Absorption of Water Through Suberized Roots of Trees

It is generally assumed that absorption of water and solutes by plant roots occurs principally in the unsuberized region back of the root cap where root hairs are usually produced. It seems probable, however, that under some conditions considerable absorption must occur through the suberized regions of roots, especially those of woody plants. For example, root elongation of most trees and shrubs almost or entirely ceases in cold or dry soil, and few or no unsuberized root tips exist at such times. Nevertheless many evergreen species such as pine, privet, holly and citrus lose considerable water by transpiration during such periods. The water lost during periods of little or no root elongation presumably is replaced by absorption through the suberized surfaces of fully differentiated, mature roots. Several investigators have presented indirect evidence that this actually occurs. Crider (5) reported absorption of nitrate and phosphate through roots of citrus and vitis after all root tips had been removed and the cut ends sealed. Chapman and Parker (2) observed absorption of nitrate by orange roots which had turned brown and were at least partially suberized. Colby (4) stated that apparently apple trees can absorb enough water through suberized roots to prevent wilting but not enough for shoot growth. Nightingale (7) found that potted apple and peach trees grown at soil temperatures of 45 and 95? F. produced no new roots and some of the old roots even died at 95? F. Nevertheless these trees absorbed water as indicated by the fact that they leafed out and made some shoot growth. Whatever absorption occurred during these experiments must have been through the old suberized roots, or even, in some instances, through dead roots. In spite of the strong presumptive evidence little direct evidence of water intake through suberized roots has been obtained. In fact the only direct measurements known to this writer are those made by Hayward, Blair, and Sk alino (6) under laboratory conditions on roots of sour orange seedlings. It was decided therefore to make a series of measurements of absorption through suberized roots of trees, especially pines, growing in the open.