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Presented as Thesis (Ph. D.)--University of Wisconsin--Madison, 1920. / Title from added collective thesis title page. Reprinted from American journal of botany. Part 1: vol. 10, no. 2 (Feb. 1923), Part 2: vol. 10, no. 3 (Mar. 1923). eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Seeds are very complex and diverse plant organs. Seed germination is the most sensitive stage of plant life and is influenced by various environmental signals including phytohormones, salt, light, temperature and water potential. Seeds have an innate mechanism called dormancy that blocks germination, and plants have developed several dormancy-inducing strategies to optimise the timing of germination. Seed germination vigour is an important factor in crop yield. Seedling vigour is defined as the sum of the seed properties which determine the level of activity and performance during germination and seedling emergence. A poor seed lot can be improved by post-harvest treatment such as hydro-priming as it is used in the seed industry, but the biological mechanism is unknown. The aim of this study is to understand the mechanism(s) of hydro-priming in order to improve seed vigour and seed germination. I set seeds at different temperatures to produce variation in seed vigour and it showed that germination of seeds with low vigour can be improved by hydro-priming. Using LC-IT-ToF/MS I characterised compounds that leach from seeds during hydro-priming, and showed that some of these are putative germination inhibitors. Adding these compounds to the water during hydro-priming showed that the inhibitory effect of these compounds is not the main mechanism that regulates germination. Also, transcriptomic analysis showed that genes involved in OPDA pathway are expressed during hydro-priming as well as during endosperm weakening cap associated genes. I concluded that hydro-priming improves the speed of germination of low temperature set and its efficiency is dependent of activation of metabolic activity.

MSc (Horticulture) / Department of Horticultural Sciences / Grafting is the process of combining two parts of plants to form a single plant. Grafting is a common technique in trees and vine crops and is becoming popular in annual vegetable production in order to control biotic and abiotic stresses, improve fruit yield and quality. The objectives of the study were to determine the (a) compatibility of the tomato scion x rootstocks combinations (b) effect of rootstocks on vegetative growth parameters (c) effect of tomato rootstocks on fruit quality and yield of tomato. The experiments were conducted under a protected environment (in a high tunnel) at the University of Venda, Thohoyandou, South Africa. Four tomato rootstocks (‘Powapak’, ‘Everest’, ‘Matterhorn’ and ‘Golf’) and two scion cultivars (‘Money Maker’ and ‘Rodade’) were used to develop 10 scion x rootstock combinations. The tube grafting method was used. Seedlings were transplanted in the polyethylene bags inside the high tunnel. Growth parameters, including plant height, stem diameter, number of leaves per cluster as well as yield attributes were measured. In addition, the chemical fruit quality parameters including the total soluble solids and pH were determined. A randomized complete block design with four replications was used for the study. Quantitative data sets of the variables were subjected to analysis of variance. There was a high plant survival rate and the number of clusters per plant showed marked improvement in some grafting combinations particularly in ‘Rodade x Everest’ and ‘Money Maker x Everest’. A few individual stunted plants were observed in the tunnel. A significant seasonal effect was observed on some of the growth and productivity attributes. The pH showed a narrow range of values and there was significant interaction between sampling time and genotype. The results demonstrated the viability of producing grafted tomatoes under tunnel conditions. / NRF

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