Calcium deficiency and auxin transport in Cucurbita pepo L. seedlings.

Hypocotyls of Cucurbita pepo L. (zucchini) seedlings grown at low calcium levels (0.1 mM) showed reduced basipetal polar transport of the auxins indole-3-acetic acid and 1-naphthylacetic acid in comparison with control plants grown at 5 mM Ca(2+). The contribution to overall transmembrane auxin transport of the symport uptake carrier was unchanged by calcium deficiency, but the efflux carrier of the auxin-anion uniport was less active and the sensitivity of auxin transport to N-1-naphthylphthalamic acid (NPA) inhibition was thereby reduced. These changes could partially be reversed by short treatments with Ca(2+) or La(3+). The NPA receptor was unaltered in level and affinity for NPA by calcium deficiency. It is suggested that the major lesion responsible for diminished polar auxin transport in calcium-deficient tissue is in flux through the auxin efflux carrier which could be subject to control by Ca(2+) by as yet unestablished mechanisms.