Physiology of Braeburn Maturity and Disorders: A Discussion
The data and thoughts presented here are based on research I have conducted over the past 6 years, my own observations, and the research and observations of others in other fruit-growing regions of the world. In large part, funding for these studies was obtained from the Washington State Tree Fruit Research Commission. In addition, I was ably assisted in technical assistance by Carol Ann Duplaga, Doris Frederic and Susan Pavelko.
I will discuss three things:
- The best maturity indices that describe the maturation process
- Calcium-related disorders
- Fruit behavior in both regular and CA storage.
After evaluating fruit maturity for more than 6 years, we have found that certain indices are much more valuable than others to define fruit maturity. First, in weekly harvests beginning about August 14 we found that although starch rating can differ quite a bit between years depending on the microclimate, in a given year it is the single best maturity index. Starch clearing provides a measure of the advancing maturity and depending on the rate at which this process occurs indicates whether the process is hastening or slowing. The important thing is that you have to monitor starch clearing weekly each year and not rely on the previous year. Nor can you assess a present season by an average from previous years. Starch progression is fairly linear during a single year but also may vary from orchard to orchard depending on microclimate. Nevertheless, starch clearing is the single best index upon which to base physiological maturity.
As far as background color rating (based on a 1 to 8 scale of 1=green and 8=yellow), again, from one year to the next, there can be as much as 1.5 to 2.0 units difference on any given date. The ground color, and in large measure the development of red color, depends mainly on weather patterns, e.g. a shift from warm weather to warm days and cold nights. Again, the rate of change gives an indication of the nearness of harvest. The ground color should be used secondarily to starch clearing.
Ethylene is the third maturity index considered in this discussion. In a previous report during this conference, I mentioned in "Delicious" one part per million ethylene is regarded as the threshold value that the fruit is advancing from maturity into ripening, e.g., the respiratory climatic. In our trials over several years, we measured ethylene 24 hours after it has been harvested and allowed to rest at room temperature. We found that internal ethylene concentration (IEC) rarely increased beyond 1 ppm. Moreover, the overall average of the block sampled never exceeded 0.5 ppm even when the starch was well beyond 3.5. Therefore, maybe we must redefine how we view ethylene in cultivars like Braeburn. It may well be that different cultivars have different thresholds. One ppm may be the magic number for Delicious, but for Braeburn, it may be that 0.5 ppm ethylene indicates movement from the physiological state of maturation to ripening.
I mentioned in a previous talk about taking a different approach to measuring ethylene because the internal measurement of ethylene (from the core) may not be physiologically meaningful. In Braeburn this is especially appropriate because the value generated may not be representative of what is actually going on within the apple because of low internal O2 and high CO2, which in many tissues inhibit ethylene. We are still investigating this.
Maturity and Internal Disorders
There is also a relationship between the stage of maturity, starch, water core, ethylene, and Braeburn Browning Disorder (BBD). In one year, we harvested fruit on October 21 and two weeks later, on November 4. On October 21, the starch was about 2.2 and two weeks later the starch was about 3.3. The water core in the earlier harvested fruit was 8%, and two weeks later, it increased to about 26%. The BBD at the first harvest, was around 3% and in the later harvest, it increased to about 10%. Even with these changes, the average IEC (Internal Ethylene Concentration) only changed from about 0.25 ppm to 0.37 ppm. Normally in Delicious when the starch levels are this different, we would expect to see dramatic changes in IEC, but in this variety, this is not the case.
There is a relationship between harvest date, cropload, watercore and BBD. Fruit harvested October 24 and examined immediately had roughly the same watercore regardless of whether it came from a tree with low, moderate or heavy cropload. 'Low' is defined as a crop of 25% to 30% of what is normal for that particular size and age tree. After storing this fruit for a month in regular storage, we found that watercore dissipated significantly, which is a good sign. Unfortunately, there was also an increase of BBD more so for the low cropping trees than for those with a high crop. This has also been noted in New Zealand.
In contrast with fruit harvested two weeks earlier, the later harvested fruit showed an increase in watercore both for the low and the high crop. Again, after a month in regular storage, the watercore decreased but was not totally eliminated. This is important for growers and packers. The amount of BBD was almost double in fruit from the low cropping trees than from high cropping trees. With later harvested fruit, however, after a month in regular storage, the BBD increased dramatically.
In summary, fruit from low cropping trees if harvested late are likely to show more watercore and BBD. Nevertheless, whereas the watercore may decrease somewhat, the BBD often increases.
The disorders associated with calcium deficiency in Braeburn are lenticel blotch pit, bitter pit and internal browning. We know they are associated with calcium deficiency because if we increase calcium usually we can reduce the disorder.
Two of the more common horticultural factors associated with calcium susceptibility are tree age and soil type. Young trees are susceptible to calcium disorders. This could be the result of something as simple as root growth. For example, when a young tree is planted, the tendency is to fertilize heavily to emphasize top growth. Possibly, the root to shoot ratio swings in favor of the shoot growth thus requiring more of root growth for nutritional support. If the root system is undersized, or the root/shoot ratio is less than optimal, the tree may be unable to absorb enough calcium to support the fruit the tree is holding. From my own observations, sandy, drought prone, and/or shallow soils are restrictive to root growth resulting in the same scenario as that mentioned above for less than optimally functional root systems. Where excessive tree vigor is a problem and cropload is low, the situation worsens because instead of one, we now have two competing sinks for root-supplied calcium. Vigorously growing shoots pull water and calcium with the transpiration stream. Fruit, too, are a calcium sink. However, they are also capable of being a source to the shoots when water availability is low. In the competition for calcium, single fruit on a spur close to a vigorously growing shoot is the loser.
To improve this condition, crop heavily and thin spurs instead of clusters. As you double the number of fruits per spur, less calcium is drawn from each apple by nearby shoots and, therefore, each is less likely to develop calcium deficiency. Control tree vigor by balancing nutrition and cropping, relying mainly on wood that is two or more years old. Manage nitrogen for proper root/shoot ratio, which means don't try to push the top part of the tree beyond what the root is capable of supplying. It has also been reported that calcium competes with potassium and magnesium, and it has been shown that high potassium soil generally results in fruit with higher BBD. If you consistently apply potash onto the soil, the condition will worsen. Attempting to develop low potassium soils would be advantageous. If you have high potassium soils, you may want to consider replanting to another variety not as susceptible to calcium deficiency disorders. Certain environmental conditions also favor high-risk fruit. These may include winter injury or winter stress, high temperature stress during the summer, drought, and cold spring weather.
New Zealand's early experience was that if they cooled the fruit in 48 hours by cold forced air, and then kept the fruit well ventilated and cold for 2 to 3 weeks, they would generally have less BBD. We still don't understand why this period of ventilation is necessary and how it affects the flesh. However, if we ventilate the fruit for 3 to 4 weeks before we do anything to modify the external atmosphere (waxing, packing, etc.), it will be advantageous. For example, if you bring fruit in from the orchard, wax and pack it and put it away in storage, chances are very high that it will develop BBD. It makes little difference if the fruit is brought in from the field at 40°F or 60°F. It is important that you ventilate the fruit--that you move air over and around the surface of the fruit.
A couple years ago workers in my lab conducted some experiments designed to either delay cooling before CA, or delay CA after cooling. We either rapidly cooled the fruit to 34°F in two days, or in 3 weeks at the rate of about 5 or 6 degrees a week starting at 50°F. It is important to know that our CA system is research oriented. The units are the size of two apple boxes and the atmosphere is only adjusted when necessary. This is not considered a ventilated system--semi-static. The results showed that if we cooled the fruit immediately and put it into CA we got significant internal browning. However, when we cooled it for two days followed by another 3 weeks ventilation in regular storage and then put it into CA, the BBD was reduced to zero. In the slow cooling regime where we essentially ventilated it for 3 weeks and put it into regular storage, BBD was 3%. On the other hand, when we cooled slowly directly in the CA environment BBD was 8%--more than double. Thus, it is the air movement--the constant circulation of oxygen containing air that reduces the BBD. Any increase in CO2 in the external environment is likely to worsen the browning.
With regard to storage temperature, we found that generally under low oxygen (1%) and high CO2 (2%), lowering the temperature from 36°F to 33°F will worsen internal browning and cavitation.
The best conditions in our experiments have been CO2 at 0.5% or less, and oxygen at 2%. New Zealand uses 3% for their oxygen level so I think anywhere between 2 and 3% oxygen will give you pretty fairly low BBD. Waxing increases BBD during the high-risk period (1 month following harvest) and, although we haven't done the experiments, I think that after you get beyond this first month of high susceptibility, you can probably wax normally. These trials are planned for this year. Don't pack and hold. That is, don't bring the fruit in, pack it and store it or you are setting yourself up for a lot of BBD.
The BBD we see on the tree at harvest, is generally the dense, dark brown kind. Sometimes these spots are small and sometimes they are large and consume the body of the flesh. We are not sure what causes this, but I think it is probably just the natural propensity of the variety and I'm not sure that we can do a whole lot about it. Some orchards have more than others so there must be a reason, but we haven't been able to identify the causative factors. We also see internal browning at 4 to 5 months coming out of the CA. We can examine the fruit in CA monthly but at about 4 to 5 months we begin to see this wave of browning. Usually it is a little less dark and a little drier or pithier. These different types of BBD are all brown, but I think they are different based on the growing conditions as well as the length of time and condition under which they are in storage.
Recently some researchers in New Zealand reported that DPA applied to fruit as a drench at harvest reduced BBD. In our preliminary trials this past year we used DPA on early and late harvested fruit. In control fruit harvested early (starch 1.7), the amount of BBD was about 9%. With a drench of DPA at 1350 ppm, BBD was reduced to about half. On the other hand, if we put on an oil wipe, which prevents oxygen from reaching the fruit, we dramatically increased internal browning.
If we waited until the starch was almost 3.0, and we repeated the experiment, we saw that the control developed about 22% BBD, whereas a DPA drench reduced it by less than half as we saw in earlier harvested fruit. Again, if we put oil on the fruit by wiping it over the surface, BBD increased significantly. DPA looks like it will help quite a bit at reducing internal browning, but it will probably depend on some other factors, such as how the fruit is treated, the harvest maturity, and so on.
As a final statement, I'd just like to say that Braeburn is a variety in which horticulturists, marketers, and warehouse people must work together. You have to know what kind of fruit you have. You must gauge each lot as to whether the risk of BBD is high or low. If you harvested early all of the low risk fruit, that is, the fruit on heavily cropping trees planted on good soil and subject to adequate light, you would get perhaps 2 to 3% BBD. You can then harvest fruit from the high risk trees and you might get 30 or 40% in your worst fruit. However, if you harvested all the fruit together, you are likely to see 15 to 16%, which might disqualify the entire lot. So, you may as well harvest your best fruit with the lowest risk of developing BBD or watercore, and market it separately. Segregate you fruit according to high and low risk, and market only your best product.
Dr. Eric A. Curry, Plant Physiologist
USDA, ARS Tree Fruit Research Laboratory
1104 N. Western Ave., Wenatchee, WA 98801
14th Annual Postharvest Conference,
March 10-11, 1998