Within potato crops some tubers are too small or too large to be marketable. There are economic and environmental incentives to improve uniformity, thereby increasing the fraction of yield that is marketable. The potato crop has a complex, three-tiered population structure consisting of plants, stems and tubers, each of which is a branch of the previous tier. Plant-to-plant variation in the number of stems, number of tubers and yield has long been noted to occur but the causes of this variability and how it impacts the uniformity of the crop are uncertain.

This thesis investigated how variability in seed tuber weight, within-row spacing and date of emergence affected the growth of individual plants in field-grown crops and how this affected the uniformity of the crops. Stem-to-stem variation in yield was described for the first time and shown to be considerably more variable than plant-to-plant variation. The causes of this stem-to-stem variation in yield and its influence on tuber size were investigated through detailed surveying of Desiree, King Edward, Maris Piper and Russet Burbank. Commercial crops grown from different seed tuber sizes and on contrasting soil types were surveyed to establish whether plant-to-plant variation was similar to that encountered in field experiments.

Within crops, plants with a higher average yield per stem were found to produce larger tubers and mean tuber size varied by c. 20 mm between plants. The number of tubers increased as the average yield per stem increased, but was insufficient to counter the influence of increased yield per stem on tuber size. These findings demonstrate that the same principles apply for explaining variation in mean tuber size between plants and between crops.

Although the average seed tuber weight correlates linearly with the average number of stems produced, for individual plants, seed tuber weight accounted for little of the variation in the number of stems per plant. In 2013 reducing variation in seed tuber weight did not reduce variation in stem number per plant nor yield per plant. In 2014 variation in the number of stems and yield per plant were decreased by reducing variation in seed tuber weight, but this did not affect uniformity. In Maris Piper, high levels of variability in within-row plant spacing did not affect uniformity in 2013, whilst in 2014 it caused a small but significant decrease of uniformity. In a separate experiment, variability in within-row plant spacing had a larger negative effect on uniformity in Markies and Marfona and possible reasons for the different responses are discussed. Variation in emergence was affected by differences in sprout development at planting, but only relatively small differences were created between treatments that had no detectable effect on uniformity.

In all four varieties where stem-to-stem variation was surveyed, there was a strong correlation between yield per stem and mean tuber weight. The number of tubers per stem was highly variable and did not increase with yield sufficiently to compensate for the effect of yield on mean tuber weight. The proportion of plant yield produced by each stem and the allocation of yield to tubers on the same stem were highly variable. Reducing variation in seed tuber weight reduced variation in plant yield, and during the initial phases of development plants growing from larger seed tubers grew at a faster rate. The implication for stems is that variation in the supply of seed tuber substrate is a substantial cause of stem-to-stem variation in yield.

Models were developed that provide a framework for analysing the contribution of variation at different scales to the overall tuber size distribution. The underlying mechanism governing uniformity is that decreasing variation in mean tuber weight between plants or stems improves uniformity. Differences in plant-to-plant variation do not have any effect on uniformity if both variation in yield and number of tubers change by similar amounts. Models predicted that reducing variation in the yield per stem would improve uniformity, particularly in varieties where there is only a weak relationship between yield and number of tubers. The implications of these findings are discussed in relation to growers improving uniformity of present varieties and for breeders developing new varieties.