Eaks and Ludi, 1 960; Willamen and Beaumont, 1 928), salt solution (Claypool,
1 938), buffer solution (Ulrich and Thaler, 1 952) or water (Gerhardt, 1 942;
Poapst
et al. ,1974). The gases evolved were entrained in a flow of inert gas
and trapped in alkali. However Fidler and North (1 971 ) reported that using
these methods they obtained inconsistent results. In a similar method, Jerie
et al.(1 979) removed previously loaded radioactive ethylene fro m tissue
segments by holding the tissue in a closed chamber over boiling water.
Cameron (1 982) in a series of experiments to test the applicability of the
method by Jerie
et al.(1 979) to sampling endogenous C2H4' found that the
amount of ethylene driven from the tissue increased with time of exposure to
the boiling water for up to 30 minutes. Cameron (1 982) found the method
unsatisfactory except for fruits such as apples which contain exceptionally
large amounts of internal ethylene. For other tissues tested, significantly
greater amounts of ethylene were found to be released than were extracted by
the vacuum extraction or direct sampling methods.
Most authors who have used t h is method h ave recognised that the a m o u nt of gas liberated from the tissue is far in excess of that o rigi nally contained in the intercellular space as determi ned by other methods (Burton,
1 950;
Den ny,1 947).
Although attem pts have bee n made to calculate theoriginal concentration of gases in the i ntercellular spaces (Burton,
1 950),
heat extraction methods a re not desirable for the esti mation of i nternal g as composition and skin resistance to gas diffusio n (Cameron,1982).
2.1 .2.6 Digestion method
In the estimation of the i nternal atmosphere, Denny
(1 947)
and Eaks and Ludi,(1 960)
attempted to digest tissue segments in strong alkali solution. The severity and i naccuracy of this method as well as the problems associated with g as solubility and calculation of original gas concentration i n the plant o rgan make this met hod unacceptable for the esti mation of i nternal g as composition and resistance coefficients (Cameron,1982).
2.2 Respiration metabolism
R e s p i rat i o n i s t h e metab o l i c process defi n e d as t h e oxi dative breakdown of complex materials such as starch, sugars and organic acids, to simpler molecules such as C02 and H20, with the concu rrent production of energy and other molecules which can be used by the cell for synthetic reactions (Hardenburg
et
al. ,1 986;
Forcieret11 987;
Wills et al. ,1 981 ).
Such metabolic reactions are esse ntial fo r maintenance of bioc h e mical processes, ce l l u l a r organ isation and m e m brane i nteg rity of livi n g cells. Maintaining the supply of adenosine triphosphate (ATP) is the primary purpose of respiration (Kader,
1 987).
Respiration takes place in the cells (cytoplasm, �itochondria) of tissues both in light and in the dark (Berrie
et
al. ,1 987;
Debney�\
al. ,1980).
The rate of respiration of pro duce is an e xcel l e nt i nd icat o r of t h e metabolic activity of the tissue and thus i s a useful guide to the potential storage life of the produce (Fidler and North, 1 97 1 ; Wills et al. , 1 981 ). Kader et al. (1 985) contended that the rate of deterioration or perishability of harvested commodities is generally proportional to their respiration rate. The respiration rate of a given com modity differs with plant part, cultivar, area of production, growing conditions and growing season (Hardenburg et al. , 1 986). According to Oebney et al. ( 1 980), if different types of produce are classified b y their botanical structure , there is a close relationship between structural type and respiration rate. High respiration rates are typical of young tissues such as growing points, (eg. asparagus), partly developed flower buds (broccoli, globe artichoke), developing seeds (green peas, green beans) and immature fruits (sweet corn). l-ow respiration rates are typical of storage organs such as roots (carrots, sweet potatoes), underground stems (potatoes), bulbs (onions) and mature fruits (apples). Intermediate respiration rates occur in u n ripe fruits (cucumbers, zucchini) and most leafy vegetables (Oebney et al. , 1 980).
Respi ration rate of a co mmod ity is dependent upon various facto rs related to the produce, which include type of commodity and genotype, stage of development at harvest, weight of commodity, and chemical composition. It is also dependent on environmental factors such as temperature, light, stress, 02' C02, carbon monoxide and C2H4 concentrations, and other hyd rocarbons such as propylene, acetylene (Oebney et al., 1 980 ; Hardenburg et al. , 1 986 ; Kader et al., 1 989). Respiration is also one of the i mportant factors affecting the i nternal at mosphere co mposit i o n of fruits, howeve r t h e re is l i m ited i nformation on the relationship between ,respiration and internal atmosphere
", a s
composition of apples. This relationship � been investigated i n the current study.
Respiration can occur in the presen ce (aerobic respiration) o r absence of 02 (anaerobic respiration, someti mes called fermentation) (Biale, 1 960a; Montgo mery et al. , 1 990; Forward, 1 965; Wills et al. , 1 981 ).
2.2.1 Aerobic respiration
Most of the e nergy required by fruits and vegetables is supplied by aerobic respiration. Aerobic respiration i nvolves a series of reactions, each of which is catalysed by a specific enzyme and involves oxidative breakdown of complex molecules (certain organic substances such as carbohydrates stored i n the tissue) to simpler molecules (Biale, 1 960a; ap Rees, 1 980 ; Forward, 1 965). Figure 2-1 taken from ap Rees ( 1 980) shows the principal pathways responsible for the respiration of carbohyd rate.
Socro� Starch
�
HeJOSe G'uck-1- pl
�
Glucose-6 ·P-+6·Phosphoguc�teFrUC
!
".
6.�
PRSa:-P
Fructose-l.6-diP Tr�e - Pl
3-F'hosphOglycerotel
CDz Phosphoenolpyruvate Oxoloocet te.
I \
AsporlOte Molote�
CDl Acetyl-CoA CilrateI'
\. .
Malate Aconltolef
1
Fumarote Iso(: It role
\
Succinate .. -Ketoglutarate
�SUCCinyl-CoA � C�
Fi g . 2- 1 . P ri n cipal pat h ways respo nsi ble for t h e res p i rati o n of carbohydrate (ap Rees, 1 980).