A dissertation submitted to the Facult:y of Science, University of the Witwatersrand, Johannesburg, for the degree of Master of Science,
Johannesburg 1994 / Rhizosheaths are sandy coatings covering ~ ~ entire length of the
roots of many grass species~ .: They consLab of a mass of sand and
silica particles en4?eddedin a mucilaginous layer and are matted
togethel.· by a meahwozk of prolific epidermal had.ns,
A study of the phenomenon of zhd aosheat.ha in South African
grasses was undertaken. Aspects which were invest.igated include
the occurrence of rhizosheaths amongSouth African grasses and
the influence of environmental ccndftions on th.e presenc~ and ,
extent of rhizosheath development. In addition, information was
~fsll!led on their possible functions in terms of their
contribution (s ) to plant vigour, through their influence on
(1 nutrient and water uptake, particularly in low nutrient soil~ in
'~\
;'';-''>"\\ arid areas.
An extensive surv,sy of herbal:"iul!lspecimens was conducted at the ..
National Herbarium in Pretoria. It Wasfound that the presence"
of rhizosheaths is a genetically fixed trait, occurring in the
majority of gra~s species in south Africa. The extent to which
sheaths develop (the thickness of the sheat.h and the degree 'to
which the soil particles are bound to the sheath), varies
between, and sometimes wi thin;) species.
~<
Ii'
Seeds of three sheath forming grass spccf.es which occur in South
Africa, Anthephora pubescens Nees, Digitaria exientib« Steud and
Eragrostis pallens Hade, were gr.ownundez different condi.t.Lona
of soil texture and different conditions of water and nutrient
(nitrogen (N') and phosphorus: (P) aVailability. Sheath
development was found to be more extensive the higher the sand
(relative/.: to clay) content in the soil. In addition,
rhizoFlheaths developed to a greater extent in sandy soil with
high watsr and nutrient availabilityu Therefore, the extent to
whi;chsheaths develop is a facultative response directly to sandy
soil, rather than toe resulting lower water and nutrient
availability in this snil.
A:fter \: Jtudying a number of physiological and morphological
factors of the indiv~dua.ls in the different water and uutrient
treatments it appeared that rhizosheaths compensate",dfor low
water availability. In additionq sheaths appeared to «compensate
-. I.'
for low N availability when this 101;1T availability was a result
of low soil water contentc
The influence of sheath thickness on immobile nutrient ion uptake
was investigated.. 'l'his was achieved by stimulating withinspeoies
variation in sheath thiokness and mea.suring the
difference in P uptake. The individuals with thick sheaths
extracted more p from the. soil, and ~rom a greater volume of
soil, than individuals with thin sheaths. This was particularly
evident in conditi.ons of low P availabili,;ty i thereby highlighting
, the important influence of thick rhizoshea,ths in soils with a low
"""" p status.
The microbial biomass (as indicat;ed by t.he microbial Nand carbon
concentrations) in the sheath soil was compared to that in the
l?}y_k soil and the rliizosphe:r.e/out.er ,rhizosphere soil of
r{ tinsheathed/sheathep, roots. Fromthi~ study it w~s concluded that
rhj..zosheaths influence the"mic;:roorganisms in the soil ~djacent,
\, to the root surface, since the l.hizosheath I')oil had a
sigl,.ificantly higher microbial biomass than the soil .1:romthe
other regions. "
Fron\.the study it was concluded that the pzeaence of rhj.z6sheaths
:maybe a mechanismemployed byce~tain grass species in order to
enhance their ability to t.olera,te dry soil, which has J.owNand
P availability. This mechanismmaybe a. sUbstitute for increased
root production and root branching as well a~ increased
mycorrhizal associations in plants in arid, low nutrient status
soils~
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/20423 |
| Date | 06 June 2016 |
| Creators | Bailey, Catherine Lara |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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