As described in the previous paper, transgenic cotton plants over-expressing spinach sucrose phosphate synthase (SPS) had improved seed and fiber weight and improved fiber quality when plants were grown under cool night temperatures. This paper describes mechanisms by which extra SPS activity can cause these effects, with possible contributions at both source (leaf) and sink (fiber) levels. In addition, this paper describes another positive effect on number of open bolls harvested leading to greatly increased seedcotton yield in an irrigated breeding nursery.