APOMIXIS
The embryo is generally produced by sexual reproduction but there are certain cases in which the embryo is produced by an asexual process. This is of great value as the resulting plant can be reproduced by seed propagation in almost the same manner as it would be by any other vegetative method. The seedlings produced through apomixis are known as apomictic seedlings. Apomictic seedlings are identical to their mother plants and similar to the plants raised through other vegetative means, as it has the same genetic make-up as that of the mother plant. Hence, propagation by means of apomictic seedlings is equivalent to vegetative propagation.
The phenomenon in which an asexual reproductive process occurs in place of the normal sexual reproductive process of reduction division and fertilization is known as apomixis.
Kinds of apomixis:
a. Obligate apomixis:
Plants that produce only apomictic embryos are known as obligate apomicts.
b. Facultative apomixis:
Plants that produce both apomictic and sexual seedlings are called facultative apomicts.
Types of apomixis:
Recurrent apomixis:
In this the embryo develops from the diploid egg cell (diploid parthenogenesis) or from some other diploid cells of the embryo sac, with out fertilization (diploid apogamy). As a result, the egg has the normal diploid number of chromosomes, as in the mother plant. Eg, Onion, raspberry, Apple etc. In some plants apomixis occurs without the stimulus of pollination, in others pollination is necessary for embryo development.
Non-recurrent apomixis:
In this type, the embryo develops directly, either from the haploid egg cell (haploid parthenogenesis) or some other haploid cells of the embryo sac (haploid apogamy). In this case haploid plants are always produced. As the plants produced by this method contain only one set of chromosomes, these are sterile and the process is not continued for more than one generation. Non-recurrent apomixis does not commonly occur and is primarily of genetic interest. Eg. Solanum nigrum, Lilium spp.,etc.
Adventitious apomixis (Adventitious embryony or nucellar embryony):
In this type of apomixis the embryo does not develop from the cells of the embryo sac, but develops from any diploid sporophytic cell, eg.,cells of the nucellus (usually), integument etc. Hence, the diploid cells of the sporophyte giverise directly to diploid new embryos. This type of apomixis is found in citrus , where fertilization takes place normally and a sexual plus a number of apomictic (nucellar) embryos develop. In opuntia also this type of apomixis occurs.
Vegetative apomixis (Bulbils):
In this case the flowers in an inflorescence are replaced by bulbils or vegetative buds, which often sprout into new plants while they are still on the mother plant. This type of apomixis is found in some species of Allium, agave, Diascorea, pao etc.
Polyembryony:
This is a type of apomixis.The phenomenon in which two or more embryos present with in a single seed is called polyembryony. When such seeds are sown, more than one seedling arises from the seed .Of them one is from the zygote (Sexual seedling).The others are asexual or apomictic seedlings.
The reasons for this phenomenon are many. The origin of these extra embryos or seedlings varies.
1) From nucellus- Nucellar embryony as in citrus and mango
2) From seed coats (integuments) or antipodals or synergids –rare—mango.
3) Occasionally more than one nucleus develops with in the embryo sac (in addition to the usual and regular nucleus).
4) Cleavage of the embryo during the early stages of development is common occurrence.
What ever may be the place of origin, the common thing is, these embryos arise from the maternal tissue of the plant. Eg.Citrus, Mango, Jamun, Rose, apple etc. The poly embryonic seedlings are uniform and true to parent like other vegetatively propagated plants. They are derived by mitosis, and come from maternal tissue (not by meiosis), but they have the characteristics of sexual seedlings like juvenility, vigour, freedom from virus diseases. Eg. Citrus, Mango, Jamun, Rose apple.
How to differentiate the poly embryonic and sexual seedlings:
It is difficult to differentiate in the nursery. Generally more vigorous seedlings are considered to be polyembryonic. By rejecting about 10% of weaker and weakest seedlings, one can have fairly uniform poly embryonic seedlings.
GRAFT INCOMPATIBILITY
When most closely related plants are grafted together they unite readily and continue their growth as one plant. When entirely unrelated plants are grafted together, the usual result is the failure of the graft union. The inability of parts (stock and scion) of the two different plants when grafted together, to produce a successful graft union and the resulting single plant to develop satisfactorily is termed as graft incompatibility. Ex. Apple on pear, pear on quince and apricot and almond.
Types of incompatibility:Graft incompatibility in fruit trees has been classified in to two types.
1. Translocated incompatibility.
2. Localized incompatibility
1. Translocated incompatibility:
This type involves phloem degeneration and this can be seen by the development of brown line or necrotic area in the bark. This is due to the movement of toxic materials through the phloem. This cannot be overcome even by inserting an intermediate stock.
2. Localized incompatibility:
Incompatibility reactions are seen at the graft joint. This incompatibility can be overcome by inserting a mutually compatible interstock in between them by avoiding direct contact between stock and scion. This may be due to repulsive action of stock and scion.