USING A POROUS-TUBE SYSTEM TO STUDY POTATO RESPONSES TO CONSTANT WATER TENSION IN A ROOTING MATRIX

A system maintaining continuous water tension on a nutrient solution h as been developed to control root zone moisture levels for plants in m icrogravity conditions, This study was conducted in a growth chamber t o characterize potato (Solanum tuberosum L,) responses to constant wat er tension compared to plants grown with no tension using a free-water technique. In three separate experiments, plants were grown in trays filled with a 4-cm layer of l-mm-diameter isolite (porous ceramic) par ticles, Ten porous stainless-steel tubes, 4 cm apart, were buried in t he medium, and nutrient solution was drawn through the porous tubes un der a constant water tension of -0.5 kPa maintained with a siphon syst em, For the free-water treatment, trays were slanted, and solution was supplied along the upper end of trays, passed under the medium, and t hen collected at the lower end and recirculated, The same nutrient sol ution was recirculated through both treatments at a flow rate of 150 m l . min(-1) through each tray and maintained at a pH of 5.6, Uniform m icropropagated 'Norland' potato plantlets were transplanted into repli cate trays and maintained at 18C, 70% relative humidity, and a continu ous photosynthetic photon flux (PPF) of 450 mu mol . m(-2). s(-1). Wat er tension reduced total plant dry weight, leaf area, leaf number, and stolen number by >75%, but specific leaf weight increased compared to free water, However, tubers enlarged more rapidly with water tension, and plants consistently partitioned a greater fraction of biomass int o tubers (than into shoots), Tuber weight was greater with water tensi on than in the free-water culture in Expt, 1 harvested 37 days after t ransplanting, however it was less in Expt, 2 when plants were grown to a larger size for 52 days before harvest, Leaf CO2 assimilation rate, stomatal conductance, and transpiration were reduced with water tensi on, although the relative water content of leaves was not significantl y affected. Also, with water tension, concentrations of N, P, Zn, and Cu in leaf tissues decreased, whereas tissue Fe increased compared to plants grown with free water, The results in this study demonstrate th at constant water tension significantly affects potato plant growth an d shifts biomass partitioning toward tubers.