MECHANISMS FOR THE INCREASE IN PHOSPHORUS UPTAKE OF WATERLOGGED PLANTS - SOIL-PHOSPHORUS AVAILABILITY, ROOT MORPHOLOGY AND UPTAKE KINETICS

Waterlogging frequently reduces plant biomass allocation to roots. Thi s response may result in a variety of alterations in mineral nutrition , which range from a proportional lowering of whole-plant nutrient con centration as a result of unchanged uptake per unit of root biomass, t o a maintenance of nutrient concentration by means of an increase in u ptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experim ent, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was af fected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root- shoot ratios, but did not reduce P concentration. Thus, uptake of P pe r unit of root biomass increased under waterlogging. Our second object ive was to test three non-exclusive hypotheses about potential mechani sms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root m orphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) c hanges in P availability induced by waterlogging; (2) specific root le ngth of waterlogged and control plants, and (3) P uptake kinetics in e xcised roots from waterlogged and control plants. The results supporte d the three hypotheses. Soil P avail-ability was higher during waterlo gging periods, roots of waterlogged plants showed a morphology more fa vorable to nutrient uptake (finer roots) and these roots showed a high er physiological capacity to absorb P. The results suggest that both s oil and plant mechanisms contributed to compensate, in terms of P nutr ition, for the reduction in allocation to root growth. The rapid trans formation of the P uptake system is likely an advantage for plants inh abiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the in creased relative abundance of P. dilatatum in the community after wate rlogging periods.