Phosphorus uptake and active growth of Elodea canadensis Michx. and Elodea nuttallii (Planch.) St. John

Two related submerged freshwater macrophyte (Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx.) were used in a combination of in- and outdoor experiments to compare growth rates and routes of phosphorus uptake, translocation and possible excretion. Elodea nuttalli had a higher growth rate than Elodea canadensis both in the field and under laboratory conditions. The uptake and translocation of phosphorus was studied using 32P in a partitioned container. Roots and leaves were supplied with 32P separately and simultaneously. Both macrophyte species were able to take up phosphorus both with leaves and roots. The uptake rate of 32P by roots was higher in Elodea nuttallii when tracer was supplied to the root compartment only. Leaf uptake was stimulated by the supply of phosphate to both compartments, and the uptake was faster and reached a higher level than when the tracer was injected to the leaf compartment only. The 32P uptake by either roots or leaves was translocated. Shoot-to-root translocation predominated over the reverse. There was no significant difference between both species in root uptake rates, but leaf uptake was significantly higher in Elodea canadensis. The two studied Elodea species do not have a strong preferential source of phosphorus.

Differences in resource allocation to stolon branches of Kopu white clover genotypes induced by manipulation of rooting

White clover (Trifolium repens L.) genotypes of the cv. Grasslands Kopu which had differing numbers of vascular bundles within their stolons were tested for potential for resource sharing between shoot branches on either side of parent axis. Genotypes with a high (10-13) number of vascular bundles (H-genotypes) had larger leaves, thicker stolons but lower node appearance rates than genotypes with a low (8-9) number of vascular bundles (L-genotypes). In the first experiment all roots on one side of the parent axis were severed two days before plants were labelled with 32P (uptake period 24 h). In H-genotypes, distribution of radioactive phosphorus (32P) exported from a nodal root was restricted largely to near-side branches, i.e. branches which were on the same side of the parent axis as the source root, with only 4% allocated to far-side branches. In contrast about 20% of 32P exported from the source root was transported to far-side branches in L-genotypes. In the second experiment, where all far-side roots were severed three weeks before harvest, growth of far-side branches was similarly reduced in L-genotypes and Hgenotypes. When root formation along one side of the parent axis was prevented during the entire experimental period the mean ratio of total dry weight between nearand far-side branches was higher in H-genotypes compared to L-genotypes. Keywords: branch development, nutrient distribution, phosphorus, vascular bundle, white clover, Trifolium repens L.

Endogenous renal dopamine production regulates phosphate excretion

We examined the effect of endogenous dopamine production on Pi and citrate excretion by Wistar rats. Carbidopa (20-40 mumol/kg ip) decreased dopamine, Pi, and citrate excretion within 20 min (86%, 47%, and 38%, respectively); Pi reabsorption increased 11 +/- 4% (P = 0.03). The decreases were sustained for at least 18 h. 3-Hydroxybenzylhydrazine (45 mumol/kg ip) reduced Pi excretion 24%. Benserazide (40 mumol/kg ip and 0.1 mumol/min iv) reduced dopamine excretion (94%) and blocked the effect of carbidopa on Pi and citrate excretion. In isolated perfused kidneys benserazide, carbidopa, and 3-hydroxybenzylhydrazine all decreased Pi excretion. Dopamine (1 mumol/l) added to cortical minceates reduced brush-border membrane vesicle (BBMV) 32P uptake by 8% (P < 0.02) and amiloride-inhibitable 22Na uptake by 19%. Carbidopa added to minceates increased 32P uptake by 12%. Carbidopa pretreatment increased (75%) amiloride-sensitive 22Na uptake into BBMV of rats fed a high-salt diet. Uptake was not increased into BBMV from rats fed a low-salt diet. Carbidopa increased (17%) basolateral membrane Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) gradually over 4 h. Na(+)-K(+)-ATPase did not increase in rats fed a low-phosphorous diet, but did increase when dopa was added to the diet. Thus endogenous dopamine appears to directly control Na(+)-Pi and Na+/H+ transport and secondarily alter basolateral membrane Na(+)-K(+)-ATPase.