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Home HortScience Nitrogen's Effect on Apples Reported
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ITHACA, NY—When choosing an apple for purchase, most consumers check the fruit's skin for color and bruising. Apple growers assess some of the same criteria to determine when to harvest fruit. Growers must also understand the effects of fluctuating nitrogen levels, which can affect apple color and firmness in different ways. Cornell University Department of Horticulture researchers conducted a study to determine how nitrogen supply affects the breakdown of flesh starch relative to the development of skin color in 'Gala' apples. The results, published in HortScience, show that increasing nitrogen delays the reddening of the apple skin but accelerates the degradation of the flesh starch.

Prior to budbreak in 2007, scientists Huicong Wang and Lailiang Cheng identified trees for the experiments. Each tree was given one of four concentrations of nitrogen twice a week from the time they bloomed until 2 weeks before harvest, except during active shoot growth when trees were treated three times per week. All trees were well watered and treated for disease and insect control in the same way. All of the trees were thinned by hand so that each tree maintained the same number of fruit through harvest.

Three weeks before harvest and at harvest time, five apples from each tree were evaluated for color on both the shaded and exposed sides of the fruit using a special tool called a chroma meter, which quantifies the amount of redness. Forty apples were picked from each tree at harvest and divided into five groups of eight. One group was used to evaluate various criteria of quality. The remainder were stored at near-freezing temperatures and evaluated every 5 weeks for a total of 20 weeks.

Instruments were used to measure the firmness of the fruit while juice was extracted to determine the concentration of soluble solids.

Leaf samples were taken at 90 days after bloom to measure the amount of nitrogen they contained. Leaves that were given the highest nitrogen concentration contained twice as much nitrogen as leaves given the lowest levels. However, the amount of nitrogen in the fruit was proportional to the concentration of nitrogen applied to each tree. The trees given the least amount of nitrogen yielded the smallest fruit, but also the firmest.

"Fruit firmness tended to decrease with increasing N [nitrogen] supply," the study reports. Fruit starch broke down faster with increasing N supply. The accelerated starch breakdown in high N fruit may have contributed to their higher soluble solids concentration at harvest, but the soluble solids concentration in these fruit showed a decreasing trend during cold storage. In contrast, apples from trees with the lowest N concentration had lower soluble solids at harvest, but soluble solids concentration tended to increase from harvest to 105 days after harvesting and only declined slightly afterward.

Trees receiving the third-highest amount of nitrogen yielded the most fruit and largest apples. The reddest apples came from trees given the lowest concentration of nitrogen.

The results of this study are similar to previous findings in that higher levels of nitrogen slow the reddening of apple skin. The researchers noted that this could be related to high nitrogen levels' tendency to increase shoot growth and therefore shade the fruit. "However, because only fully exposed fruit on the south side of the tree canopy were used in the current study, this indirect effect would be small for the fruit measured, if any."

"Increasing N supply delays skin red color development but accelerates flesh starch degradation in 'Gala' apples. These differential effects of N supply should be taken into account when assessing fruit maturity for optimizing harvest time", said Wang and Cheng.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

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Original Article:

Differential Effects of Nitrogen Supply on Skin Pigmentation and Flesh Starch Breakdown of 'Gala' Apple
Huicong Wang and Lailiang Cheng
HortScience 46:1116–1120. [Abstract][Full Text][PDF]

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