Flower Promotion in Pinus maximinoi and Pinus tecunumanii in a tropical environment and Artificial Screening of High-Elevation P. tecunumanii for Resistance to Fusarium circinatum

Abstract

Pinus tecunumanii and Pinus maximinoi are two important commercial species for Smurfit Kappa Cartón de Colombia (SKCC). However, seed production in tropical regions can be very problematic for pine species. For SKCC, these problems create a severe limitation to cone and seed production of pine species used for commercial plantations. An effective and inexpensive method to increase seed crops would be very valuable. The literature suggests that application of exogenous gibberellin (GA4⁄7) can be effective in promoting flowering in pines.

Two experiments involving stem injections of hormones to enhance female flowering were conducted in an 11-year-old of P. maximinoi clonal seed orchard and in a 5-year-old low-elevation (LE) P. tecunumanii clonal seed orchard located in Colombia. Experiment 1 was conducted in August 2007 using 15 clones of both species and applications of Provide® 10 sg (AI GA4⁄7); Experiment 2 was conducted in September 2007 using 12 clones of both species and applications of Procone® (AI GA4⁄7). Both manufactured by Valent Bioscience Corporation, Libertyville, IL, U.S.A. In both experiments one ramet per clone was randomly assigned to each of four gibberellin treatments: 0, 50, 100, or 300 mg⁄tree of active ingredient. In Experiment 1, the treatments were lower than the target amount of the hormone. In addition two branch girdling treatments were applied to investigate the effect of girdling in various locations within the crown on pollen production.

In Experiment 1, trees treated with GA4⁄7 produced significantly more female strobili than the controls for both species. Trees of P. maximinoi treated with 300, 100, and 50 mg of GA4⁄7 averaged 1193, 968, 1128 total female strobili per tree respectively vs. 870 on average for the controls, whereas for P. tecunumanii, trees treated with the same doses of GA4⁄7, averaged 353, 301, and 297 total female strobili per tree respectively vs. 211 for the controls. In Experiment 2, trees treated with GA4⁄7 produced significantly more female strobili than the controls for P. maximinoi, but not for P. tecunumanii. Pinus maximinoi trees treated with 300, 100, and 50 mg of GA4⁄7 averaged 859, 878, 838 total female strobili per tree respectively vs. 623 on average for controls, an increase of 38%. There is some evidence that branch girdling increased pollen production in the middle of the tree crown.

Fusarium circinatum is a serious disease threatening many economically important pine tree species throughout the world. Fourteen open-pollinated families of high-elevation (HE) P. tecunumanii and three bulk seedlots (P. patula and P. tecunumanii) were screened for resistance to pitch canker using artificial inoculation on seedlings that were 16 and 18 weeks old. Consistent with previous results reported in the literature, P. tecunumanii (LE) shows essentially no stem dieback, P. tecunumanii (HE) shows intermediate resistance, and P. patula is very susceptible. Heritability estimates for the four variables used to assess response to pitch canker (stem dieback at 3 and 5 months after inoculation) were quite high, ranging from 0.48 to 0.58. There was little family x experiment interaction, with rBg values ranging from 0.77 to 0.91 for the four response variables. There was substantial genetic variation among the P. tecunumanii (HE) families for resistance to pitch canker infection; the range in GCA predictions for percent stem dieback 5 months after inoculation was 12% to 63%.