Maize grain yield components and source-sink relationship as affected by the delay in sowing date

Delaying maize (Zea mays L.) sowing date can diminish grain yields through reductions in the number, size and activity of growing grains (sink strength) and/or reductions in the assimilate supply (source capacity) to grains during the grain filling period. Whether the source capacity or the sink strength is the limiting factor for grain yield in late sown maize still remains unclear. Understanding source-sink relationships is relevant to optimize crop management practices, to identify critical processes for crop modelling and to develop breeding strategies. The objective of this work was to assess the effect of delays in maize sowing date on grain yield components and on the source-sink relationship during the grain filling period. Three well irrigated and fertilized maize field experiments were conducted at Balcarce, Argentina (37° 45′ S, 58° 18′ W; 130 m a.s.l.) during 2009-10; 2010-11 and 2011-12 cropping seasons. Sowing dates ranged from October to January covering a broad range of the seasonal photo-thermal variation. Grain yield was affected by sowing date and varied from 1680 g m−2 (early sowings) to 203 g m−2 (late sowings). Grain number per unit area was reduced proportionally less than weight per grain as sowing date was delayed. Variations in grain yield were related to the harvest index, and were closely associated with dry matter accumulation during the post-silking period. The variation of source capacity was higher than that of sink strength during the grain filling period and the source/sink ratio decreased from early to late sowing dates. Results indicate that crop growth during the grain filling period was limited by the sink strength in early sowing dates and by the photosynthetic source capacity in the late ones.