Ph.D Thesis

Ph.D StudentAsli Sare
SubjectEffects of Colloids in the Root Environment on
Apoplastic Water Transport and Plant Growth
DepartmentDepartment of Civil and Environmental Engineering
Supervisor PROFESSOR EMERITUS Peter Neumann
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Colloids from recycled sewage effluents and natural colloids accumulate and are transported in soil solutions. The first interaction between soil solution colloids and transpiring plants is via the root cell walls. These walls consist of polymer fiber networks penetrated by pores found here to have mean diameters of 6.6 nm.  Root cell walls may act as filters which reduce the passage of soil colloids with diameters above 6.6 nm. Plant cell walls may be clogged by the build up of filtered colloids.

The research aims were 1.) To characterize hydraulic and physiological effects of selected individual colloids and the mixed colloids in domestic effluents on root hydraulic conductivity 2.) To assay associated effects on plant transpiration growth and ability to withstand water deficits.

Water flow through excised primary roots, live or dead, was reduced by 20-40%, together with root hydraulic conductivity, within 70 min of exposure to inorganic or organic colloidal suspensions at 1 g L-1. Root cell-wall pore diameter was reduced by 50%. Root hydraulic conductivity was also reduced by exposure to primary or secondary sewage effluents before and after dialysis to remove low molecular weight solutes. All of the colloids tested were shown to reduce the ability of soil grown plants to withstand water deficits. Colloids reduced leaf growth and transpiration (20% reduction) but did not reduce root growth. However, the growth of   roots was specifically inhibited by effluent treatments.

            The reduction of root hydraulic conductivity by colloids and associated whole plant responses appeared to involve physical reduction of flow through nano-sized root cell wall pores since: - 1) flow inhibitory effects of external colloids on root hydraulic conductivity were only partially reversible. 2) Colloid treatments reduced water flow through killed roots left after hot alcohol disruption of the cell membranes. 3.) Average particle exclusion diameter of root cell wall pores was reduced from 6.6 nm to 3.0 nm by colloid treatments.  However, the growth-inhibitory effects of effluents on plant roots appeared to be related to effects of colloids and other as yet unidentified inhibitory solutes.

In summary this research reveals novel physical effects of colloids on plant water transport and directs attention to overlooked environmental implications of increasing agricultural use of sewage effluents for crop irrigation.