Abscisin II and , some. Hormone-Regulated Plant Responses

Abscisic acid is known to control different phases of plant growth processes, and its physiological effects have been recently reviewed (1, 12). Although its inhibitory effects are numerous (1), cases of growth promotion by ABA are scarce (10,11). Some interactions of ABA with other growth substances, mainly with gibberellic acid, in promoting growth of cucumber hypocotyls (2), and with kinetin, in the growth of soybean callus (3), were also reported.While studying the development of citrus buds in vitro, we noticed a formation of callus in bud cultures which was induced specifically by ABA. The effects of ABA alone, and in combination with IAA and 6-benzylaminopurine are reported.MATERIALS AND METHODS Plant Material. Typical branchlets of spring flush, bearing 5 to 7 leaves and their axillary buds, were cut from 40-year-old Shamouti orange trees (Citrus sinensis [L.] Osbeck), washed with tap water and detergent, rinsed with distilled water, dipped momentarily in 80% ethanol, and surface-sterilized by immersing for 30 min in a filtered calcium hypochlorite solution (90 g/liter) with 0.1% Tween-20. Explants were prepared aseptically: the petiole was excised and then the explant was removed from the branchlet by a shallow longitudinal excision penetrating somewhat into the wood. These explants therefore consisted of the whole bud, portions of bark and wood from the branchlet, and the stump end of the petiole (Fig. 1A). In some experiments portions of the leaf petiole and blade, with or without the bud, were also examined. Bud explants were planted in 25-x 95-mm culture tubes containing 10 ml of nutrient medium with the stem end in the agar.Nutrient Media. The Murashige and Skoog medium as adapted for citrus tissues (6, 7) was used throughout the experiments. The medium, in addition to the original inorganic salt composition, contained the following (mg/liter): glycine, 2.0; nicotinic acid, 0.5; pyridoxine-HCl, 0.5; thiamine-HCl, 0.1; and myoinositol, 100.0, together with 5% sucrose and 1% Difco Bacto-agar. To this basal medium were further added varying concentrations of ABA, IAA, and BA.' The pH of the medium was adjusted to 5.7, and sterilization was accomplished by autoclaving for 25 min at 124 C (1.4 kg/cm2).Cultures were allowed to develop for up to 2 weeks in a controlled growth chamber at 25 + 1 C and under a 16-hr photoperiod in weak white light (400 ft-c). Explants were then removed for examination and fresh weight determination. Each experiment consisted of 10 separate cultures and was repeated at least three times.

Section: Resultsmentioning

confidence: 99%

Promotion of Callus Formation by Abscisic Acid in Citrus Bud Cultures

Altman¹,

Gören²

1971

“…Comparison between the behavior of isolated leaves and those attached to the plant then brought out the important part played by transport phenomena (22,32,38 (e.g. 18), it was natural to look for a function of this hormone in senescence, and indeed it was reported early to increase leaf senescence (6,12, 27). Our own experiments (14) '…”

mentioning

confidence: 99%

“…Since AbA accumulation is well known to cause stomatal closure (e.g. 18), it was natural to look for a function of this hormone in senescence, and indeed it was reported early to increase leaf senescence (6,12,27). Our own experiments (14) ' Supported in part by grant (33), and causes increases of respiration and of ATP, along with the loss of Chl-the typical senescence syndrome (31).…”

mentioning

confidence: 99%

The Role of Ethylene in the Senescence of Oat Leaves

Gepstein¹,

Thimann²

1981

140

The evolution of ethylene, both from the endogenous source and from added 1-aminocyclopropane-1-carboxylic acid (ACC), has been foUlowed in close relationship with the senescent loss of chlorophyll from seedling oat leaves. In white light, where chlorophyll loss is slow, the ethylene evolution increases slowly at first, but when the loss of chlorophyll becomes more rapid, ethylene evolution accelerates. CoCI2 inhibits this increase and correspondingly maintains the chlorophyll content, with an optimum concentration of 10 micromolar. The rapid rate of chlorophyll loss in the dark is slightly decreased by 3-aminoethoxyvinyl glycine (AVG), by cobalt, and slightly stimulated by ACC. The slower chlorophyll loss in white light, however, is almost completely inhibited by silver ions, greatly decreased by cobalt and by AVG, and strongly increased by ACC. Since the chlorophyll loss is accompanied by proteolysis, it represents true senescence. Chlorophyll loss in light is also strongly antagonized by C02, 1% CO2 giving almost 50% chlorophyll maintenance in controls, while in the presence of added ACC or ethylene gas, the chlorophyll loss is 50% reversed by about 3% CO2. The ethylene system in leaves is thus more sensitive to CO2 than that in fruits. Indoleacetic acid also clearly decreases the effect of ACC. It is shown that kinetin, C02, Ag+, and indoleacetic acid, all of which oppose the effect of ethylene, nevertheless increase the evolution of ethylene by the leaves, and it is suggested that ethylene evolution may, in many instances, mean that its hormonal metabolism is being prevented.Abscisic acid somewhat increases ethylene evolution also, but its action in promoting senescence in light is antagonized only partially by Ag+, C02+, or AVG. For this and a number of other reasons it is concluded that ethylene and abscisic acid both independently control leaf senescence in the light.The study of leaf senescence continually brings to light more and more interactions. The earliest researches tended to concentrate on the nucleic acids (1 1, 24, 43, 44), then proteolysis came to the fore, together with the balance between proteolysis and protein synthesis, and the specific roles of individual amino acids (29 and see 35). Comparison between the behavior of isolated leaves and those attached to the plant then brought out the important part played by transport phenomena (22,32,38 (e.g. 18), it was natural to look for a function of this hormone in senescence, and indeed it was reported early to increase leaf senescence (6,12, 27). Our own experiments (14) '

“…PCM 76-83126. 2 Present address: Universidad Nacional, Belgrano (S) 1912, 4200 Santiago, del Estero, Argentina. some inhibiting it.…”

mentioning

confidence: 99%

Relation between Respiration and Senescence in Oat Leaves

Satler¹,

Thimann²

1983

The respiration of excised oat (A vena sativa cv Victory) leaves and their sensitivity to inhibitors was followed during senescence under varied conditions. The respiration rate, which in controls reaches its peak on the third day in darkness, is lowered at the time of fastest loss of chlorophyll (as reported earlier) by seven unrelated reagents that all delay dark senescence. When senescence is delayed by white light or by cytokinins, the respiratory rise is correspondingly delayed. Kinetin and L-serine, which act as antagonists on senescence, also act as antagonists on the respiratory rate. However, an exception to this close correspondence between senescence and the respiratory rise is offered by the lower aliphatic alcohols, which delay dark senescence and yet accelerate the onset of the respiratory rise.The respiration of freshly cut leaves is insensitive to KCN up to 8 millimolar, but sensitive to benzhydroxamate (BAM), 1 to 2 millimolar BAM causing 25% promotion and higher concentrations inhibiting. At the respiratory peak, however, part of the respiration becomes KCN-sensitive. Low concentrations of alcohols in darkness, or 3-(3,4dichlorophenyl)-1,1-dimethylurea, diuron, in light, also render part of the respiration KCNsensitive, but this sensitivity soon disappears again. Some 10 to 15% of the respiration is insensitive to both inhibitors. Thus, cyanide sensitivity comes and goes, while BAM sensitivity is always present. The current concept of the cyanide-resistant pathway as an overflow, therefore, does not fit well with behavior of these leaves. The respiratory rise in leaf senescence is similar to, but not identical with, the climacteric in ripening fruits and the aging phenomenon in tuber slices.During senescence in the dark, detached leaves of seedling oats (A vena sativa cv Victory) undergo a marked increase in respiratory rate, which reaches a peak at about 250%o of the initial rate on the 3rd d (23). By that day, the Chl loss and proteolysis are well advanced, and by the 4th d, when the respiratory rate falls, more than 65% of both Chi and protein have been lost. Cytokinins, at concentrations that inhibit or delay senescence, correspondingly delay this respiratory rise (23). It was therefore concluded that the respiratory rise was either itself the cause or perhaps the direct result of the senescence process. A first suggestion was that both are due to respiratory uncoupling and the consequent loss of phosphorylative energy, but direct determination of the ATP content of the leaves showed that this was not the case (13 (1, 2, 6-8, 16, 24, 29). It has also been established that opening of the stomata invariably accompanies, and indeed seems to cause, the delay of senescence in darkness, while closure of the stomata causes accelerated senescence in light (29). White light is itself an important factor in delaying senescence and causes immediate stomatal opening which is followed by very gradual closure (29). Its influence reaches saturation at irradiances well below those that saturate ph...