However, this target has been difficult to achieve
due to the existence of a negative correlation
between grain yield and GPC (Lawlor, 2002; Triboi
and Triboi-Blondel, 2002). This negative correlation
occurs because grain yield is mainly a consequence
of starch accumulation, while protein accounts for
less than 10–15% of the grain dry weight, and any
increase in starch accumulation causes a dilution of
the protein content if it is not accompanied by an
equivalent increase in N accumulation. Consequently,
any environmental factor that affects grain
yield also affects GPC. Due to the existence of this
important interaction between ambient conditions
and final GPC, it is accepted that more than half of
the final GPC is determined by the environment,
and only a smaller proportion is determined by
genetic factors (