Promotion of aerenchyma formation in Pinusserotina seedlings by ethylene

Abstract

The following study was designed to investigate a possible regulatory role for ethylene in aerenchyma and lenticel formation in pond pine (Pinusserotina Michx.) seedlings, a moderately flood-tolerant species. Seedlings were grown in noncirculating, continuously flowing solution culture under aerobic or hypoxic solution conditions; half the seedlings were treated with exogenous ethylene for the last 23 days of the 30-day treatment period. Ethylene stunted root growth, induced hypertrophy of stems and taproots, and initiated adventitious root and lenticel formation in aerobically grown seedlings. Histological studies indicated that taproot hypertrophy and resultant fissures were partially due to both an increase in number and radial enlargement of loosely packed parenchyma cells in the pericycle region. Exogenous ethylene also promoted the predominantly schizogenous formation of intercellular air spaces in the pericycle region of aerobically grown seedlings, enough to allow longitudinal diffusion of atmospheric O2 to submerged roots, as indicated via an indigo-carmine dye technique. Ethylene-treated seedlings grown under hypoxic conditions exhibited signs of reduced root vigour, with more pronounced taproot hypertrophy and fissuring than in the aerobic + ethylene treatment. Taproot fissures were a result of a proliferation of undifferentiated, loosely packed parenchyma cells in the pericycle region, with little continuous periderm development or evidence of a well-defined phellogen. Our results suggest that ethylene promotes those morphological changes in flooded roots that increase internal porosity, enhancing internal aeration.