Assessing genetic variation in growth and development of potato

Key words: Potato (Solanum tuberosum L.), segregating population, canopy cover dynamics, tuber bulking dynamics, beta function, thermal time, components of variance, genotype-by-environment interaction (GE), heritability, QTL mapping, QTL-by-environment (QTLE) interaction, complex traits, ecophysiological crop model, GECROS, cultivar choice, maturity type, ideotype breeding, tuber yield. Due to increasing food demand and changing diets potato (Solanum tuberosum L.) is becoming a subsistence crop in many regions. However, the agronomy and whole crop physiology of tuber yield production is extremely complex, due to genotype and environment specific effects on crop physiological and morphological characteristics. Such intrinsic complexities complicate the manipulation of yield determining traits and make their prediction a challenging task. The genetic improvement of tuber yield can be understood more mechanistically by investigating and interpreting the relationships between the main attributes of crop growth.<br /> This thesis aims to develop an approach to quantify the yield of individual genotypes and to estimate parameters which may reveal the effects of genetic and environmental factors on the important plant processes controlling tuber yield variation among a large set of F1 (SH83-92-488  RH89-039-16) genotypes of potato and a set of standard cultivars covering a wide range of maturity types.<br /> It first presents a model approach to analyse the time course of canopy cover and tuber bulking during the entire crop cycle as a function of thermal time in terms of large number of physiological component traits and explain their inter-relationships and impact on crop maturity and tuber yield production across six contrasting field experiments. The results indicated that the length of the canopy build-up phase (DP1) was conservative with respect to genotype&rsquo;s maturity type, but the duration of maximum canopy cover (DP2) and the decline phase (DP3) varied greatly, with later genotypes having longer DP2 and DP3 and thus a higher area under whole green canopy curve (Asum). Values of tuber bulking rate (cm) were highest for early maturing genotypes followed by mid-late and then late genotypes. Late maturing genotypes had longest effective duration of tuber bulking (ED) followed by mid-late and early genotypes. As a result tuber yield (wmax) was higher in late genotypes than in early genotypes. The radiation use efficiency (RUE) values were highest for early maturing genotypes followed by mid-late and late genotypes whereas nitrogen (N) use efficiency (NUE) was highest in late maturing genotypes followed by mid-early and early genotypes.<br /> High genetic variability and high heritability for most of these traits were found. Results indicated that increased tuber yield by indirect selection for optimal combination of important physiological traits can be achieved. While using these traits as a criterion for selection, the causal physiological relationships and trade-offs must be considered simultaneously.<br /> Our molecular dissection of traits determining the dynamics of canopy cover, tuber bulking, and resource (radiation, nitrogen) use efficiencies identified several QTLs, the mapping position of each identified QTL, the interaction of QTL with environment (QTLE) and the magnitude of the QTL effect in explaining genetic variance in both SH and RH parental genomes. The QTL results indicated that one particular chromosomal position at 18.2 cM on paternal (RH) linkage group V was tightly linked to the genotype&rsquo;s earliness and controlling nearly all the traits and explaining the phenotypic variance by up to 79%. This suggested the pleiotropic nature of the QTL for most of the traits determining crop maturity and tuber yields. A number of QTLs for traits were not detected when tuber yield per se was subjected to QTL analysis. The phenotypic variance explained by the QTLs for tuber yield per se was also lower than for other traits.<br /> The physiological and quantitative knowledge gained was used to evaluate the conventional system of maturity type and to quantify and re-define the concept of maturity type on a physiological basis for a large set of genotypes. Four new physiological based maturity criteria were developed based on four canopy cover and tuber bulking traits. Physiological maturity type criteria tended to define maturity classes less ambiguously and were easily and clearly interpretable compared to the conventionally used method of defining maturity.<br /> The capability of an ecophysiological model &lsquo;GECROS&rsquo; was tested to analyse differences in tuber yield of potato. The model yielded a reasonably good prediction of differences in tuber yield across environments and across genotypes. Model analysis identified the genotypic key-parameters affecting tuber yield production and Nmax (i.e. total crop N uptake) contributed most to the determination of tuber yield. The results concluded that genotypes with higher Nmax and lower tuber N content exhibited higher tuber dry matter yield. Further analysis of the genotypic parameters should be performed in conjunction with molecular markers in order to determine their genetic control and to proceed towards QTL-based crop modelling approach.<br /> This thesis identified the dominant component traits mostly involved in the formation of a tuber yield and gave insight into the possibilities of genetically and physiologically manipulating the size or number of such traits. The information obtained should help in marker-assisted selection as well as in designing ideotypes for specific and/or diverse environments. However, to make significant contributions for breeding, there is a need for further research efforts to evaluate the combined physiological and genetic approach. &nbsp