Chimeras in Horticulture: Apical Origin, Ontogeny, and Propagation, Study notes of Gardening and Horticulture

Download Chimeras in Horticulture: Apical Origin, Ontogeny, and Propagation and more Study notes Gardening and Horticulture in PDF only on Docsity! CHIMERA Apical Origin, Ontogeny and Consideration in Propagation 1 CHIMERA (ki' mer a) Chimera - a plant or plant part composed of genetically different layers. The most common example is a "variegated" plant where different regions of the leaf are yellow or white due to the lack of chlorophyll synthesis, i.e. these are chlorophyll mutants. However, there are many kinds of chimeras. Thornless blackberries are chimeras where the L-I epidermis lacks the ability to produce thorns. Some fruits have sweet and sour regions of flesh, which may be a chimera. Tunica-Corpus Model of Apical Organization Stable chimeras usually occur in species that exhibit a tunica-corpus type of apical organization, which is common in many dicot and monocot species. The outer layer of the tunica gives rise to the L-I layer, the second layer of the tunica gives rise to the L-II layer, and the corpus gives rise to the L-III layer. Chochicine-Induced Tetraploid Chimeras Chochicine is a chemical that allows doubling of the chromosomes, but not cytokinesis; thus, tetraploid cells are produced. If chochicine is applied to the apical dome, tetraploid cells can be induced to form. If only one layer of the apical meristem is receptive to the chochicine, then a distinguishable chimera is produced. (Fig. 8a from Dermen 1960) Reed HORT 604 4 Portions of Stems Derived from L-I, L-II AND L-III Layers of the Apical Meristem Cross-section of two peach stems. The epidermis is L-I. The wavy line denotes the border between L-II and L-III. L-III is differentiated by its larger chochicine-induced tetraploid cells. L-I: encompasses the epidermis L-II: usually encompasses most of the cortex, a variable portion of the vascular tissue, and some of the pith. L-III: usually encompasses most of the pith, a variable portion of the vascular tissue, and some of the cortex. 2-2-4 Chimera - Chochicine-Induced Tetraploid L-III (Fig. 5 from Dermen 1960) Reed HORT 604 5 Portions of Flower Derived from L-I, L-II AND L-III Layers of the Apical Meristem Longitudinal section through an apple flower just before opening. The epidermis is L-I. The wavy line denotes the border between L-II and L-III. L-III is differentiated by its larger chochicine-induced tetraploid cells. L-I: comprises the epidermis L-II: comprises the mesophyll of the petals, and the non-epidermal portions of the stamens, and most of the pistil and ovary tissue; i.e. the reproductive portions. L-III: comprises the center of the peduncle and receptacle. 2-2-4 Chimera -Chochicine-Induced Tetraploid L-III (Fig. 11 from Dermen 1960) Reed HORT 604 6 Ontogeny of Chimeras Each layer of the tunica corpus gives rise to specific anatomical regions of the plant body. There are key differences between dicots, monocots and gymnosperms. Dicots and monocots - developmentally they are similar, with the most pronounced differences expressed in the leaves as detailed below. Gymnosperms – most gymnosperms do not follow a strict tunica-corpus organization; rather have a mantle-core arrangement. Anticlinal cell divisions are less strictly followed in the mantle. Thus, typically, gymnosperm chimeras are less stable. Layer Region of Tunica-Corpus Gives Rise To L-I first (outer) cell layer of tunica epidermis - of all organs dicot leaf - L-I usually gives rise to only the epidermis. Epidermal cells usually are colorless (i.e. lack chlorophyll), thus an L-I mutation in dicots usually cannot be seen. In some species, L-I may give rise to small islands of tissue along the margin. You can verify a L-I chlorophyll chimera by looking for chloroplast in the guard cells monocot leaf - L-I contributes to the outermost region of the leaf mesophyll giving rise to a strip along the leaf margin. gymnosperm leaf – L-I and L-II may have anticlinal and periclinal cell divisions, thus L-I L-II contribute to a variable amount of the epidermis and subepidermal tissue. For this reason, chimeras of gymnosperms typically are not very stable. flower, fruit and seed – L-I gives rise to the epidermis. L-II second cell layer of tunica stem and root - outer and inner cortex and some of vascular cylinder. leaf - mesophyll in outer region or perimeter of leaf. flower, fruit and seed – L-II gives rise to the mesophyll of petals and sepals, and the internal tissue of the stamens and pistil and resultant pollen and seeds. L-III corpus stem and root - inner cortex, vascular cylinder and pith leaf - mesophyll in central region of leaf. flower, fruit and seed – L-III typically does not contribute to any part of the flower, fruit or seeds. Reed HORT 604