EFFECTS OF DROUGHT AND SUBSEQUENT REHYDRATION ON THE STRUCTURE, VITALITY, AND PERMEABILITY OF ALLIUM-CEPA ADVENTITIOUS ROOTS

Drought stress was induced in onion (Allium cepa L.) plants that had been sprouted from bulbs initially planted in moist vermiculite. The medium received no further water, and its free water content fell from 50 to 0% over a period of 35 d. The water stress thus imposed killed the root apices and accelerated the death of the epidermis. However, other regions, including the short cells of the dimorphic exodermis, the remainder of the cortex, and the stele (except for the mature vessels), remained alive for periods of prolonged drought (up to 200 d after cessation of watering). During this time, symplasmic continuity from the exodermis to the vascular tissues of the stele was maintained through the living cells as judged by the transport of fluorescein from an ambient solution into the stele. After the death of the epidermis, the presence of Casparian bands in all cells of the exodermis and suberin lamellae in its long cells presumably reduced the flow of water from the root to the dry external medium. A dieback pattern in which the cortex is retained would place an increased respiratory demand on the plant compared with dieback to the endodermis. However, this may be offset by positive functions such as preserving a site for future food storage and (or) providing a hospitable environment for mycorrhizae when conditions outside the root are hostile. Loss of a viable apex under conditions of drought induced the development of many lateral root primordia that did not emerge but remained dormant within the root cortex. When water was added to the growth medium, these grew out quickly and along with the new adventitious roots that sprouted from the bulbs, rejuvenated the root system. Stress periods longer than 200 d resulted in death of the majority of the roots, and the plants did not recover after watering.