COFFEE TREES...............................................................................86
5.1 Introduction...............................................................................................86
5.2 Material and methods................................................................................88
5.2.1 Soil-plant moisture contents...................................................................88
5.2.2 Hydraulic measurements.........................................................................89
5.3 Statistical analysis.....................................................................................91
5.4 Results.......................................................................................................91
5.4.1 Soil moisture...........................................................................................91
5.4.2 Leaf water content...................................................................................93
5.4.3 Hydraulic conductance............................................................................95
5.4.4 Relations between growth and hydraulic parameters...........................101
5.4.5 Hydraulic resistance..............................................................................104
5.5 Discussion................................................................................................108
6 GROWTH AND DEVELOPMENT OF COFFEE SEEDLINGS......................114
6.1 Introduction.............................................................................................114
6.2 Material and methods..............................................................................117
6.2.1 Microclimatic measurements................................................................117
6.2.2 Phenological parameters.......................................................................118
6.2.3 Leaf and branch orientations.................................................................118
6.2.4 Leaf chlorophyll contents.....................................................................119
6.2.5 Shoot and root morphology..................................................................119
6.2.6 Response to light regimes.....................................................................120
6.3 Statistical analysis...................................................................................121
6.4 Results.....................................................................................................122
6.4.1 Characteristics of potting medium........................................................122
6.4.2 Microclimatic variables........................................................................122
6.4.3 Seedling emergence and early growth..................................................124
6.4.4 Growth characteristics of coffee seedlings...........................................129
6.4.5 Biomass production and partitioning....................................................145
6.4.6 Growth rate of coffee seedlings............................................................153
6.4.7 Growth response to light regimes.........................................................159
6.5 Discussion................................................................................................167
6.5.1 Microclimate.........................................................................................167
6.5.2 Growth characteristics of coffee seedlings...........................................168
6.5.3 Biomass assimilation and distribution..................................................173
6.5.4 Growth rate of coffee seedlings............................................................175
6.5.5 Growth response to light regime...........................................................177
7 WATER RELATIONS AND HYDRAULIC CHARACTERISTICS OF
COFFEE SEEDLINGS.......................................................................................183
7.1 Introduction.............................................................................................183
7.2 Material and methods..............................................................................186
7.2.1 Determination of moisture content.......................................................186
7.2.2 Percent loss of hydraulic conductance..................................................187
7.2.3 Recovery from drought stress...............................................................187
7.2.4 Soil-plant chemical compositions.........................................................189
7.2.5 Hydraulic measurements.......................................................................189
7.2.6 Stomatal characteristics........................................................................191
7.3 Statistical analysis...................................................................................192
7.4 Results.....................................................................................................192
7.4.1 Soil moisture content............................................................................192
7.4.2 Leaf water potential..............................................................................194
7.4.3 Relative leaf water content....................................................................200
7.4.4 Percent loss of conductance..................................................................203
7.4.5 Recovery from drought stress...............................................................206
7.4.6 Soil-plant chemical compositions.........................................................217
7.4.7 Hydraulic measurements.......................................................................227
7.4.8 Stomatal characteristics........................................................................246
7.5 Discussion................................................................................................253
7.5.1 Soil-plant water relations......................................................................253
7.5.2 Soil-plant chemical compositions.........................................................262
7.5.3 Hydraulic resistance..............................................................................263
7.5.4 Hydraulic conductivity..........................................................................267
7.5.5 Stomatal characteristics........................................................................271
8 OVERALL SUMMARY.....................................................................................276
9 GENERAL CONCLUSIONS AND RECOMMENDATIONS..........................280
9.1 Conclusions.............................................................................................280
9.2 Recommendations...................................................................................282
10 REFERENCES....................................................................................................286
11 APPENDICES.....................................................................................................301
ACKNOWLEDGEMENTS
LIST OF ABBREVIATIONS AND UNITS

Abbreviation Description Unit
BA Basal area cm2
BD Bulk density g cm-3
CA Crown area cm2
CV Coefficient of variation %
FC Field capacity PMa/bar
K Hydraulic conductance kg s-1 m-2MPa-1
h
k Hydraulic conductivity kg s-1m-1MPa-1
h
LA Leaf area cm2
LAI Leaf area index cm2 cm-2
LAR Leaf area ratio cm2g-1
LARMR Leaf area root mass ratio cm2g-1
LMR Leaf mass ratio g g-1
LWP Leaf water potential MPa
OM Organic matter %
PWP Permanent wilting point MPa/bar
RD Root density g cm-3
RGR Relative growth rate cm time-1
R Hydraulic resistance MPa m2 s kg-1
h
RH Relative humidity %
RLWC Relative leaf water content %
RMR Root mass ratio g g-1
SD Standard deviations %
SLA Specific leaf area cm2g-1
SSL Specific stem length cm g-1
SLM Specific leaf mass g cm-2
SMC Soil moisture content % vol
SMR Stem mass ratio g g-1
TN Total nitrogen %
WHC Water holding capacity % (dry wt)