TRANSPIRATION DOES NOT CONTROL GROWTH AND NUTRIENT SUPPLY IN THE AMPHIBIOUS PLANT MENTHA-AQUATICA

Mentha aquatica L. was grown at different nutrient availabilities in w ater and in air at 60% RH, The plants were kept at 600 mmol m(-3) free CO2 dissolved in water (40 times air equilibrium) and at 30 mmol m(-3 ) CO2 in air to ensure CO2 saturation of growth in both environments, We quantified the transpiration-independent water transport from root to shoot in submerged plants relative to the transpiration stream in e mergent plants and tested the importance of transpiration in sustainin g nutrient flux and shoot growth. The acropetal water flow was substan tial in submerged Mentha aquatica, reaching 14% of the transpiration s tream in emergent plants, The transpiration-independent mass flow of w ater from the roots, measured by means of tritiated water, was diverte d to leaves and adventitious shoots in active growth, The plants grew well and at the same rates in water and air, but nutrient fluxes to th e shoot were greater in plants grown in air than in those that were su bmerged when they were rooted in fertile sediments, Restricted O-2 sup ply to the roots of submerged plants may account for the smaller nutri ent concentrations, though these exceeded the levels required to satur ate growth, In hydroponics, the root medium was aerated and circulated between submerged and emergent plants to minimize differences in medi um chemistry, and here the two growth forms behaved similarly and coul d fully exploit nutrient enrichment. It is concluded that the lack of transpiration from leaf surfaces in a vapour-saturated atmosphere, or under water, is not likely to constrain the transfer of nutrients from root to shoot in herbaceous plants, Nutrient deficiency under these e nvironmental conditions is more likely to derive from restricted devel opment and function of the roots in waterlogged anoxic soils or from l ow porewater concentrations of nutrients.