Population Studies of the Monoterpene Composition of 4- to 12-Year-Old Half Siblings of P. ponderosa

The study included about 1,500 half-siblings, 4 to 12 years old, of 21 P. ponderosa that were selected primarily on the basis of monoterpene composition. Three of the ecotypic regions (monoterpene races) of P. ponderosa were represented, and results were based on 25 to 200 siblings per parent. Composition of individual trees of a set of half-siblings usually varied greatly in two or more major components. Average composition of a set of progeny tended to be broadly intermediate between the parent tree and the average of the local stand. Parents with similar composition produced progeny with quite similar arrays of compositions and frequency distribution characteristics. However, limonene and myrcene were lower than the calculated intermediate amounts; α-pinene and ß-pinene were higher. A juvenile effect is postulated that was more or less evident in all young growth. The effect of this juvenile factor was much the same as that shown by plants under stress. Therefore, all young trees or young parts of older trees may be under some level of stress— the stress of rapid growth. Nearly one third of the half-siblings of a high limonene parent could be classed as high limonene or potentially so. Some high-limonene trees were found among the progeny of a parent that could be classed only as near-high limonene. A rare tree was found with ultra-high limonene, though such trees have not been found in mature, natural stands. There were some differences between the three geographic regions in the comparative ratios between parent and progeny for a given monoterpene, suggesting that the locality of a parent tree may have an effect on the juvenile response or that there were minor genetic effects that are distinctive to a region, or that there was an effect on a young tree grown far from its seed source. Two forms of stress—over-application of a phytocide and prolonged growth in a restrictive container—caused a sharp change in quantitative composition, usually evidenced by lower levels of limonene and myrcene and higher levels α-pinene and ß-pinene. Results, in general, indicated that the quickest and cheapest way to obtain trees with a diverse array of compositions was the use of wind-pollinated seed from localities with much variation in individual tree composition. Research on young half-sibling populations served three purposes: (1) to better understand natural populations; (2) to provide a readily available supply of trees with a diverse array of monoterpene compositions for research on host resistance, and for studies of monoterpene genetics; and (3) to guide the selection of parent trees for breeding programs and to make preliminary allele assignments for tree breeding studies.