Differences in size and surface ultrastructure of pollen grains are useful for distinguishing species of tree fruits. Peach (Prunus persica [L.] Batsch) and nectarine (P. persica var. nectarina [Ait.] Maxim.) have the largest pollen grains, about 62 × 32 μm, and have similar surface markings. Apricot (P. armeniaca L.), like peach, pollen has ridges of exine which are generally longitudinal and shallow, but the grains are slightly smaller. Pollen of European plum (P. domestica L.) is slightly shorter and comparatively wider than that of peach and has the most prominent and complex exine ridging of the tree fruit species. Japanese plum (P. salicina Lindl.) pollen has similar conformation and surface appearance to that of peach but is only one-half as large. Sweet cherry (P. avium [L.] L.) and apple (Malus domestica Borkh.) pollens are about three-fourths the dimensions of those of peach and have narrower ridges. Apple has the least coarse ridges of any of the tree fruits. Sweet cherry has many abortive grains and its exine ridges are more distinct than those of apple. Sour cherry (P. cerasus L.) pollen is about one-half the size of peach pollen and has prominent ridges and many abortive grains. Pear (Pyrus communis L.) pollen is also about one-half the length of peach pollen but comparatively wider; it has shallow but distinct and complex ridges and some abortive grains.
Twenty sample clones of peach (Prunus persica [L.] Batsch) and ten clones each of nectarine (P. persica var. nectarina [Ait.] Maxim.), European plum (P. domestica L.), sweet cherry (P. avium [L.] L.) and apple (Malus domestica Borkh.) were compared by scanning electron microscope (SEM) for characteristics of their pollen exine. The 4 taxa were separated into 3 groups by pollen grain size. Exine differences distinguished apple and sweet cherry pollens, which had similar size, and separated clones within each of the species. Peach and nectarine clones varied within the same ranges of pollen size and exine characteristics. Within P. persica, two patterns of ridging and the relative frequency, depth of impression, and size of pores categorized the clones individually. Plum exine patterns were distinct from those of other tree fruits, but ridging and pore characteristics of the exine separated plum clones into distinct categories. Apple cultivars, which had the narrowest ridges, were separated by the abundance and placement of pores in the exine. Sweet cherry clones also were distinguished by exine characteristics.
The fruit surface characteristics of 15 nectarine clones were examined by scanning electron microscopy at 200 and 2000× magnifications. The surface of nectarines appeared very different from surfaces of fruits previously examined. The relatively unprotected surface, the apparent ridging, and minute cracks starting at stomates of nectarines have not been previously described. Among clones, differences were evident in prominence and coarseness of ridges, number and elevation of stomates, and cracking of the cuticle of mature fruit. The appearance of minute cracks coincided with the time of natural brown rot infection; thus the cracks are regarded as the most likely entry sites for the brown rot organism. Stomates seemingly were suberized in maturing fruits. The study revealed that relatively small differences in surface structures are apparently heritable.
The growth curves of eastern-grown nectarines (Prunus persica (L.) Batsch) do not follow closely the three-phase sigmoid curve established for peaches. All clones completed phase I at the same time. However, many clones did not have a well-defined ‘final swell’ and several showed gradual increases in growth through phases II and III. Percentages of final fruit size attained during the final few weeks of growth or percentages of calcium in leaves, fruits or peels were not closely associated with cracking of fruits. Slower growth rate appears linked to the nectarine character, although the linkage seems to have been partially broken in some clones. Growth rates of 41 nectarine clones were not closely associated with field cracking or minor surface cracking.
Sprays of 1,1,5,5-tetramethyl-3-dimethylaminodithiobiuret (MATB), when applied to peach trees during cytokinesis, thinned fruits without causing excessive leaf chlorosis or defoliation during 3 years of testing. The concn that thinned fruits satisfactorily in the cultivars tested varied between 100 and 300 ppm. MATB at 100 ppm thinned ‘Ranger’ fruits without excessive damage to leaves while at 200 and 300 ppm it produced comparable results in normal and vigorous trees, respectively, of the ‘Earlired’ cultivar. ‘Rio Oso Gem’ was thinned satisfactorily by 300 ppm. MATB appears to be a promising peach fruit-thinner for such commercially important, hard-to-thin cultivars as ‘Earlired’, ‘Rio Oso Gem’, ‘Redskin’, and ‘Redhaven’.
‘Earliscarlet’ nectarine [Prunus persica (L.) Batsch] was released in Apr. 1985 because of its attractiveness, high quality, and early season of ripening, a season when relatively few high quality nectarine cultivars for the eastern United States are available.
‘Bounty’ peach [Prunus persica (L.) Batsch] was released because of its large fruit size, excellent flavor (as judged by us), and productivity, particularly under dry soil conditions of eastern Texas. Its ability to produce fruit of uniform maturity throughout the canopy makes it especially suitable for once-over harvesting. ‘Bounty’ has outstanding potential as a mid-season fresh-market peach for the south-central United States, particularly Texas, and is suggested for trial in the mid-Atlantic and eastern United States.