Fruit ethylene production

Tomato fruits (Lycopersicon esculentum Mill. var. Castlemart ) were collected from vines grown in the field at the University of California, Davis. Pericarp discs were cut from surface sterilized MG stage fruit (10). Droplets (10 n ) of test solutions (see below) were applied to the cut surface of discs and disc ethylene production was measured as described previously (11). The amounts of test materials used were based on colorimetric assay (6) of uronic acid content. 

Fig. 4. Ethylene production by MG pericarp discs treated with 50 /rg (uronic acid equivalents) of partially purified G7 citrus oligomers (a) or 30 rg of individual B fruit oligomers purified by HPLC. Water was used for the control. Peak numbers correspond to those shown in Fig. 2. Bars indicate SEs for the means of measurements of 8 discs/treatment.

If the endogenous material is a participant in the initiation of ripening, however, it is unlikely the fruit PG is responsible for its origin. Several studies (e.g. 8, 20) have shown that tomato fruit synthesis of PG follows (by as much as a day or more) the increase in fruit ethylene production that is accepted as a marker of ripening s onset. Of course, ripening is not a simultaneous, whole fruit phenomenon. It is a developmental event the passes through the fruit (its different tissue areas, perhaps even from cell to... 

The synthesis of endo-PG occurs in the ripening stage after an increase of ethylene production [21] and its appearance has been correlated with an increase in soluble pectin and softening [22]. Exo-PG is suggested to participate in the initiation of climacteric ethylene production [23]. Strawberry fruit has been accepted to be a non-climacteric fruit and ethylene... 

In spite of the often constitutive activity of AGO in the majority of plant tissues, an increase in its activity may regulate ethylene production especially associated with ripening and senescence of leaves, fruits, and flowers (see Sections 5.04.2.3 and 5.04.4.2.3, and Figure 3). 

Ethylene is now considered to be one of the main plant-hormones involved in fruit development. Many responses formerly believed to result from the presence of auxins are now ascribed to induced ethylene production.425 The biosynthetic pathway for formation of ethylene from methionine, in a wide variety of plant tissues, including shoots of mung bean,426 tomato,427 and pea427 carrot427 and tomato428 roots and the fruits of apple,429,430 tomato,427 and avocado,427 has been elucidated, and is as follows. 

Galacturonanases in the parenchyma have been shown to be wall-bound in peach (both endo- and exo-enzymes),625 apple (exo-enzyme only),621 tomato (endo-enzyme),499 and pear (unspecified activity).641 Increases in the level of polygalacturonase activity during ripening have been demonstrated in tomato,499,642-644 avocado,615,633 peach645 date,636 cucumber,631,646 and pear,641 and, in tomato,644 avocado,615 and cucumber,631 this increase is associated with the respiratory climacteric following a transient burst of ethylene production. In the mutant rin tomato fruit, which does not ripen, this increase in galacturonanase does not occur.644... 

The concentration gradient across the fruit skin was related by Burg" to the rate of ethylene production by a factor of 2 ppm/pl./kg./hr., and the response was related to the log of the internal content of ethylene. 

The production of ethylene in fruit tissue and in small amounts in leaves may justify its consideration as a hormone, functioning in the gaseous state, Cherimoyas and some varieties of pear produce 1000 times the effective physiological concentration. Ethylene formation is closely linked to oxidation and may be centered in the mitochondria. Its effects are to promote cell-wall softening, starch hydrolysis, and organic add disappearance in fruits—the syndrome known as ripening. Ethylene also decreases the geotiopic responses of stems and petioles. 

The tolerance limitation of fruit for irradiation establishes the maximum acceptable dose. If this dose controls decay organisms, the use of irradiation for a particular fruit may appear promising. Response to irradiation may be influenced by fruit maturity, variety, pre- and postharvest temperatures, handling, and extent of fungus growth. Climacteric fruits irradiated prior to the normal rapid increase in respiration usually show an immediate increase in respiration and the production of ethylene. These fruits are frequently retarded in ripening. 

Fig. 4. Ethylene production by wounded leaves and ripening fruit of normal tomato plants and those transformed with EFE (pTOM 13) antisense genes. A, Ethylene production by wounded leaves of plants with none ( ), 1 (O), or 2 ( ) antisense genes B, ethylene production by ripening fruit from plants with none ( ), 1 (O), or 2 ( ) antisense genes (after Hamilton et al., 1990).

Feedback Relationship Between Ethylene and Other Plant Hormones If ethylene production in ripening fruit is an index of aging and senescence, then its suppression should result in retardation, or antagonism to ripening, aging, and senescence. 

---