DA Meter Assessment of Apple Maturity: Myths, Realities and Challenges

There has been much hype about maturity assessment of tree fruit using the DA meter developed at the University of Bologna, Italy and distributed by T.R. Turoni S.R.L. While the use of the instrument has been relatively straightforward for peaches and pears, the story has not been so simple for the apple. However, with continued research, we are beginning to get a better understanding of issues regarding mode of use, variability of the fruit and relationship of IAD values (generated by the DA meter) to other maturity indices such as internal ethylene, firmness and starch clearing.

One of the issues causing difficulty for progress of adaptation for this instrument to apples is the myth that all maturation processes are synchronized for all varieties of apples. In fact, we are finding that the synchronicity of various maturation indices in Ambrosia® and a few other new apple cultivars can occur in some years, but not others. For example, for Ambrosia®, starch clearing index and IAD values were correlated in a similar manner in 2012 and 2013 in the Okanagan Valley, however the relationship was quite different in 2014 with starch clearing advancing more quickly than the decline in IAD values.

It is important at this point to discuss ripening indices and just what IAD values represent in regards to maturity. While some claim that IAD values correlate to firmness, that is generally considered to be a secondary association, i.e. firmness changes during maturation parallel changes occurring with IAD values but the two processes are not directly linked with each other. It has to be emphasized that all IAD tells you is how much chlorophyll is in the peel of the apple - nothing more. Johnston et al. (2009) published a very good paper discussing how ethylene might regulate different ripening process in apples. In essence they state that flesh softening and peel colour change (i.e. loss of greenness in the background colour) are processes having stronger dependencies on ethylene concentrations, but neither process is very sensitive to ethylene (i.e. you have to have high ethylene levels to induce the change). In contrast, starch degradation and loss of acidity are not strongly dependent on ethylene levels, but changes in these indices are induced by very low concentrations of ethylene. However, there can be other environmental and biological factors influencing these four maturity indices that occur during ripening. In regards to biological factors, many of the new apple varieties do not produce a lot ethylene until well after they are mature for harvest (e.g. 'Fuji', Ambrosia®, Aurora Golden Gala™, Salish™, etc). As a consequence, the coordination of ripening events seen for older cultivars, which show an earlier climacteric rise, does not necessarily apply with the new cultivars. In a cultivar such as 'Gala', the internal ethylene concentrations, starch clearing and IAD indices are fairly well coordinated and so IAD may well be used as a surrogate for internal ethylene or starch clearing, however year-to-year variability in this correlation has yet to be assessed. The operational word is "surrogate", which emphasizes that the correlation is not a true estimate for the other two measures of maturity, it only works as long as the other two indices change in coordinated manner with the decline in IAD index (i.e. the loss of chlorophyll as the background colour of the peel turns from green to yellow).

The picture can get more complicated with the use of treatments such as Harvista™ in the field. Since Harvista™ blocks ethylene action and catalytic production, it blocks the changes associated with ripening induced changes in ethylene production. As a consequence, if Harvista™ is applied, then the relationship that existed between internal ethylene concentrations and IAD in untreated fruit will disappear. Researchers at Dr. Chris Watkins laboratory at Cornell University reported this observation at the International Horticultural Congress in Brisbane this past August. The disassociation of IAD with internal ethylene in Harvista™ treated apples is supported by the understanding developed by Johnston et al. (2009). Peel chlorophyll loss was not influenced by the internal ethylene in the apple, since that process has low sensitivity to ethylene any ethylene produced internally would rapidly dissipate, and not accumulate to induce chlorophyll loss of the peel even in a conventional early climacteric type apple (e.g. 'Gala'). Since ethylene was not an issue in the untreated apple, treatment to block ethylene would not have any effect on peel degreening. We have made a similar observation when measuring IAD values over time in storage. SmartFreshSM has no significant effect on the rate of chlorophyll loss (i.e. decline in IAD values) in air or CA storage (Toivonen and Hampson, 2014).

The question that might be raised is whether the DA meter is of any value for maturity assessment. I believe the answer is yes, but the users need to understand what the measures are telling us. In general the measures are telling us how the peel itself is maturing on the tree. That information is useful for understanding susceptibility in storage to peel-related disorders, not firmness, not starch clearing, etc. Our data to date is showing that two peel-related disorders that can affect Aurora Golden Gala™ in some years, soft scald and lenticel breakdown, are associated with harvesting the apples at a very low IAD value (i.e. advanced peel maturity). Starch index has been found not to be as sensitive as IAD and since this apple shows a very late ethylene climacteric that index cannot be used to assess susceptibility to these two disorders. Similarly, IAD may be useful to assess maturity in Ambrosia to reduce risk of soft scald in years where harvest maturity can mitigate the disorder. DeLong et al. (2014) show a similar relationship in 'Honeycrisp'.

However, even with this understanding, there are yet a few other issues that must be reconciled for effective use of the DA meter, whether it is used as a surrogate for other maturity indices or as a direct indicator of peel maturity. First and foremost, the supplier of the instrument and some researchers underestimate the influence of stray sunlight on the accuracy of the measurements in the field or in a sunlight-exposed window of a lab. In our early work we realized that since this is a light-based instrument, that stray light impinging from the sun or from a bright lamp can alter the reading. Why? The DA meter measures the difference in absorbance of two wavebands of light 670 and 720 nm and this difference is used to calculate the IAD value or "relative chlorophyll content". Sunlight has significant amounts of a wide range of light wavelengths and these overlap the two used by the instrument - therefore the apple peel section being measured has to be shaded from sunlight exposure during measurement. We have designed a light shroud from soft rubber and attach it to the instrument with Velcro® (see Figure 1). The shroud-fitted DA meter will allow reliable measurements in the field. We have three years of experience with this configuration in research plots and commercial orchards.

Figure 1: A light shroud-fitted DA meter in use in an orchard

Figure 1: A light shroud-fitted DA meter in use in an orchard

Chlorophyll physiology must also be considered in order to get reliable measures of maturity using this instrument. Chlorophyll content of the peel has been recognized as an indicator of maturity Blanpied and Silsby (1992), however that work did not consider the use of an instrument such as the DA meter, which only measures a very small area of whole apple (~ 1 cm diameter). Chlorophyll content of any one section of the peel can be affected by many factors including light exposure levels, excessive light (i.e. sun damage), and disorders on the peel. While many like to make measures on two sides of an apple using the DA meter, we have concluded that measuring one side of the fruit is a better approach in order to reduce variability. Please refer to the report on the Summerland Varieties Corporation website regarding the variability encountered when measuring two sides (Toivonen, 2011). The intent of the use of the DA meter is to monitor declines in peel chlorophyll that are only related to maturity change. Chlorophyll production is very much influenced by light; therefore a deeply shaded side of an apple (a side with little or no blush colour) will have less chlorophyll at any time than a more light-exposed side of the apple. Therefore, it is important to avoid using the shaded side for making measures. In contrast, the side of the apple which is exposed to direct sunlight will likely suffer lethal or sub-lethal sunscald or heat injury. Such injury causes chlorophyll bleaching which may not be visible to the naked eye, but results in much lower chlorophyll values than on sides not exposed to direct sun. The best, most reliable place to measure the apple is a side not exposed to direct sun or deep shade, preferably on the north-facing side of the fruit. It is important to always measure on the same side of all fruit measured. Location on the tree, or, of the tree in a block can also influence the readings. Apples at the top of the tree can have different chlorophyll levels than those lower in the tree if there is significantly more foliage in the lower part of the tree. An outside row of a block will also have different chlorophyll levels than a tree in a block due to higher light levels on the outside of a block.

Keeping all of the cautions in mind, it is possible to get reliable and repeatable measures of IAD values that can allow you to measure peel maturity in apples. In some varieties and some cases (when Harvista™ is not used), the IAD values can be used as a surrogate for other maturity indices. But you need to evaluate each variety and situation before moving ahead. This brings me to my final point, there needs to be more research to evaluate use of this instrument on a case by case basis, until our knowledge has improved well beyond our current understanding.

References for further reading

  • Blanpied, G.D. and K.J. Silsby. 1992. Predicting Harvest Date Windows for Apples. Cornell Cooperative Extension Information Bulletin 221, 12 pp.
  • DeLong, J., Prange, R., Harrison, P., Nichols, D. and Wright, H. 2014. Determination of optimal harvest boundaries for Honeycrisp™ fruit using a new chlorophyll meter. Canadian Journal of Plant Science 94: 361-369.
  • Johnston, J.W., K. Gunaseelan, P. Pidakala, M. Wang and R. J. Schaffer. 2009. Co-ordination of early and late ripening events in apples is regulated through differential sensitivities to ethylene. Journal of Experimental Botany 60: 2689-2699.
  • Toivonen, P.M.A., C.R. Hampson 2014. Relationship of IAD index to internal quality attributes of apples treated with 1-methylcyclopropene and stored in air or controlled atmospheres. Postharvest Biology and Technology 91: 90-95.
  • Toivonen, P.M.A., Mostofi, Y., Wiersma, P.A., and Hampson, C.R. 2012. Evaluation of Non-Destructive Instruments for Assessing Apple Maturity and Quality: 2011 Results. Agriculture and Agri-Food Canada (AAFC)/Agriculture et Agroalimentaire Canada (AAC), 21 pp. Available at: www.summerlandvarieties.com/research_and_development

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca