Effects of Root Anaerobiosis on Ethylene Production , Epinasty , and Growth of Tomato Plants 1

Experiments were performed to determine the source(s) of ethylenecausing epinasty in flooded tomato plats (Lyc_perskoa escultm MilL). Simultaneous measurements were made of ethylene synthesized by the roots and shoots of tomato plants exposed to either aerobic or anaerobk atmospheres in the root zone. When the root zone was made anaerobic by a flowing stream of N2 gas, petiole epiaty ad accelerated ethylene synthesis by the shoots were observed. In soil-grown plats, ethylene synthesis by the root-soil complex Increased under anerobic codtions; but when grown in inert media under the same conditions, ethyle synthesis by roots remained constant or declie during the period of rapid epinastic growth by the petioles. Other characteristic symptoms offloding, e.g. reduced growth and chlorosis, were also observed In plants with anaerobic roots. Pretreatment of pants with AgNO, an nibtor of ethylene action, completely prevented epnsty, demotrating that ethylene is the agent responsbe for waterog symptoms. These results nicate that deprivation of 02 to the roots is the primary effect of soil flooding, and that this Is sufficiet to came increased ethylee synthesis In the shoot. The basis of the observed root-shoot communication is unknown, but root-synthesized hormones or specific ethylee-promoting factors may be involved. cause epinasty (1 1) and proposed that ethylene diffusing from the soil into the plant under waterlogged conditions may be responsible for the rise in shoot ethylene content and symptom expression. Using tomato plants grown in nutrient solution, Jackson and Campbell (9, 12) found that low 02 (less than 3%, v/v) in the root zone caused epinasty and elevated ethylene levels in the shoots. Since no ethylene was evolved from the nutrient solution alone, this suggested that root 02 deficiency can stimulate shoot ethylene production. The first two hypotheses for the origin of the ethylene increase (i.e. root and soil) are passive models which require a concentration gradient between the root and the shoot to allow diffusion of ethylene up the stem. The plant is essentially gassed from within by root-synthesized ethylene, or from without by microbially synthesized ethylene. The third hypothesis requires some form of root-shoot interaction whereby a root stress stimulates ethylene synthesis in the shoot. This study tested these hypotheses by using N2 ventilation of the root zone, rather than flood water, to impose an 02 deficiency upon the root without blocking gas diffusion. Microbial contribution of ethylene was prevented by using inert growing media. A special apparatus maintained the desired atmosphere in the root zone and allowed simultaneous measurement of root and shoot ethylene synthesis. Waterlogging of higher plants can cause petiole epinasty, chlorosis, stem hypertrophy, reduced growth, and adventitious rooting (16). Epinastic growth of tomato petioles commenced within 12 hr following flooding of the soil (12). Exposure to ethylene will also cause petiole epinasty in a variety of plants (5). Recent investigations have revealed that flooding-induced epinasty is due to elevated ethylene concentrations in the shoots (9, 12, 14). Several hypotheses have been proposed suggesting that this excess ethylene may be produced in the soil, the root, or the shoot (11, 12, 15). Kawase (13) found that submersion increased the ethylene content in a variety of plant tissues. Ethylene content of flooded intact sunflower plants increased first in the submerged portion, then in the aerial parts, and was correlated with the development of epinasty, adventitious rooting, and stem hypertrophy (14). Kawase (15) suggested that blockage of ethylene diffusion from the submerged roots allows the accumulation of ethylene which then moves up the plant, causing visible symptoms of flooding injury. Zeroni et al. (27) reported that ethylene may be diverted throughout a plant if escape is blocked by a diffusion barrier. Following the discovery (26) of relatively high concentrations (>20 ,ul 1-1) of ethylene in anaerobic soils, Jackson and Campbell (8) suggested that microbially synthesized ethylene may enter the plant and cause epinasty. They later showed that concentrations of ethylene in the root zone in excess of 2 1. 1` were sufficient to ' Michigan Agricultural Experiment Station Journal Article No. 8215. MATERIALS AND METHODS Plant Material. Dwarf tomato plants (Lycopersicon esculentum Mill. cv. Tiny Tim) were used for some experiments (Figs. 3, 4, and 5) and a nondwarf cultivar, Chico III, for others (Figs. 2 and 6). Growing Conditions. Plants were grown in a greenhouse at 18 C minimum with natural lighting between October 1976 and February 1977. Plants were grown in a silty loam soil (pH 6.7), organic matter 5% (w/w), in perlite, or in Turface (BASF Wyandotte Corp.) an inert clay medium. Plants were watered with 0.5 Hoagland solution (7) and were acclimated in the controlled environment room at least 3 days prior to an experiment. A 16-hr photoperiod was used with incandescent and fluorescent lights at an intensity of 1,000 to 1,300, ft-c (4-5 mw cm 2). Temperature was maintained at 25 ± 2 C. Six-week-old plants were generally used, when flower buds were just visible on the dwarf plants. Data for Figures 2 and 3 were obtained from plants (eight/treatment) grown in 453-ml canning jars with metal lids fitted with rubber serum stoppers and tubing for introducing gases. The stem of a young plant was sealed to the lid with lanolin or with RTV-11 silicone rubber (General Electric) with a nonphytotoxic catalyst (Harter TI, Wacker Chemie Gmbh, Munich). Thus, the roots were isolated from the atmosphere, while the shoots were not enclosed. Ethylene-free air or N2 gas at a rate of 3 to 4 ml min-' was introduced to the bottom of the root zone through a stainless steel tube and exited through an outlet in the 506 www.plantphysiol.org on October 23, 2017 Published by Downloaded from Copyright © 1978 American Society of Plant Biologists. All rights reserved. ROOT AERATION AND ETHYLENE PRODUCTION lid. Ethylene analyses were made on the effluent gas or by sealing the root chamber for I hr and sampling the accumulated ethylene. The rooting media were initially brought to field capacity and subsequently maintained by adding nutrient solution back to the original weight. The jars were covered with aluminum foil to exclude light. For other experiments, a special apparatus was constructed to permit simultaneous measurement of ethylene synthesis by the roots and shoot (Fig. 1). The shoot chamber accommodated a 6week-old dwarf tomato plant without restraint. Twelve plants were used/treatment. The test unit held 30 chambers and provided irrigation through tubes connected to each chamber. The roots were maintained in darkness and were irrigated with nutrient solution daily. The root zone was ventilated at a rate of 1.5 ml min-' employing capillary flow meters. Since the free air space of the filled tube was approximately 30 ml, this rate supplied about 3 volumes/hr. Assuming complete mixing, this would remove 95% of the gas originally in the root zone after I hr (17). Romell (22) calculated from rates of CO2 production and levels of CO2 found in soil atmospheres that normal aeration completely exchanges the gas in the top 20 cm of soil once every hr. Thus, the selected flow rate represents a realistic rate of supply of 02 and removal of CO2 in the root zone. Ethylene, C02, and 02 were sampled from the effluent gas under steady-state conditions. The shoot chamber was ventilated at a rate of 45 to 60 ml min-', sufficient to provide one chamber volume approximately every 10 min. For ethylene sampling the flow gas was stopped and the chamber sealed for 2 hr. Otherwise, the shoot and root chambers were flushed continuously with humidified, ethylenefree air. Treated roots received 99.9% N2 which was also free of ethylene. Flow gas to the roots was filtered with Ascarite to remove CO2. This system maintained O2 levels of less than 0.5% in the root zone of N2-treated plants and allowed simultaneous measurement of ethylene production by both the roots and shoot of a single plant over the course of several days. Ethylene, C02, and O2 Determination. Ethylene was determined shoot exhaust root sampling port