Many Factors Can Affect Almond Size And Quality

This image shows a cross section of a developing nut during stage two. This period usually lasts about a month and is completed by late May or early June. Photo credit: David Doll

This image shows a cross section of a developing nut during stage two. This period usually lasts about a month and is completed by late May or early June. Image is sourced from Integrated Pest Management for Almonds (UC ANR Publication 3308, pg 6.)

After harvest, Extension offices often receive almond kernel samples from farmers. With the exception of a few snacks, most samples show obvious defects and are brought in with the question “Why?” This is often difficult — and sometimes impossible — to answer, as each farmer’s practices and growing season differs. If the farmer, however, has a good understanding of the challenges faced in the past year, we can usually narrow it down to a time when the damage might have occurred. This is possible if the nut has defects that are characteristic of deficiencies or stresses experienced at certain stages of almond development.

Development Stages

Advertisement

There are three stages of almond fruit development. Stage one is characterized by the rapid growth of the hull, shell, and integuments. Although the hull and shell are known and easily identified, the integument is the white enclosure that forms around the embryo and develops into the pellicle or skin. This rapid period of growth occurs during the first two months after almond bloom, and typically is completed by the end of April or early May. It can be lengthened or shortened depending upon the weather conditions; high temperatures reduce the time in stage one, cool temperatures lengthen the period. For the most part, the almond remains soft even though the fruit and seed reach maximum size during this period.

Stage two includes the process of shell hardening and embryo (kernel) expansion. This period usually lasts about a month and is completed by late May or early June. The shell will harden, and the kernel will begin to fill with the nucellus (watery jelly), becoming translucent at the apical end with the formation of the endosperm. As the endosperm continues to develop, the white, opaque embryo begins to form. The embryo continues to expand through May, reducing the nucellus and endosperm, and pushing it outward against the skin of the almond. At the end of stage two, kernel dry weight begins to accumulate and the shell fully hardens.

Top Articles
Have a Plan For Climate Change? Why Fruit Growers Need To Act Now

Stage three is post-kernel fill through harvest. During this stage, the cell development of the hull, shell, and kernel is complete, and dry weight of the hull and shell do not change. The kernel, however, will continue to accumulate photosynthate as long as the vascular bundles remain connected to the nut. The carbohydrates stored within the kernel are converted to fats and proteins, and as the almond enters into hull-split, the process of ripening begins. The kernel remains affected by the tree’s status until the vascular bundles are broken by either the formation of the abscission layer or the harvesting process.

Size/Temperature

Through understanding the stages of almond growth, farmers can help determine why various conditions influence kernel yield, quality, and harvest timing. For example, as observed this year, almond kernel size was negatively affected by higher than normal heat in the spring. Heat reduces the amount of photosynthate that can be diverted toward the developing fruit. This is due to photosynthesis being maximized between temperatures of 65°F and 90°F while respiration rates generally double in response to every 18°F increase in temperature.

In other words, photosynthesis is maximized as long as the temperature is above 65°F, but respiration rates continue to increase as temperatures increase. Therefore, when warm (greater than 75°F) days are experienced in March and April, more photosynthate is needed for respiration and less is directed toward crop development during the critical period of stage one, leading to a smaller kernel. This process also compresses stage one of growth, reducing the number of cells that have divided, and shortens the growing season.

On the contrary, cooler springs will lengthen the season and increase nut size. A good example of this was in 2010 when almonds of the Fritz variety were being harvested in late October/early November. Although the late harvest was unwanted, kernel size was larger than normal. In 2010, the spring time degree-days (40°F minimum, no maximum) were the lowest in 10 years, in contrast to this year, which was above average.

Don’t Stress Too Much

Tree growth conditions can also impact kernel quality in stage two. Water stress or disease can create conditions that lower the quality and reduce the weight of the mature nut. These nuts are often fully or partially shriveled at harvest. In some cases, excess vigor during this period may lead to rapid kernel development, which pushes against the developing shell, rupturing tissues. This can lead to gumming on the exterior of hulls, an aborted nut, or a weakened shell seal.

Stress during stage three can reduce kernel weights. This timing comes during the hottest part of the year when there is high demand for water. Drying the trees out too much can cause kernel shrivel. This often gives the nuts a “textured” look. Furthermore, as many farmers approach harvest, trees are stressed to aid in the harvesting process which can worsen the problem. The difficulty lies in trying to apply enough stress that it aids in the harvesting process, but not severely stressing the trees. This severe stress can also create stick-tights due to preventing the formation of the abscission layer.

Every year presents a different set of challenges. Being aware of the weather conditions and stresses at certain stages may help explain or reduce damage experienced at the end of the year. Although many of these factors are out of our hands, being able to adjust orchard practices to help manage these conditions can lead to a more successful, larger yielding harvest.

6