The life cycle of the potato (Solanum tuberosum L.) : from crop physiology to genetics

This thesis describes the results of an analysis of the major developmental events taking place during the life cycle of the potato, based on records from 250 genotypes, from the highly diverse CxE population. The aim was to generate a massive amount of data to build a framework of crop physiological relationships, and use the data to begin with the identification of genetic factors controlling major developmental processes like senescence. Physiological ageing of seed tubers during storage influences tuber initiation and plant development. Since the majority of the CxE population had short or no tuber dormancy, seed tubers sprouted during storage. Physiological ageing is associated with the intensity of sprouting during storage and with tuber formation after planting. Therefore, it was necessary to investigate the association between sprout length, as an indicator of physiological ageing, and the timing of occurrence of events related to tuber formation and the duration of the plant cycle. Among genotypes of the CxE population, sprout length at the end of the storage period was not strongly related to tuber initiation or other events related to tuber formation. Associations were not found between sprout length and the duration of the plant cycle, plant size and several other variables recorded. We conclude that the variation in sprouting of seed tubers was not the main factor responsible for the large variation observed in the CxE population in the timing of occurrence of tuber formation and plant developmental events. Despite the large variation found in the CxE population and control varieties in the duration of the plant cycle, all began to form stolons and to flower within a remarkably short period of time. Results strongly suggest that the onset of stolon formation and flowering are not necessarily triggered by an environmental stimulus. The peak production of stolons and swelling stolon tips took place during flowering. However, while in most genotypes, stolon formation was confined to the flowering period, stolon tip swelling proceeded beyond the end of flowering and ended during the senescence period. The number and size distribution of tubers was largely influenced by the degree of stolon branching and the length of the stolon swelling period, and apparently to an smaller extent by the resorption of tubers. Tuber initiation did not have a determinant influence on the timing of stolon formation, stolon tip swelling, flowering and the duration of the plant cycle, contradicting extensive literature supporting the determinant influence of tuber initiation on plant development. Differences in the timing of events related to plant development and tuber formation were further analyzed after grouping the CxE population according to the duration of the plant cycle. Genotypes with life cycles shorter than 90 days had a small plant size, a short period of stolon formation, stolon tip swelling and flowering, early tuber initiation, very little or no stolon branching and fast progress of canopy senescence. In contrast, genotypes with life cycles longer than 170 days, had a large plant size, profuse and long flowering period, large production of stolons during most of the plant cycle, profuse stolon branching, late but long period of stolon tip swelling and subsequent tuber initiation and slow progress of canopy senescence. Differences were also found between groups of genotypes in the chronological order at which tuber formation and plant developmental events took place during the plant cycle. Our results show that although in every potato plant there is a period when there is a simultaneous presence of newly formed stolons, swelling stolon tips, tuber incipients and growing tubers, there was no chaos, but a well-orchestrated order influenced by the duration of the plant cycle. The large variation found in the CE genotypes and varieties in the order of occurrence of morphological events related to plant development and tuber formation demonstrates the remarkable plasticity present in potato and the influence of the selection process during the developing of varieties on the timing of events leading to high yields. Very high correlations were found between tuber formation and plant developmental events and the duration of the plant cycle (end of the senescence process). In addition, very large differences were found in the CxE population in the onset, duration, mid point and rate of the senescence process. We decided to investigate the genetic factors underlying these variables, describing the process of senescence in potato. We found between two and four QTLs associated with each one of these senescence variables, located on chromosomes 5, 6, 9 and 12. Each one of these QTLs was found in common to at least two of these senescence variables. The effect of these QTLs on different senescence variables was in the same direction, suggesting pleiotropic effects rather than close linkage interactions between these genetic factors. From the QTL analysis of the progress of senescence (amount of yellowing of the plant canopy) during the life cycle of the CxE population, new QTLs were identified, which were present either at the beginning, at the end or during most of the senescence process. Our results show that the evaluation of variables describing complex traits in a large mapping population, can be a powerful strategy to describe a process at both, physiological and genetic levels.