PhD

The term 'polyembryony' (PE) encompasses a set of processes whereby
adventive embryos are formed along with zygotic embryo resulting in the emergence
of multiple seedlings from a single seed. Understanding the histological and
histochemical changes underlying the PE, a developmentally fascinating phenomenon,
generally observed in a number of mango cultivars of south west coast of India is
critical for the successful utilization of the same in crop improvement and other
aspects. The project on "Histological and biochemical characterization of
polyembryony in Muvandan and Vellaikolumban mangoes" - two homestead mango
cultivars of Kerala - was hence taken up at the Department of Pomology and
Floriculture, College of Horticulture, Kerala Agricultural University during 2002-2004
for tracing and characterizing the various histological and biochemical changes taking
place during embryogenesis in polyembryonic Muvandan and Vellaikolumban
mangoes vis-a-vis monoembryonic Bangalora; for characterizing the polyembryony
involving isozymes as genetic markers and for studying the effect of different seed
treatments on the expression of polyembryony at the field level and the 'subsequent
seedling growth and vigour.
Histological investigations revealed that the sequence of events in zygotic
embryo formation was similar in both polyembryonic and monoembryonic cultivars.
Fertilization was observed within five days after anthesis in polyembryonic and
monoembryonic cultivars. Endosperm formation began by 15 days after fertilization
(DAF) when the embryos were at near heart shaped stage In Muvandan and
Vellaikolumban. Radicle and plumule initials were visible by 20 DAF when the
endosperrn development was near complete in Muvandan. Adventive embryonic
initial cells (AEICs) appeared in the nucellus by 10 DAF and 15 DAF in
Vellaikolumban and Muvandan, respectively, when the zygotic embryo was at near
heart shaped stage and this could be the turning point as far as adventive
embryogenesis is concerned. Nucellar embryogenesis was found to be invariably
preceded by and closely dependent on zygotic embryogenesis. Such nucellar embryos

were distinct with prominent nucleus and dense cytoplasm. The number of nucellar
initials ranged from one to eight in the polyembryonic cultivars.
Histochemical localization of total insoluble polysaccharides showed that
the content increased with the advancement of embryogenesis in both polyembryonic
and monoembryonic cultivars up to 25 DAF. The content of RNA showed an
increasing trend up to 25 DAF in polyembryonic cultivars during embryogenesis. The
protein content also showed an increasing trend up to 15 DAF in both the groups and
thereafter it declined.
The phenotype of isozyme banding pattern of first sprout was different
from that of the mother plant in Muvandan and Vellaikolumban whereas that of
subsequent sprouts were identical with the mother plant.
The seed stones subjected to different mechanical and hormonal treatments
had an overall significant promotive effect on the rate (number of sprouts/seed) of
polyembryony and the vigour of subsequent sprouts.