Effect of Surface Roots Drying on Water Relations, Gas Exchange, Nodulation, Nitrogen Fixation and Growth of Faba Bean (Vicia faba L.)

SOIL columns in which the root system was divided into two equal layers, each 24 cm in diameter and 33 cm high were used to examine the influence of drying different proportions of the root system on ihc water relations, gas exchange, nodulation. nitrogen fixation and growth of faba bean (Vicia faba L.). The treatments were: (i) Adequate water in both soil layers (Well-watered) and (ii) Water withhelded from the upper layer only (limited water). The two watering treatments were imposed from 61 to 81 days after sowing (DAS) when the plants were flowering. Drying the soil surface reduced the photosynthesis rate over the measurement period by 15% as a result of the reduction in stomatal conductance from 568 to 44S mmol/m/s. Plant height, branches and leaf number, shoot biomass, were not affected significantly by drying the topsoil (0-30 cm) profile, while the length and width of leaflets were reduced. Total root biomass and root length density were reduced by 23 and 37%, respectively, when (lie topsoil was dry, while the proportion of root- lo—total biomass in the top and bottom layer of the soil was not affected by the watering treatments. Water restriction drastically affected nodulation. and N2 fixation of faba bean plants. The reduction in nodulation weight and number, nitrogen derived from atmosphere, net photosynthetic rate, stomatal conductance and leaf water potential when the top 30 cm of the soil in the pots was dried did not result in significant reduction in %N and dry weight of faba bean plant