Effects of different inbreeding levels on seed characteristics and growth of slash pine.
Objective: Slash pine (Pinus elliottii) was introduced in China 8 decades ago, and has become one of the most important exotic tree species for commercial usage in Guangdong. Inbreeding management is important at multiple cycles of genetic improvement. Most Pinus elliottii breeding programs have progressed to advanced-generation improvement in China. However, limited efforts have been made to evaluate the effects of inbreeding on the important traits of current genetic materials. In current study, the effect of inbreeding on the fitness of Pinus elliottii genetic material in Guangdong and the variation in inbreeding depression (ID) among different parents was analyzed. Method: C2 and C8 were two super clones chosen based on backward selection. Cone development and field growth were measured in 16 outbreeding and inbred progenies, and in 5 open-pollinated progenies from the C2 and C8 founder clones. There are 4 levels of inbreeding: outcrosses (F=0), half-sib (F=0.125), full-sib and backcrosses (F=0.25), and selfed (F=0.5). Seed characteristics including cone length, cone width, seed number per cone, seed weight per cone, filled seeds number per cone, weight of filled seeds per cone, thousand seed weight, proportion of filled seeds and so on, were measured in 1999. The progeny testing stand with a randomized completed block design was established in Taishan, Guangdong in 2000. From 2002, the growth traits were determined, and included height, diameter at breast height (DBH), individual volume, and plot volume. Result: Inbreeding hinders cone development in Pinus elliottii, few cones were obtained in 4 inbred families and the rate of seed was obviously reduced at F=0.5. Furthermore, Inbreeding severely decreased the filled seed number and weight per cone. In C8, the ID at F=0.125, 0.25, and 0.5 was 20%, 35%, and 57% for filled seed number, respectively, which was highly related to the inbreeding coefficient (R2=0.99). In C2, the filled seed number decreased to 50% and 67% at F=0.25 and 0.5, respectively, when compared to that of F=0.125. The other traits related to cone morphology and seed production were not consistent in C2 and C8. The ID for survival was not obvious for 4-year-olds, but at F=0.125, 0.25 and 0.5 it was 4%, 8% and 19% for survival to 13-year-olds, respectively. There were significant differences between the two founder clones and the related families in growth response to inbreeding. The ID for overall growth was severe at F=0.125, which was 22% for individual volume at 13 years. The response to inbreeding was higher in C2 than that in C8, and the IDs in C2 followed certain rules: the ID degree was the highest in individual volume, followed by DBH, and the tree height was lowest; The mean IDs in DBH are twice as height. The ID at F=0.125, 0.25, and 0.5 from the C2 founder clone was 13%, 15%, and 30% for individual volume at 4 years, respectively, whereas it was 26%, 20% and 43% for plot volume at 13 years, respectively. The IDs in C8 were not obvious, and the IDs in plot volume unexpectedly increased by 2.1%. Two specific inbred progenies (C8-2 × C8-1 and C8-2 × C8-2) showed better growth performance than the C8 and C8-2 open-pollinated progenies. Conclusion: The inbred progenies exhibited IDs for cone development and growth, which especially caused large economic losses due to the reduction in the survival and plot volume at the mature stage. It was thus indicated that inbreeding management in Pinus elliottii should be given attention during seed orchard establishment. Variations were found in IDs from different parents. The elite inbred progenies could be selected for advanced-generation breeding.