Salt Resistance and Adaptation to Root-Zone Hypoxia in Sunflower

Abstract

Sunflower plants (Helianthus annuus L.) were grown in temperature controlled (20-21°C) nutrient solutions under greenhouse conditions for up to 50 days. Root-zone oxygen status was varied by either continuous aeration or periodic gassing with nitrogen to simulate root response to anaerobiosis under flood irrigation. Salt treatments (up to 200 mM final concentration in culture solution) were imposed either during sustained aeration v. hypoxia, or else initiated at the time of reciprocal transfer between those two levels of oxygenation. Continuously aerated v. hypoxic cultures served for comparison.

Salt tolerance threshold, and especially Na⁺ ion exclusion, was lowered by hypoxia, while selectivity for K⁺ over Na⁺ ions also diminished. Such plants showed partially reversible decline in laminar expansion with reduction in aeration status and, due to a much reduced root : shoot ratio, were prone to wilting.

Adaptive responses to hypoxic culture solution, viz. minor osmotic adjustment within laminar tissues plus formation of root aerenchyma tissue external to an endodermal layer, appeared to confer improved salt tolerance when salinization coincided with lowered oxygenation. Time lag, between decline in root-zone O₂ status, and onset of plant response, would limit relevance of this adaptation for improved salt resistance in cropping situations to periods of low transpirational demand.