Keywords: cell division, endoreduplication, cell expansion, cyclin, cyclin dependent kinase, growth theory, systems biology. Okello, R.C.O (2015) Multi-level analysis of the impact of temperature and light on tomato fruit growth. PhD thesis, Wageningen University, Wageningen, The Netherlands, 166 pp, with English and Dutch summaries. Tomato fruit growth commences with an increase in cell number followed by cell expansion. Upon cessation of cell division, a spectacular increase in nuclear DNA content without cell division (endoreduplication) occurs, which is positively correlated with cell size. Evaluation of the relative importance of each cellular process during fruit growth is important for attempts aimed at manipulating fruit size. In this thesis, the genetic and physiological basis for the differences in fruit size between cultivars and their response to fruit temperature was studied. In addition, the effects of darkness, white, blue, and red light around the fruits on tomato fruit growth were investigated. Temperature or light treatments were applied at the fruit level in all experiments in order to separate plant and fruit level responses. Fruit phenotype was assessed at whole fruit, cell and gene level. Expression patterns of 20 different genes encoding regulators of cell division, endoreduplication or cell expansion were analysed. Besides the experimental work, a literature review of the role of light in the regulatory networks of cell division, endoreduplication and cell expansion was conducted. Results from experiments were then placed into context of other studies in order to identify processes that drive fruit growth. Experiments showed that differences in fruit size between cultivars can result from differences in both cell number and cell size. Increased cell number in the larger fruited cultivar was corroborated by an increase in the expression of three cell division promoters (CDKB2, CycA1 and E2Fe) and a decrease in the expression of an inhibitor (fw2.2) of cell division. The observed smaller fruit size in heated compared with non-heated fruits appeared to stem from a reduction in cell size even when cell number tended to increase. The expression of three promoters (CDKB1, CDKB2, and CycA1) and one inhibitor (fw2.2) of cell division increased when fruits were heated. However, the expression of genes encoding proteins known to regulate endoreduplication and cell expansion did not corroborate observations on cell size in the temperature experiment. Fruits subjected to different light treatments did not differ in either fruit size or carbohydrate content. However, cell division was strongly stimulated at the expense of cell expansion by light. This thesis shows that cell division is stimulated by light irrespective of the organ under consideration while endoreduplication and cell expansion responses are organ specific. It is proposed that light effects on cell division, endoreduplication and cell expansion stem from either degradation of transcription factors or inhibitory competition between transcription factors for promoter regions of target genes. It is also argued here that the commonly observed positive correlation between cell number and fruit size does not imply a causal relationship. In addition, the thesis argues that fruit growth is dependent on cell-autonomous and non-cell-autonomous regulatory mechanisms as well as a global coordinator, the target-of-rapamycin and, consequently, the increase in fruit size follows the neo-cellular theory of fruit growth. This thesis provides clues on the link between gene expression and cell and fruit level observations. It also provides in depth knowledge on the role of environmental factors on the regulation of cell division, endoreduplication and cell expansion. Further studies at the level between genes and the cells will be necessary to quantify the relationship between gene expression and cell and fruit phenotype.