Abstract
In an attempt to increase source capacity, transgenic corn was generated by expressing a truncated maize sucrose phosphate synthase (ZmSPSΔ482) under two leaf mesophyll cellspecific promoters (CAB and PPDK). The endogenous and truncated SPS proteins from transgenic leaf extracts were distinguishable by protein immunoblot analysis. The expression of transgenic SPS protein across events varied from very low to very high and included several cosuppressed events. SPS activity showed a diurnal pattern in both transgenic and wild-type maize leaves. In greenhouse experiments, transgenic maize had higher leaf sucrose and lower leaf starch, suggesting a shift in carbon partitioning from starch to sucrose. Conversely, cosuppressed events had lower leaf sucrose and higher leaf starch. A field test was performed to compare sucrose and starch in positive and negative isolines of hybrid maize CAB and PPDK ZmSPSΔ482 events. In the field, many positive isolines had higher levels of both leaf sucrose and starch than the negative isolines. This suggests that in the field, with higher light intensity the shift in carbon partitioning from starch to sucrose, observed under greenhouse conditions did not occur. This in turn suggests that the environment affects the phenotype of the transgenics and that in the field, there was an overall increase in carbon assimilation. Six events from each construct were tested in a pilot multi-density yield trial but overall, no effect on yield was observed. Therefore, although the transgenic plants had more source capacity, this did not translate into higher seed yield.