Factors influencing controlled pollination of Pinus patula.

Abstract

A study of factors contributing to successful controlled pollinations of Pinus patula Scheide et Deppe was undertaken. The pollen morphology of P. patula, P. oocarpa, P. greggii, P. elliottii, P. tecunumanii, P. caribaea and P. radiata was studied and the mean size of pollen grains was determined for these species. Clonal differences in pollen size within P. patula were also determined. The impact of pollen management practices on pollen viability was highlighted and a protocol for in vitro pollen viability testing of P. patula and other pine species was determined. A one percent agar solidified distilled water medium gave the best germination results after 72 hours incubation at 30 °C for a number of different Pinus species and P. patula clones. The addition of boric acid increase germination, although not significantly. The addition of sucrose to the pollen germination medium had a negative effect on pollen germination of P. patula, P. greggii and P. caribaea. Re-hydration of pollen for two hours prior to in vitro germination testing improved germination significantly. Incubation temperatures of above 38 °C were detrimental to germinating pollen grains. Stored pollen with low humidity (less than 10 %) of P. patula, P. greggii and P. caribaea could tolerate temperatures of up to 70 °C while still retaining some level of viability. The initiation and growth of the pollen tube was also studied and differences in pollen tube-lengths germinated at 30 °C for 72 hours were found between species studied. Flowering of different P. patula clones was monitored over seven seasons. Flowering periods varied in length between 4 and 14 days amongst five clones over the different seasons. The best cone-survival after controlled pollination was achieved with breathable micro-fibre material. Seed yields were also highest when breathable material was used for controlled pollination. The role of pollen viability in controlled pollination was also determined in pollination studies with low viability resulting in low cone survival and low seed yields. The temperature and relative humidity inside isolation bags were monitored and temperatures above 40 °C were reached inside bags constructed of nonbreathable material. These temperatures were lethal to pollen germinating in vitro. Relative humidity of between 80 and 100 % was maintained in non-breathable bagging material, constituting a risk of diseases causing cone-mortality. The application of fungicide before, during and after controlled pollination was ineffective in improving cone survival.