Planta, 216 (3), 366-376, 2003.
Abstract
Modern corn (Zea mays L.) varieties have been selected for their ability
to maintain productivity in dense plantings. We have tested the possibility
that the physiological consequence of the selection of the modern hybrid, 3394,
for increased crop yield includes changes in responsiveness to auxin and light.
Etiolated seedlings in the modern line are shorter than in an older hybrid,
307, since they produce shorter coleoptile, mesocotyl, and leaves (blade as
well as sheath). Etiolated 3394 seedlings, as well as isolated mesocotyl and
sheath segments, were less responsive to auxin and an inhibitor of polar auxin
transport, N-1-naphthylphthalamic acid (NPA). Reduced response of 3394 to auxin
was associated with less reduction of elongation growth by light (white, red,
far-red, blue) than in 307, whereas the activity of polar auxin transport (PAT)
and its reduction by red or far-red light was similar in both genotypes. NPA
reduced PAT in etiolated 3394 seedlings much less than in 307. A characteristic
feature of 3394 plants is more erect leaves. In both hybrids, light (white,
red, blue) increases leaf declination from the vertical, whereas NPA reduces
leaf declination in 307, but not in 3394. Our results support findings that
auxin and PAT are involved in elongation growth of corn seedlings, and we show
that light interacts with auxin or PAT in regulation of leaf declination. We
hypothesize that, relative to 307, more erect leaves in the modern hybrid may
be primarily a consequence of a reduced amount of auxin receptor(s) and reduced
responsiveness to light in etiolated 3394 plants. The more erect leaves in 3394
may contribute to the tolerance of the modern corn hybrid to dense planting.