Department of Horticulture
University of Georgia
Athens, Georgia 30602
Pecan fruit growth occurs in three distinct stages: elongation, expansion, and kernel development. The purpose of this article is to discuss the stages of elongation and expansion. These are the stages of fruit growth that determine nut size.
June
through mid-August is the major nut sizing period in the southeastern United
States (Fig.1). There is some variation in the time from
year to year and more variation from early vs. late maturing cultivars. In
spite of these variations, generalizations can be made.
June is the month of fruit elongation. During this month much of the final length of the nut is determined. The fruit becomes an elongated cylinder with diameter being much less than length.
July is the month of major fruit expansion. During this month, the elongated cylinder increases in diameter. Nut size is fixed by mid-August when shell hardening has begun. The nut does not continue to increase in volume once hardening has occurred.
The fruit continues to increase in size after shell hardening. However, this increase in fruit size is due to an increase in shuck thickness, not in nut size.
Nut size or volume in
pecan is primarily a function of soil moisture. The relationship of
size to soil moisture is illustrated in Figure 2. The relationship
is especially striking considering each data point is a different year.
The final size and shape of the nut depend on the availability of soil moisture during the elongation and expansion stages of the sizing period. Optimum rainfall or irrigation throughout the sizing period causes maximum nut elongation and expansion. The net result is a large and normal shaped nut. Soil moisture deficit throughout the elongation and expansion stages causes minimum nut elongation and expansion. The result is a nut with a normal shape but small. A soil moisture deficiency during the elongation stage, but optimum during the expansion stage results in a short, fat nut, that is, roundish. On the other hand, when soil moisture is optimum during elongation but deficient during the middle of the expansion stage, the basal or stem end of the nut does fully expand relative to the blossom end. The result is a nut with an obovate shape or the nut is said to have a pinched butt. Variation from normal size and shape of the mature nut can easily be used to track the soil moisture conditions that existed during the two stages of nut sizing.
Large nut size is desirable in the southeastern United States because of the premium paid by the market. However, a large nut is sometimes a detriment to kernel quality, because a large nut is more difficult to fill with a high quality kernel than is a small nut. Such is the case simply because more energy is required to fill a large nut than a small nut. The inverse relationship between nut size and kernel development is often apparent in nuts from cultivars versus seedlings. The small nut of seedlings is usually better filled than the larger nut of cultivars. Among cultivars, nuts from cultivars that have a small nut, for example Elliott, usually have better quality than cultivars that produce a large nut such as Mohawk and Mahan.
Large nut size becomes a detriment to quality primarily when there is a full or excessive crop and soil moisture is less than optimum during the period of kernel development, primarily September. Thus, in years of high or excessive production, a small nut is an advantage and especially in orchards with marginal soil moisture during kernel development.
The importance of a small nut in a year of excessive production was illustrated during 1993 when Georgia produced a record of 150 million pounds. A drought during the nut sizing period resulted in a small nut. Quality was acceptable in spite of the high production and moderate soil moisture during kernel development. A large nut would have resulted in a disaster in most orchards and especially in non-irrigated ones.
Because nut size is mainly a function of soil moisture, growers with irrigation can regulate nut size to varying degrees depending on the amount and timing of rains. Water management should be based on crop load and type of irrigation. If the crop is off, but still commercial, the objective should be to irrigate for maximum nut size in order to make up for less than desired number of fruits per tree. There is little risk in maximizing nut size on trees with a low crop load. A large nut usually can be filled if production is off even with marginal soil moisture during kernel development. In fact, a small nut during a low production season may split if soil moisture is optimum during kernel development. The nut splits near the time of nut maturity and does so because the nut is not large enough to accommodate the kernel.
The decision of how much to control nut size by irrigation comes when crop load is near optimum. The decision depends, in turn, on how well the soil moisture can be controlled. Unless the crop is excessive, maintaining adequate soil moisture during kernel development of the major cultivars of the southeast will largely override the negative effect of large nut size on nut quality. Thus, quality is ensured, in most years, in orchards with solid-set sprinklers and in which irrigation is scheduled by pan evaporation.
The quality problem comes in orchards with drip irrigation systems because of limited water supply. Most drip systems in the southeast supply about 2400 gallons of water per acre per day. This is only about half of the water needed to fill a large crop if a soil moisture deficiency exists during kernel development. Typically, the major month of kernel development, September, is the second driest month of the season. Thus, in drip irrigated orchards with a full crop, the system should be used primarily to supply water during kernel development and not as a tool to maximize nut size.
A gamble exists in attempting to regulate nut size with drip irrigation. September may have above average rain and the extra rain plus the drip may be sufficient to fill a large nut in spite of high production. Regardless, in recent years, some growers have successfully used drip irrigation to limit nut size during years of high production. The irrigation scheduling varies, but basically involves increasing the irrigation frequency as the season progresses. Specifically, as the water requirement increases for fruit growth the irrigation is increased, for example, 4 hr per day in May, six in June, eight in July, 10 in August, and 12 or more in September. Some judgment has to be used. If a severe drought occurs during the nut sizing period, an adjustment will have to be made. Another option is to run the system only in September. However, this may not result in maximum use of the water due to insufficient time to develop an adequate root mass near the emitter. Other growers prefer to use drip to maximize nut size regardless of crop load and take the chance that between the water from drip irrigation and rains, soil moisture will be sufficient to fill a large nut. The latter program will not be successful in the long run simply because rainfall in September is often inadequate. Regardless of the pros and cons of regulating nut size, soil moisture during the June to mid-July period exerts a dominating influence on final crop production by affecting nut size, and, in some situations, indirectly by influencing kernel quality.
Blackmon, G.H. 1925. Pointers from the Florida Experiment Station. Proc. Natl. Pecan Growers Assn. 24:25-35.
Hunter, J. H. and H. E. Hammar. 1947. The results of applying different fertilizers to the Moore variety of pecan over a ten-year period. Proc. Southeastern PecanGrowers Assn. 40:10-32.
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