ABSORPTION OF WATER THROUGH SUBERIZED ROOTS OF TREES

Abstract

It is generally assumed that absorption of water anid 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 somne conditions considerable absorption must occur through the suberized regions of roots, especially those of woody plants. For example, root elon-gation of most trees and shrubs almost or entirely ceases in cold or dry soil, alnd 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 suLberized surfaces of fully differelntiated, 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. CHAP-MAN 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 950 F. produced no new roots and some of the old roots even died at 950 F. Nevertheless these trees absorbed water as indicated by the fact that they leafed ouit and made some shoot growth. Whatever absorptioni ocetrred during these experiments must have been through the old suberized roots, or even, in soilme instances, through dead roots. In spite of the strong presumptive evidence little direct evideniee 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 SKALING (6) under laboratory conditions on roots of sour oralnge seedlings. It was decided therefore to make a series of measurements of absorption through suberized roots of trees, especially pines, growing in the open. Methods and results Potometers were made from both glass and rubber tubing. They are shown in figure 1. The glass potomieters were slipped over the ends of cut roots and sealed over split stoppers bored to fit snugly. around the roots, thus surrounding the root with a water chamber about 2.5 cm. long and 1.0 cm. in diameter. The joints were made watertight with graftinog wax or Fisher's Pyseal, the latter being particularly useful in hot weather. Each potometer had two side tubes. To one was attached a small pipette graduated in 0.05 37