The use of agar nutrient solution to simulate lack of convection in waterlogged soils

Abstract

Agar at 0.1% in nutrient solution ('stagnant solution') was used to prevent turbulence (convection), thus simulating the slow gas movements which occur in waterlogged soils. Wheat, aged between 6 and 16 d at the start of the treatment, was used to test plant growth and development in this stagnant solution for 8-15 d. K-MES buffer at 5 mol m-3 was used to retain the pH of the rhizosphere in the stagnant solution at pH 6.5. The prevention of convection reduced dissolved oxygen concentrations in the bulk solution from 0.275 to below 0.05 mol m-3 after 1 d, while ethylene accumulated over 10 d to 6.5 x 10-6 m-3 m-3 (ppm). Aerenchyma of nodal roots grown in stagnant solution comprised 22% of the cross sectional area of the root 50 mm behind the root tip; this was similar to values recorded earlier for nodal roots of wheat in waterlogged soil and contrasts with 7.6% for roots in non-flushed solution without agar (referred to in this paper as 'semi-stagnant solution') and 2.4% in N2-flushed solution. Increases in dry weight and numbers of nodal roots with time were larger for stagnant and N2-flushed, than for semi-stagnant or aerated solution. In contrast, seminal roots did not grow in stagnant solution, while seminal roots in N2-flushed solution grew much less than in semi-stagnant or aerated solution. In the stagnant solution, relatively high concentrations of N, K and P were required to avoid limitations in mineral uptake into the roots, due to the long diffusion pathway from the bulk solution imposed by the lack of convection. Nevertheless, our data show that the slow growth imposed by the lack of convection was due to factors other than low mineral nutrition. The most likely cause was the change in the dissolved gas composition of the root media, particularly of the rhizosphere. In conclusion, in terms of anatomy and morphology the roots grown in the stagnant solution more closely resembled those from waterlogged soil than did those grown in either semi-stagnant or N2-flushed solution.