THE USE OF AGAR NUTRIENT SOLUTION TO SIMULATE LACK OF CONVECTION IN WATERLOGGED SOILS

Agar at 0.1% in nutrient solution ('stagnant solution') was used to pr event turbulence (convection), thus simulating the slow gas movements which occur in waterlogged soils. Wheat, aged between 6 and 16 d at th e start of the treatment, was used to test plant growth and developmen t in this stagnant solution for 8-15 d. K-MES buffer at 5 mol m(-3) wa s used to retain the pH of the rhizosphere in the stagnant solution at pH 6.5. The prevention of convection reduced dissolved oxygen concent rations 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 2 2% 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 i n waterlogged soil and contrasts with 7.6% for roots in non-flushed so lution without agar (referred to in this paper as 'semi-stagnant solut ion') and 2.4% in N-2-flushed solution. Increases in dry weight and nu mbers of nodal roots with time were larger for stagnant and N-2-flushe d, than for semi-stagnant or aerated solution. In contrast, seminal ro ots did not grow in stagnant solution, while seminal roots in N, flush ed 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 th e lack of convection. Nevertheless, our data show that the slow growth imposed by the lack of convection was due to factors other than low m ineral nutrition. The most likely cause was the change in the dissolve d gas composition of the root media, particularly of the rhizosphere. In conclusion, in terms of anatomy and morphology the roots grown in t he stagnant solution more closely resembled those from waterlogged soi l than did those grown in either semi-stagnant or N-2-flushed solution . (C) 1997 Annals of Botany Company.