What takes place during these periods of development determines the size the nuts attain, the degree to which they are filled, and finally the quality at harvest. These three developmental stages are interdependent, because the size of the nuts may affect the degree of filling, and that, in turn, the time and nature of their maturity. They are not entirely separate and distinct but overlap in that there is more or less development of the kernel, varying with the species, while the nuts are growing in size. In general, however, there is not appreciable kernel development until after the nuts have attained approximately full size, except in the chestnut.
The outstanding example of this situation is the pecan. There is practically no growth of the kernel until after the shell of the nut has started to become hard. At that time growth of the embryo, which constitutes the kernel, become rapid. The major portion of the kernel is formed during a period of approximately one month, starting at Beltsville, Maryland about the middle of September. The final stages of filling occur just before the nuts mature, and the first nuts to fall usually have the best filled kernels. Later maturing nuts are generally poorly filled; their shells and kernels are often discolored, and the shucks fail to open properly, if at all.
The development of walnuts, hickory nuts, and filberts, so far as is known, is in all essentials the same as that described for the pecan nut except that the kernel or embryo begins to grow somewhat earlier in the season. However, the major portion of the filling, which consists in the thickening of the cotyledons, takes place late in the season, and only a month or a little more before the nuts mature.
The period of the maturing of the nuts generally closely follows the completion of the filling of the kernels. During this period in the pecan, certain other species of hickory, the Persian walnut, chestnut, and others, food reserves are transferred from shucks, hulls, or burs to the nuts. Abscission layers are formed and shucks, hulls, or burs split open on drying out, thus partially or wholly releasing the nuts. There is a very direct relationship between the degree to which the nuts are filled and their time of and normality of maturing; well filled nuts mature early and normally, whereas poorly filled nuts mature late, if at all, and shucks, hulls, or burs fail to open properly.
+Growth in Size+
The size of the nuts produced by a tree is determined by a number of factors, one or all of which may operate during the course of the season. These are: (1) Age of tree; (2) position of the nuts on the tree; (3) fertility of the soil and moisture supply, or the nutritional status of the tree; (4) size of the crop borne.
In general, old trees bear smaller nuts than do younger trees. Hence size of nut for a particular variety is only relative. The first few crops produced by a tree usually consist of nuts large in size for the variety; and then, as the tree attains age, nuts become smaller in size. Young trees make longer and thicker shoot growth than do older trees. There is, then, under normal conditions, a direct relationship between the growth made by a tree and the size that the nuts attain. The more vigorous trees not only produce larger nuts than those produced by less vigorous trees, but the hulls and shells of such nuts are thicker and constitute a higher total percentage of the total weight of the fruit.
The position of the nuts on a tree has an important effect on the size that they ultimately attain. In general, the nuts in the top are larger than those nearer the ground; and those on the strongest and most vigorous shoots of the top or lateral branches will attain a larger size under normal conditions than those located on weaker and shorter shoots or on the inside of the tree. Here again there is a direct relationship between growth of the tree and growth in size of nuts. All normal trees make longer and stronger shoot growth in the top than they do on the terminals of lateral branches, and the shortest and weaker shoots as well as the smallest nuts are generally on the lateral branches inside of the tree top.
Fertility of soil and moisture supply determine in large measure both the growth made by the tree and the size of nuts. The nuts borne on trees growing on fertile soils adequately supplied with moisture are generally much larger in size than those borne by trees on infertile soil or soil poorly supplied with soil moisture. Deficiency of either nitrogen, or moisture, or both is particularly effective in limiting the size of nuts produced. Pecans grown under soil conditions in which both nitrogen and moisture were deficient have been known to attain only about one-fourth the size of nuts of the same varieties grown in the same orchard but under conditions of clean cultivation and supplementary nitrogen applications. A prolonged drought during the time that the nuts are increasing in size very frequently causes them to be much smaller than they would have been had the moisture supply been adequate.
The size of the crop borne by a tree determines in a very large measure the size that the nuts attain at maturity. There is generally an inverse relationship also between the number of nuts borne in a cluster on a shoot and the size they attain. In this respect nut crops are little different from apples and peaches, which, too, are sold on the basis of size. In order to produce fruits of large size having a high market value, the crops are thinned in years of a heavy set of fruit. In the case of pecans, for example, thinning the crop at the time the nuts are growing in size on heavily producing trees is a very effective method of increasing the average size of the nuts allowed to remain on the trees. The earlier the thinning is done the more effective it is; however, it will increase the size of the nuts even when done as late as when the shells have started to become hard. No practical and economical method of thinning the crop of nuts has as yet been found; nevertheless it is well to bear in mind that a large crop borne by a tree generally means reduced average size of the nuts at harvest.