Influence of nutrient solution concentration and a root pathogen (Pythium aphanidermatum) on tomato root growth and morphology

Young roots, especially those in the root hair zone, are most important for nutrient uptake. However, these roots are also sensitive to high nutrient solution concentration and root pathogens, major problems in soilless culture systems. Supply of nutrient solution at a constant electrical conductivity (EC) can increase the EC in the root environment up to 10 dS m(-1). Pythiaceae are among the most important root pathogens in soilless systems. Therefore, investigations were carried out to quantify and compare root damage by high nutrient solution EC and by Pythium aphanidermatum (Edson) Fitzp. Three tomato plants at two-leaf-stage (Lycopersicon esculentum [Mill] L. cv. Counter) were transferred to a 21 container filled with aerated nutrient solution. They grew in climate chambers for 14 days (16 h light 600 mumol m(-2) s(-1), 30/25 degreesC day/night). In one experiment, a basic nutrient solution of 1.5 dS m(-1) was supplemented with either NaCl or macronutrients to give an EC of 1.5, 5 or 9 dS m-1. In another experiment, plants were inoculated with P. aphanidermatum (0, 1 02, or 10(4) oospores ml(-1) solution). All treatments were replicated five and four times, respectively. On days 3, 7, 10, and 13 after treatment one plant was sampled from each container. Fresh and dry mass of shoots and roots, total root length, number of adventitious roots and of all tap root laterals decreased with increasing nutrient solution EC. Dry matter content of roots and tap root diameter were not influenced while shoot dry matter content increased with increasing EC. NaCl enhanced root fresh matter, tap root diameter,. total root length and number of adventitious roots compared with the macronutrient treatment. On the contrary shoot dry matter content and number of all laterals were reduced by NaCl. Three days after inoculation with P. aphanidermatum the number and length of roots were reduced significantly by the treatment 10(4) oospores ml(-1). P. aphanidermatum effects were greatest on young and thin roots, particularly those of the second and third laterals. Reduction increased with time and increasing number of inoculated oospores. Root diameters were greatest in the treatment with 10(4) oospores ml(-1). Results confirm that attempts to extrapolate results obtained on average root systems to single root formation could be misleading. Furthermore, a tomato plant can compensate to some extent its water uptake of a root system reduced by nutrition or pathogen effects. (C) 2002 Elsevier Science B.V. All rights reserved.