PhD

Rubber tree (Hevea brasiliensis Muell. Arg.) is the only commercial
source of natural rubber and the species is well suited to the equatorial region
with plenty of well distributed rainfall and minimum fluctuations in temperature.
With increasing global demand for natural rubber, attempts have been made to
extend the cultivation of this tree to agroclimatically marginal lands such as
drought and cold prone areas. But the very narrow genetic base of the cultivated
Hevea resulted from the development of the species from a limited number of
seeds introduced by Sir Henry Wickham, limits the scope of extending the clone
to such marginal areas. A broad genetic base is a prerequisite for developing new
varieties tolerant to various stress conditions. Plant genetic resources provide the
requisite genetic variability and are the most important and vulnerable basic
materials to meet the current and future needs of plant breeding programmes.
The wild germplasm accessions collected by an expedition of
International Rubber Research and Development Board (IRRDB) during 1981,
into the primary centre of origin of the crop, the Amazon forests provide a rich
source of natural variability in this tree species. Introduction of genes from such
wild progenitors is an ideal method of broadening the genetic base of cultivated
Hevea species.
To make more efficient use of these germplasm materials, it is
necessary to identify morphophysiological traits and structural and biochemical
traits associated with tolerance to different abiotic stresses in these materials.
Such a detailed study for traits associated with drought tolerance has not been
made so far using wild Hevea germplasm materials. Assessing the genetic
variability for such traits in these germplasm materials will help further to identify
superior genotypes which can be used as potential parents in future breeding
programmes for developing varieties suitable for drought stress conditions. Hence

the proposed research work entitled "Genotypic evaluation and screening for
drought tolerance in wild Hevea germplasm" was carried out with the objectives
of assessing the genetic variability present among the wild Hevea germplasm
materials for characters associated with drought tolerance and screening of wild
Hevea germplasm to identify genotypes for future breeding programmes of
developing a drought tolerant clone.
The entire study was conducted with 99 wild Hevea germplasm lines
conserved in the Rubber Research Institute of India, Kottayam for a period of
three years from 1998-2000. Three separate experiments were carried out. In
Experiment I a preliminary screening of 99 wild Hevea germplasm accessions
(representative of Acre and Mato Grosso states of Brazil) for drought tolerance
based on cell membrane stability was conducted using RRll 105 as control clone.
The relative injury to cell membrane varied from 30 to 80 per cent with a mean of
53 per cent and the heritability (H2) for this character was very high (87%). Based
on relative injury to cell membrane, the entire accessions were classified and 10
accessions (AC 1044, MT 55, AC 446, MT 41, MT 76, MT 66, MT 938, AC 650,
AC 652, AC 728) representative of all classes were selected for detailed study
under Experiment II along with the control clones RRll 105 (a popular high
yielding clone), RRIM 600 (a drought tolerant clone) and Tjirl (a drought
susceptible clone).
Experiment IT was conducted under field conditions to assess the
genetic variability among the 10 selected accessions for drought tolerance using
various indices. The polybag plants were grown under induced water stress for 15
days, 30 days and 45 days. Drought related physiological parameters viz., leaf
temperature, stomatal conductance, transpiration rate, afternoon leaf and soil
water potentials and chlorophyll fluorescence parameters were studied along with
the plants under non-stress. Under water stress condition, leaf temperature was
increased and stomatal conductance, transpiration rate and afternoon leaf and soil

the proposed research work entitled "Genotypic evaluation and screening for
drought tolerance in wild Hevea germplasm" was carried out with the objectives
of assessing the genetic variability present among the wild Hevea germplasm
materials for characters associated with drought tolerance and screening of wild
Hevea germplasm to identify genotypes for future breeding programmes of
developing a drought tolerant clone.
The entire study was conducted with 99 wild Hevea germplasm lines
conserved in the Rubber Research Institute of India, Kottayam for a period of
three years from 1998-2000. Three separate experiments were carried out. In
Experiment I a preliminary screening of 99 wild Hevea germplasm accessions
(representative of Acre and Mato Grosso states of Brazil) for drought tolerance
based on cell membrane stability was conducted using RRll 105 as control clone.
The relative injury to cell membrane varied from 30 to 80 per cent with a mean of
53 per cent and the heritability (H2) for this character was very high (87%). Based
on relative injury to cell membrane, the entire accessions were classified and 10
accessions (AC 1044, MT 55, AC 446, MT 41, MT 76, MT 66, MT 938, AC 650,
AC 652, AC 728) representative of all classes were selected for detailed study
under Experiment n along with the control clones RRll 105 (a popular high
yielding clone), RRIM 600 (a drought tolerant clone) and Tjirl (a drought
susceptible clone).
Experiment n was conducted under field conditions to assess the
genetic variability among the 10 selected accessions for drought tolerance using
various indices. The polybag plants were grown under induced water stress for 15
days, 30 days and 45 days. Drought related physiological parameters viz., leaf
temperature, stomatal conductance, transpiration rate, afternoon leaf and soil .
water potentials and chlorophyll fluorescence parameters were studied along with
the plants under non-stress. Under water stress condition, leaf temperature was
increased and stomatal conductance, transpiration rate and afternoon leaf and soil

water potentials were decreased. Components of chlorophyll fluoresence viz.,
initial fluorescence (Fo) showed a substantial increase under water stress and all
other components - maximal fluorescence (Fro>, variable fluorescence (Fe), FjFm
and FJFo showed a remarkable decrease. The selected accessions exhibited
significant genotypic difference for all the physiological parameters studied and
the effect of water stress and non stress on genotypes as well as the interaction
effect between genotype and stress levels were also significant for most of the
characters. Significance of genotypic difference was worked out statistically
following Completely Randomised Design (CRD) and Factorial CRD.
Morphological, biochemical and anatomical (leaf and bark) parameters
related to drought tolerance were recorded from the polybag plants under non
stress conditions, in order to assess the genotypic difference for these parameters.
Morphological characters studied include plant height, basal diameter, number of
flushes, total number of leaves, inter flush distance, single leaflet area and specific
leaf weight. All these characters exhibited significant genotypic difference except
for specific leaf weight and inter flush distance.
Epicuticular wax content, chlorophyll content and chlorophyll
reduction percentage were studied under biochemical parameters. Here also there
was significant genotypic difference for all these characters.
Leaf anatomical characters studied include stomatal density on the
lower surface, leaf thickness, midrib diameter, mesophyll tissue thickness,
palisade tissue thickness and palisade cell number per unit length of palisade
tissue. All these exhibited significant genotypic difference.
Total bark thickness; proportion of soft bast, total number of latex
vessel rows (L VR) and proportion of L VR in the soft bast were the bark structural
characters studied. There was no significant genotypic difference for these bark

structural characters which might probably due to the juvenile stage of the plants
selected.
A glass house study was conducted as Experiment ill in order to avoid
the influence of untimely rains occurred during the recording period.
Observations on growth of basal diameter and dry matter production under
stressed and non stressed conditions were studied. Under water stress, the basal
diameter of most of the genotypes was reduced and the rate of change was
significant. On the basis of dry matter produced, dry matter stress tolerance index
(OMSI) was worked out which showed significant genotypic difference. The
highest OMSI was noticed in the wild accession AC 652.
Genetic parameters viz., phenotypic and genotypic coefficients of
variability (PCV and GCV)~ broad sense heritability (H2) and genetic advance
(GA) as percentage of mean were worked out for the selected characters. As
expected most of the morphological characters had high pev and the other
parameters had low to medium PCV indicating less influence of environment for
the expression of these characters. Among the characters studied, physiological
parameters viz., stomatal conductance and transpiration rate exhibited fairly good
GCV. Broad sense heritability (H2) indicated medium to high values for all the
characters considered. Genetic advance (GA) was low to medium for almost all
the characters studied, except for LVR where the GA was high (58%).
Character association studied helped to understand the relationship of
various characters with each other. Genetic divergence existing in the population
of wild genotypes was assessed in terms of "generalised group distance" using
Mahalanobis 02 analysis. Two separate analyses were done (a) for morphological
and structural characters and (b) for physiological and biochemical characters. In
both, the genotypes were grouped into 6 clusters each indicating the wide genetic
distance among the genotypes.

A ranking of these genotypes was done for a better understanding of
the worth of individual genotype, as they differed from each other for different
drought related parameters. Some of the highly related parameters with drought
tolerance were selected and based on their relationship with drought tolerance,
genotypes were ranked for each selected character. Comparing the rank sum
obtained for each genotype, the superiority of the accessions was assessed. Based
on this, the best genotype identified was MT 41 followed by MT 55 and AC 650.
From this study it is concluded that drought resistance mechanism is a
complex factor involving a number of physiological, morphological, biochemical
and anatomical parameters with unknown inheritance. An understanding of these
characters, which are more directly related to drought tolerance helps in easy
identification of genetic materials which can be used for extensive study in this
area. From the present study it was found that, the parameters such as stomatal
conductance under water stress, FJFm ratio of chlorophyll fluorescence, growth of
,
basal diameter under water stress, chlorophyll and epicuticular wax contents,
thickness of palisade tissue and palisade cell number per unit length of palisade
tissue are some of the reliable parameters for identifying genotypes having
drought tolerance. Extensive studies using these parameters are further required
in this area, to identify a simple and fast screening technique for the large number
of Hevea germplasm.