Plants of Zea mays L. cv TX5855 were grown in a complete, well oxygenated nutrient solution then subjected to nutrient starvation by omitting either nitrate and ammonium or phosphate from the solution. These treatments induced the formation of aerenchyma close to the apex of the adventitious roots that subsequently emerged from the base of the shoot, a response similar to that shown earlier to be induced by hypoxia. Compared with control plants supplied with all nutrients throughout, N-or Pstarvation consistently depressed the rates of ethylene release by excised, 25 mm apical segments of adventitious roots. Some enzymes and substrates of the ethylene biosynthetic pathway were examined. The content of 1-amino cyclopropane-1-carboxylic acid (ACC) paralleled the differences in ethylene production rates, being depressed by N or P deficiency, while malonyl-ACC showed a similar trend. Activity of ACC synthase and of ethylene forming enzyme (g-1 fresh weight) was also greater in control roots than in nutrient starved ones. These results indicate that much of the ethylene biosynthetic pathway is slowed under conditions of N-or P-starvation. Thus, by contrast to the effects of hypoxia, the induction of aerenchyma in roots of Zea mays by nutrient starvation is not related to an enhanced biosynthesis and/or accumulation of ethylene in the root tips.Many dryland species are exposed to temporary periods of oxygen deficiency during their growth following heavy rain, irrigation, or flooding, when the soil becomes water saturated (10,17). Some species respond to this situation by forming roots with continuous, gas-filled channels in the roots (19). These channels improve the internal supply of oxygen, the oxygen originating from the atmosphere or photosynthesis and passing from leaves to roots down a concentration gradient (1). The flooding resistance of a wide range of monocot and dicotyledonous species, both herbaceous and woody, is often associated, in part, with the capacity to develop such aerenchymatous roots (8,9,16,17,19,23 The gaseous plant hormone, ethylene, is associated with a wide variety of responses in higher plant cells (3). In earlier reports we showed that aerenchyma formation in adventitious (nodal) roots of maize by cell lysis is induced by a partial oxygen deficiency (hypoxia) and is closely related to enhanced endogenous concentrations of ethylene (1 1). Hypoxia clearly stimulates the biosynthesis of ethylene in growing maize roots (2,11,12,20) as in other responsive plant tissues (4, 5, 7). Low exogenous concentrations of ethylene under aerobic conditions (1-5 uL L-' in air) induce aerenchyma (18) that is structurally indistinguishable from that induced by hypoxia (1 1, 12); furthermore, hypoxically induced aerenchyma formation in the apices of growing roots is blocked by inhibitors of ethylene biosynthesis or ethylene action (12,18,20).However, Konings and Verschuren (21) showed that formation of aerenchyma in the seminal roots of maize was stimulated under fully aerobic conditions by omission o...

Adventitious roots of maize (Zea mays L. cv TX 5855), grown in a well-oxygenated nutrient solution, were induced to form cortical gas spaces (aerenchyma) by temporarily omitting nitrate and ammonium (-N), or phosphate (-P), from the solution. Previously this response was shown (MC Drew, CJ He, PW Morgan [1989] Plant Physiology 91: 266-271) to be associated with a slower rate of ethylene biosynthesis, contrasting with the induction of aerenchyma by hypoxia during which ethylene production is strongly stimulated. In the present paper, we show that aerenchyma formafion induced by nutrient starvation was blocked, under noninjurious conditions, by addition of low concentrations of Ag+, an inhibitor of ethylene action, or of aminoethoxyvinyl glycine, an inhibitor of ethylene biosynthesis. When extending roots were exposed to low concentrations of ethylene in air sparged through the nutrient solution, N or P starvation enhanced the sensitivity to exogenous ethylene at concentrations as low as 0.05 microliters ethylene per liter air, promoting a more rapid and extensive formation of aerenchyma than in unstarved roots. We conclude that temporary deprivation of N or P enhances the sensitivity of ethylene-responsive cells of the root cortex, leading to cell lysis and aerenchyma.The roots of many plant species, especially those well adapted to wetland conditions, respond to inadequate oxygenation of their environment by formation of aerenchyma (16,17). In maize, lysigenous aerenchyma can form in young seminal (18) as well as nodal (adventitious) roots (11,12) when they extend under conditions of hypoxia into nutrient solution that is partially oxygen deficient. Aerenchyma, comprising an interconnected series ofgas-filled cavities or lacunae (1 1), improves the internal aeration of the roots (13). At an early stage in aerenchyma formation, cells of the root cortex, located about 10 mm behind the apex, undergo premature lysis and disintegration to form lacunae (7). Hypoxia stimulates ethylene biosynthesis, and the increase in internal ethylene concentration is closely associated with induction of cell lysis and aerenchyma formation (1,11 (10,19). In an earlierpaper, we examined cell lysis and ethylene biosynthesis in the root tip of maize during temporary deficiency of N or P and compared the responses to those induced by hypoxia (10). We found that temporary shortage of these nutrients consistently inhibited the rate of ethylene production of the excised tips to only one-half to one-third that of unstarved controls. The activity of ACC3 synthase and the ACC concentration were likewise depressed by N or P starvation. Induction of cell lysis by nutrient starvation, unlike hypoxia, was thus not related to a greater endogenous concentration of ethylene.In the present study, we used inhibitors of ethylene biosynthesis and ethylene action to determine whether ethylene is associated with the onset of cell lysis in nutrient-deprived roots. We also examined the response of such roots to exogenous ethylene. We show that nutr...

Cerminated maize (Zea mays 1.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by externa1 pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1 -aminocyclopropane-1 -carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. l h e synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.

Either hypoxia, which stimulates ethylene biosynthesis, or temporary N starvation, which depresses ethylene produdion, leads to formation of aerenchyma in maize (Zea mays 1.) adventitious roots by extensive lysis of cortical cells. We studied the activity of enzymes closely involved in either ethylene formation (l-aminocyclopropane-1 -carboxylic acid synthase [ACC synthase]) or cellwall dissolution (cellulase). Activity of ACC synthase was stimulated in the apical zone of intact roots by hypoxia, but not by anoxia or N starvation. However, N starvation, as well as hypoxia, did enhance cellulase activity in the apical zone, but not in the older zones of the same roots. Cellulase activity did not increase during hypoxia or N starvation in the presence of aminoethoxyvinylglycine, an inhibitor of ACC synthase, but this inhibition of cellulase indudion was reversed during simultaneous exposure to exogenous ethylene. Together these results indicate both the role of ethylene in signaling cell lysis in response to two distind environmental factors and the significance of hypoxia rather than anoxia in stimulation of ethylene biosynthesis in maize roots.

The rate of ethylene production by intact, attached leaves of cotton plants (Gossypium hirsutum L.) during aging and senescence was studied using a continuous flow system that allowed air around enclosed leaves to be scrubbed to collect and assay ethylene. Senescence of lower leaves began around 150 d after planting in a controlled environment room. A progressive decline in the ethylene production rate was observed when comparing the 3rd, 6th, and 10th leaves from the base with each other. Ethylene production rates of individual leaves also declined over a 50-d period. However, as leaves began to appear chlorotic, a peak of ethylene production occurred that lasted for about 4 d followed by abscission. This peak involved a 3-fold or greater increase in the rate of ethylene production. The data indicate that intact leaves experience a climacteric-like surge in ethylene production after visible symptoms of senescence appear. This "ethylene climacteric" is apparently the signal that initiates hydrolysis of cell walls in the abscission zone. A more important reason arises from the fact that leaf age often alters sensitivity to ethylene (13, 23); thus, it is possible that increased sensitivity to ethylene, rather than increased concentrations of ethylene, signals the abscission process. This possibility suggests that there would be no need for a peak in ethylene production to occur for ethylene to mediate abscission. Finally, young leaves or other organs generally produce ethylene at higher rates than older leaves (7,11,14), a fact attributed to the well-known ability of auxin to promote ethylene synthesis (1,15). This correlation raises the question of what cue might initiate ethylene production in an aging leaf in which auxin levels are presumed to be declining. Although auxin levels have been shown to decrease in leaves with age (20,22,24), the analyses have not been frequent enough to eliminate the possibility of a transient rise in auxin levels. Logically, before solution to the question of what cues the presumed rise in ethylene production should be sought, it would be desirable to determine with certainty that ethylene production rates increase in intact leaves on intact plants before abscission.Availability of an apparatus to collect ethylene from intact plants via a flowing air stream (16) allowed us to enclose individual leaves on intact plants and determine their ethylene production rates with age. Plants were grown under conditions in which older leaves abscised at around 150 d after planting. We measured ethylene production of several leaves from 90 d after planting until abscission and detected a consistent burst in ethylene production just before abscission. MATERIALS AND METHODSCotton plants (Gossypzum hirsutum L. v. Stoneville 213) were grown in 7.6-L pots in growth rooms (Environmental Growth Chambers, Chagrin Falls, OH) using a 12-h photoperiod as described (16). Fluorescent and incandescent lamps provided 800 to 1000 gtmol photons.m-2 s-l at the upper canopy level with height-adjustable ...