No abstract

SUMMARYDrought and low soil water potentials depress nutrient uptake by roots, but their capacity for recovery when the soil is re-wetted has received less study. We investigated the uptake in barley of labelled phosphate and calcium from sand culture following periods of drought or treatment with polyethylene glycol 4000 (PEG) of the surface layers of the sand, with the lower portion of the root system receving ample nutrients and water. Drought diminished uptake and translocation of both nutrients and caused death of fine lateral roots, but after 3 days of re-wetting, uptake per unit fresh weight of roots was comparable with that of well-watered control plants despite apparent collapse of some of the cortical cells of the seminal axes. The activity of new nodal roots that grew during the 3-day re-wetting period alleviated the earlier depression of nutrient absorption. Effects of PEG (-0-8 MPa) were less severe and nodal roots grew through this medium with no apparent damage. The tips of these roots seemed to become osmotically adjusted largely by enhancement of their sugar content, but we found little evidence of such adjustment in those parts of the seminal axes exposed to PEG. Rapid growth of nodal roots assisted by osmotic adjustment may be of adaptive significance in ensuring resumption of nutrient and water uptake following periods of water deficit.

SUMMARY Polyethylene glycol 4000 (PEG) at an osmotic potential of −0.3 MPa had no effect on uptake of 14CO2 and partitioning between shoots and roots in barley. When an osmotic potential of −0.8 MPa was applied to the upper zone of the root system, total 14CO2 uptake and partitioning were unaffected but assimilate accumulated in the tips of nodal roots exposed to PEG. There was a parallel accumulation of soluble sugars. Transport of assimilates to the seminal root system declined. Exudation of soluble, labelled compounds by the roots was markedly diminished. When PEG was supplied to the upper root zone at −0.8 MPa and the lower zone at −0.6 MPa total 14CO2 uptake and assimilate transport declined, but there was still some accumulation of labelled compounds and soluble sugars in the nodal root tips and root exudation was diminished. The results are discussed in relation to assimilate partitioning, growth and osmotic adjustment in nodal roots.

No abstract