This thesis is an investigation of net carbon exchange rate (NCER) and assimilate partitioning of lower-ploidy 'Fragaria' species, synthetic octoploids (SO), F1 (SO x cultivar) and first outcross (OC1) (F1 x cultivar) hybrids. Plants were grown in two field trials, a greenhouse trial and a growth chamber trial. NCER was measured at different developmental stages in the field and greenhouse and different day/night temperature regimes in growth chambers. NCERs of the species were related to ecological background and NCERs of hybrids were consistent with predominant parental species. The highest NCERs were observed in the F1 and OC1 hybrids with nine of 13 hybrids having higher NCERs than the mid-parent values suggesting heterosis. Yield of OC1 hybrids increased six-fold from the F1 hybrids and in some cases were comparable to cultivars. NCER of cultivars can be increased through breeding with lower-ploidy species and exploiting heterosis without compromising horticultural characteristics.