Research on fruit production under deficit irrigation has become increasingly important as the value of water as a resource is recognised and the impact of excess irrigation on groundwater contamination becomes apparent. A series of experiments investigation the effects of plant water status on fruit composition, growth and water relations of ‘Braeburn’ apples was conducted between 1993-1995. A primary focus being the evaluation of water deficit, induced at various times of the season, on fruit quality attributes. In general plant water status was reduced in trees given deficit irrigation treatments. Specifics of deficit irrigation treatments are available upon request.

Fruit quality is strongly influenced by fruit maturity indicating a need to evaluate the influence of deficit irrigation on the latter. Numerous methods of fruit maturity evaluation are currently used for apple. These include measurement of total soluble solids (TSS), blush and background colour, and ethylene evolution. Although both TSS and colour are widely used maturity indices for apple some researchers believe that these attributes tell us little about the physiological maturity of the fruit and are more useful as indicators of commercial maturity. Commercial maturity indicates the quality of fruit for eating whereas physiological maturity may not. For fruit to be physiologically mature they must be able to continue ontogeny even if detached from the plant. The climacteric rise of fruit ethylene evolution is a good indicator of fruit physiological maturity. Physiological maturity become important if fruit are to be stored as picking prior to the climacteric is desirable. It is suggested that the onset of the climacteric rise may be an effective marker to indicate the closing date for commercial ‘Braeburn’ harvests.

Fig 1 Placement of vial for non-destructive ethylene measurements of apple fruit.

Measurements of fruit ethylene evolution has been extensively studied in recent years with several methods being developed. In the first experiment ethylene evolution was evaluated on fruit harvested that morning by taking a gas sample from the core cavity using a gas tight syringe and injecting this into a portable photoinonization gas chromatograph. The following season ethylene samples were collected non-destructively. A modified glass vial was attached to the fruit surface while the fruit was still on the tree (Fig 1). This vial was left in place for at least three days to allow the ethylene concentration within the vial to equilibrate with that of the fruit. A 100 µml gas sample was then removed through the rubber septum using a gas tight syringe and injected into the gas chromatograph. Values of ethylene evolution recorded using these two methods are compared along with the evaluation of ethylene evolution differences between irrigation treatments.

Data collected during 1993 indicated that reduced plant water status during later stages of the season increased TSS and red colour in ‘Braeburn’ apple fruit thus indicating an advancement in commercial maturity. Despite differences in the commercial maturity of these fruit no difference in ethylene evolution was apparent between treatments (Fig 2). Increased ethylene evolution as the season progresses was recorded in both treatments. Similar results were recorded in the following season using the non-destructive technique for measuring ethylene. Total soluble solids were also increased in this second experiment again indicating that commercial but not physiological maturity has been advanced. Fruit colour was not evaluated in the second experiment as colour development was poor in all treatments and may have been due to lower light levels in the greenhouse compared to the field. A lack of difference in physiological maturity, as indicated by ethylene evolution, between deficit irrigated and control fruit is in conflict with other published data. However, I believe that this may be due differences in ethylene evolution between apple cultivars. Braeburn show low levels of ethylene evolution compared to other cultivars.

Fig 2 Mean ethylene evolution throughout the season for experiments conducted during 1993 (A) and 1994 (B). C1 = control, D1 = deficit irrigation, C2 = control, D2 = deficit irrigation, LD2 = late deficit irrigation for experiments 1 and 2 respectively.

The implications of these findings are as follows. Firstly, the method of ethylene evolution has little effect on the total amount of ethylene evolution recorded. Secondly, deficit irrigation which influences commercial maturity (TSS and colour) does not affect physiological maturity. This is an important point as an advancement of physiological maturity may be undesirable due to poor storage quality of fruit harvested after the climacteric rise in ethylene. The enhancement of commercial maturity may constitute an advantage as TSS is closely associated with fruit flavour. Enhanced fruit colour development is also advantageous as export ‘Braeburn’ fruit must obtain a minimum 40% red blush colour of the skin. This data highlights results presented by other researchers who show that ethylene as indicative of physiological maturity does not correlate well with commercial maturity indices such as TSS and colour.