Reduced Kernel Size - 2021 Harvest

Horticulture

Sep 26

There have been several reports of smaller than expected nut size for the 2021 harvest. Nonpareil sizes have been reported to be in the range of 28-30/ounce rather than the typical 24/25 nuts per ounce. Many have suggested that this is due to drought induced water shortages, but this has occurred even on fully irrigated orchards. Water stress has played a role, but earlier conditions of the season should also be considered.Nut weight is influenced by a few different factors. These include:

Reduced number of cells;
Reduced cell size;
Reduced cell weight.

Cell number is mostly impacted by temperatures that occur during cell division, which occurs early in the season, typically within 6-8 weeks following fertilization. Cell enlargement is influenced by plant stress. It begins in April and continues through May. Lastly, weight accumulation begins in May and continues until early August. It is influenced by plant stress, which reduces the production of carbohydrates.

To maximize nut size, carbohydrates (sugars) must be available during these periods for cell building processes. Carbohydrate availability is influenced by the rates of photosynthesis and respiration. These two processes are heavily influenced by plant temperature, which are influenced by ambient air temperature and water availability.Temperature influences photosynthesis and respiration. Both processes increase rapidly as temperatures warm, and begin to drop as temperatures increase above a certain point. The peaks, however, are different, with 86F and 103F for photosynthesis and respiration, respectively (figure 1). Since these peaks are different, there is a net gain of carbohydrates when temperatures are below 86F, but a net loss as temperatures increase above this point. Furthermore, as temperatures warm towards 86F, there is more energy being directed towards respiration, which reduces availability to be used for other processes, like nut development and plant growth.Internal plant temperature is influenced by two factors: ambient air temperature and plant-water availability.

Figure 1: The effect of temperature on the rate of photosynthesis and respiration. Adapted from The Almond Production Manual.

Ambient air temperature and plant temperature are relatively correlated. The exception is as temperatures increases, transpiration (of water) will provide an evaporative cooling effect, reducing internal temperatures. If, however, water stress occurs and transpiration slows due to stomatal closure, the plant tissue temperature will increase, leading to reduced carbohydrate production AND increased respiration rates. Furthermore, during some periods of the season, air temperatures are too high to be offset by transpiration, leading to a net loss of carbohydrate from elevated rates of respiration.It is thought that the basic processes of respiration and plant growth (shoot and root) utilize 90-95% of the total carbohydrate produced through the season. The remaining is used for fruit development during the growing season, and carbohydrate reserves and flower bud development in the post-harvest and dormant period. This is why plant stress during the season impacts the season’s nut production, while postharvest stress impacts future years growth and production potential.

So, what was different in 2021? Spring temperatures were much warmer than normal. Average maximum temperatures ranged from 3-5 °F above the 100-year average for the Central Valley during the months of April and May (Table1). The higher temperatures experienced at this point increased internal plant temperatures, increasing the amount of energy needed for respiration (Figure 2). This resulted in less energy being directed for nut development and growth. In many orchards, this was further exacerbated by water shortages that occurred in the mid- to late season, which led to less availability of carbohydrates to be used for the creation of fats, influencing cell weight.