Water Uptake by Different Regions of the Barley Root. Pathways of Radial Flow in Relation to Development of the Endodermis

The water uptake by different lengths of lateral roots and 1.0 cm or 5.0 cm lengths of the seminal axes from differentregions of the root were measured in potometers with the shoot in air at two humidity regimes. A model of the contribution by these different regions of the root to the total water absorption by the plant agreed well with measurements of water uptake by the whole root system. According to this model, about one half of the water taken up by the main axis came from the older suberized regions further than 10 cm from the tip, and together with its associated lateral roots this region provided 75% of the total water transpired. The development of State III endodermal cells was correlated with decreases in both the water uptake by the older regions of the root and the translocation of calcium. Thus in the younger regions of the root where water uptake is maximal, the flow of water is principally apoplastic although there is also likely to be flow via the symplast Despite a 43% difference in transpirational demand between the two humidity treatments, the leaf water potentials remained constant, implying a change in root resistance. This change in resistance might be explained if there were an apoplastic pathway within the suberin lamellae of State III endodermal cells. The response to the increased transpirational demand is met by the older regions of the root, in particular by the zone of lateral emergence where an apoplastic pathway is known to exist as the Casparian band in the endodermis breaks down with the emergence of the lateral roots.