A study of incompatibility within the genus Zantedeschia : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science at Massey University, Palmerston North, New Zealand

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Zantedeschia includes many important varieties for New Zealand cut flower and pot plant production. The genus is divided into two sections. The first section consists of a single species Z.aethio pica, the common white calla, and a few selections such as the dwarf 'Childsiana'. The second section includes the remaining five species whose spathes range in colour from ivory, yellow and pink to maroon. Interspecific hybridization following hand pollination readily occurs between species of Section II and all of the coloured Zantedeschia cultivars have been derived from this section. Z.aethio pica, however, has never been successfully crossed with any of the species from Section II. Z.aethio pica has a different flowering period, is more floriferous and more tolerant of Erwinia (a soft rot of tubers) than the other species. The ability to produce hybrids from the two sections would greatly increase the potential for improved cultivars. This project was initiated to explore interspecific incompatibility by examining pollen physiology, embryogenesis of compatible and incompatible crosses, and rescue of embryos from incompatible crosses using embryo culture techniques. Zaethio pica 'Childsiana' pollen was similar in morphology to pollen from the coloured species of Section II. Pollen from all species examined were oval in shape with a light-yellow, waxy pollen coat. The exine was smooth with no aperture, while the cytoplasm was full of starch grains. The pollen of Zantedeschia is trinucleate. In contrast to other trinucleate pollen lower concentrations of sucrose (5-10%) were preferable for pollen germination and high concentration of sucrose (30%) sharply reduced germination. Pollen germination of Zantedeschia was unaffected by agar concentration and pollen even germinated well in liquid media. The in vitro medium containing sucrose (100 gl−¹) , boric acid (50 mgl−¹) , calcium chloride (100 mgl−¹) and agar (10 gl−¹) with pH 6 was optimal and was selected for further pollen germination studies. The mature megagametophyte of Zantedeschia was a typical seven-celled embryo sac, with a three celled egg apparatus, three antipodal cells and a secondary nucleus formed by the two polar nuclei. The secondary nucleus was present at the chalazal end of the embryo sac before fertilization. Ovules were anatropus i.e. inverted, and had an inner and outer-integument. The apical cells of the nucellar epidermis divided periclinally to form a nucellar cap (or epistase) 4 - 5 cells in thickness. Control ovaries of unpollinated (Section II) Zantedeschia hybrids 'Best Gold' and 'Chromatella' grew for 2 weeks and then started to shrink and turn yellow. These ovaries eventually died 8 to 10 weeks after anthesis. Incompatible crosses had far less ovule development compared with compatible crosses. In incompatible crosses ovary size was much smaller than compatible crosses from four weeks after pollination. The embryos of compatible crosses had a steady growth rate up to 12 weeks. The embryo was globular at 4 weeks but cotyledon, radicle and a leaf primordium were differentiated by 6 weeks. In the incompatible crosses globular embryos were also found at 4 weeks, but their growth rate was much slower thereafter. Most of the incompatible embryos remained at a globular stage of development. A small percentage of incompatible embryos continued development but the endosperm surrounding these embryos often became necrotic. By 8 weeks after pollination 1-2% of embryos from the Z.'Chromatella' X Z.aethio pica 'Childsiana' were larger than 0.7 mm with distinct cotyledon development. Compatible embryos at this time were about 2 mm long. The endosperm in compatible crosses did not contain starch during the first two weeks after pollination, but small starch grains were seen in the outer endosperm from week 4 and starch levels increased throughout the endosperm to week 12. In the incompatible crosses starch levels were always lower and starch grains were confined to the outer region of the endosperm; the central region was poorly developed. In the incompatible crosses necrosis of the endosperm and embryo was clearly seen in most ovules from week 6. Even in embryo sacs with well developed embryos, the endosperm had shrunk and the seeds dried by week 8. The arrest of growth of embryos from incompatible crosses appeared to be related to degeneration of the endosperm and failure of nutrient metabolism and transport. Young seeds were removed from ovaries at various stages (2 to 12 weeks after pollination) and cultured in vitro. The embryos from compatible crosses taken at 6 weeks or later grew well in vitro on Murashige and Skoog (1962) medium with sucrose (30 gl−¹). At this concentration of sucrose most embryos produced shoots, roots and cotyledonary haustorium tissue within one month of culture. High concentrations of sucrose (90 gl−¹) delayed growth of most of the cultured embryos. Most embryos from incompatible crosses (dissected 6 to 10 weeks after pollination) were small (0.1−0.2 mm) and remained globular in culture. Higher concentrations of sucrose (60−90 gl−¹) did not improve embryo growth irrespective of the age of the embryos. Some incompatible embryos were larger (0.7 mm) when dissected, and in culture these eventually produced shoots but all tissues were albino.
Calla lilies Pollen, Calla lilies Development, Calla lilies Varieties, Physiology