Question

One morning, in an effort to measure the rate of water movement through the stem of a mature sunflower (Helianthus annuus), a graduate student carefully injected tritiated water (^3H_2O) into the secondary xylem (and only the xylem) at the base of the stem. She subsequently monitored movement of the tritiated water, and found that a majority of it quickly migrated into and evaporated from the leaves. She also discovered, however, that a small amount eventually (i.e., after several hours) found its way into the xylem of the plant's roots. To investigate the cause for that seemingly odd finding, she repeated the study on plants that had either been completely defoliated or girdled (by severing all tissues outside the secondary xylem, around the full circumference of the stem). No tritiated water was detected in the roots of those damaged plants. Please provide her with a comprehensive, mechanistic explanation for her unanticipated observation. In your explanation, include a rationale as to why no tritiated water was found in the root xylem of sunflowers that had been defoliated or girdled.

Answer 1

Cell to cell radial transport of water occurs in roots via parallel transport. There is a radial transport of water in the roots of plants, and the hydraulic conductivity of roots is affected by a variety of factors like abiotic stresses, droughts or nutritional stresses.

In sunflower roots, abscisic acid promotes , flow of water in the roots and this results in release of ions in xylem vessels. Defoliation led to severe reduction in the concentration of ABA in the xylem roots. Thus, no tritiated water was found in the defoliate roots of xylem.