The development of seedless cultivars of fruit such as grape (Vitis vinifera), sweet orange (Citrus sinensis), and watermelon (Citrullus lanatus) has significantly increased the consumption of these fruit (Pollack, 2001). The economic potential of doing the same for stone fruit (Prunus) is promising considering the marketing possibilities for pitless cultivars (lacking stone and seed) of cherry (P. cerasus and P. avium), peach (P. persica), plum (P. domestica and P. salicina), and apricot (P. armeniaca), coupled with decreased production costs for canned and dried fruit. Because fruit trees are vegetatively propagated and the seeds are not eaten, pits are generally considered agricultural waste by the processing industry and must be disposed of, typically through burning. Thus, pit removal and disposal substantially raises production costs and contributes to pollution. In addition, the presence of pits and pit fragments in dried and processed fruit is a matter of concern for processors and can lead to product rejection as well as injury to consumers and consequent lawsuits.
Developing pitless stone fruit would require the elimination of the seed and the stone, a hard woody layer surrounding the seed. The goal of developing stoneless plums was first approached by the visionary breeder Luther Burbank in the early 1900s using a wild-type plum that was partially stoneless (Burbank, 1914). He was able to integrate higher fruit quality with the stoneless trait through breeding and even found lines without seeds, but was never able to completely eliminate the stone or the seed. While Burbank may have failed in his attempt to revolutionize plum production in this way, he left an important legacy by demonstrating that the stone can be essentially eliminated without loss of fruit quality or yield.
Burbank first identified the partially stoneless trait in a wild plum found in France called ‘Sans Noyau’. Burbank described this plant as bushy, thorny, and bearing small fruit with incomplete stones about the seed and no known effects on seeds (Burbank, 1914). He crossed this wild plum to commercial cultivars in California but, given that knowledge of genetics was limited at the time, Burbank did not report segregation ratios, thus it is unknown whether it is a single or multigenic trait.
While the function of the stone has not been systematically studied, there is ample evidence that it serves to protect the seed from stress and disease (Doster and Michailides, 1999). In the early 1960s, Ryugo first recognized that stones are composed of lignin (1961, 1963). His work documented the fact that peach stones are rich in lignin, the seasonal pattern of lignin accumulation, and the presence of lignin biosynthesis intermediates. These studies and others have shown an increase in stone dry weight and lignification that begins in the second stage of fruit development until maturity (Nakano and Nakamura, 2002; Ryugo, 1961). Studies in peach directed toward increasing fruit size, especially of fruit in which the embryo was destroyed by cold temperatures, have shown the effect of growth regulators on embryo and stone development (Zucconi and Bukovac, 1978). Crane et al. (1961) showed that stone development was related to fruit growth, which was affected by applied gibberellic acid (GA) concentrations. Crane (1963) concluded that subnormal growth and ultimate size of the stone in parthenocarpic peaches was due to restricted cell enlargement, with cell division a contributing factor. He found that lignification proceeded normally, regardless of the ultimate growth of the stone.
The majority of studies on lignin and its regulation have been performed on tree crops for pulp and paper production or on forage crops for improved digestibility. The recent interest in biofuels has sparked intense interest in lignin research, as large amounts of mostly inaccessible energy are stored in plants as lignin. This work has identified most of the genes for the major enzymes in the pathway and the potential regulatory points (reviewed by Boerjan et al., 2003). Lignin is formed from the phenylpropanoid pathway, the end products of which are coniferyl and sinapyl alcohols. These lignin monomers serve as the basis for lignification, which is the process of producing the lignin polymer via oxidative processes aided by peroxidases and laccases.
Here, we set out to study the physiological basis for the stoneless trait. The lack of hard stone tissue could be the result of two different mechanisms: 1) hardening via lignification does not effectively take place in ‘Stoneless’, leaving regions where the endocarp layer is present but does not harden or 2) incomplete endocarp formation that results in fragmented stones.
Burbank, L. 1914 The stoneless plum: An experiment in teaching a plant economics 21 Nov. 2008 <http://digicoll.library.wisc.edu/cgi-bin/HistSciTech/HistSciTech-idx?type=article&did=HISTSCITECH.0011.0029.0009&isize=M>.
Callahan, A.M., Morgens, P.H., Wright, P. & Nichols K.E. Jr 1992 Comparison of pch313 (pTOM13 homolog) RNA accumulation during fruit softening and wounding of two phenotypically different peach cultivars Plant Physiol. 100 482 488
Crane, J.C. 1963 Parthenocarpic peach development as influenced by time of gibberellin application Proc. Amer. Soc. Hort. Sci. 83 240 247
Crane, J.C., Rebeiz, C.A. & Campbell, R.C. 1961 Gibberellin-induced parthenocarpy in the ‘JH Hale’ peach and the probable cause of ‘Button’ production Proc. Amer. Soc. Hort. Sci. 78 111 118
Doster, M.A. & Michailides, T.J. 1999 Relationship between shell discoloration of pistachio nuts and incidence of fungal decay and insect infestation Plant Dis. 83 259 264
Gradziel, T.M. & Dandekar, A.M. 2001 Field performance of seed and endocarp based resistance to preharvest aflatoxin contamination in almond Proc. 14th Aflatoxin Elimination Workshop 82
Iglesias-Fernandez, R., Matilla, A.J., Roriguez-Gacio, M.C., Fernandez-Otero, C. & de la Torre, F. 2007 The polygalacturonase gene PdPG1 is developmentally regulated in reproductive organs of Prunus domestica L. subsp. insititia Plant Sci. 172 763 772
Labrecque, M., Barabe, D. & Vieth, J. 1985a Developpement de la drupe de Prunus pensylvanica L.f. (Rosaceae) Annales des Sciences Naturelles Botanique 7 39 53
Nakano, M. & Nakamura, M. 2002 Cracking and mechanical properties of the stone in peach cultivars after severe thinning Acta Hort. 592 531 536
Pfeiffer, D.G., Bergh, J.C., Fells, R.D., Yuan, R., Yoder, K.S., Derr, J.F., Weaver, M.J. & Parkhurst, J. 2008 2008 Virginia, West Virginia, and Maryland commercial spray bulletin 12 Nov. 2008 <http://www.ext.vt.edu/pubs/treefruit/456-419/456-419.html>.
Pollack, S. 2001 Consumer demand for fruits and vegetables: The U.S. example 49 54 Regmi A. Changing structure of global food consumption and trade Economic Res. Serv., U.S. Dept. Agr. Publ. WR-S-01-1
Ryugo, K. 1961 The rate of dry weight accumulation by the peach pit during the hardening process Proc Amer. Soc. Hort. Sci. 78 132 137
Ryugo, K. 1963 Changes in methoxyl content in the peach endocarp and some of its soluble phenolic constituents during lignification Amer. Soc. Hort. Sci. 84 110 115