Triploid production via heat-induced diploidisation of megaspores in Populus pseudo-simonii

Triploid breeding in Populus enhances the commercial properties of cultivated trees. Sexual polyploidization through the production of diploid gametes produces elite triploids with heterosis and ploidy vigour. Populus pseudo-simonii is a species indigenous to China. It is highly adaptable and resistant drought, cold, and saline-alkaline stresses. To improve the properties in growth and pulpwood traits of P. pseudo-simonii, we used heat treatments to induce chromosome doubling in the megaspores as a first step in the production of triploid saplings. The heat treatments were applied during development stages of the female flower buds. We produced 117 triploids via the diploidisation of megaspores. The highest proportion of triploids among the offspring (51.43%) was obtained by applying a heat treatment of 40 degrees C over 4h to flower buds in which one-third of the catkins were outside the bract scales. The proportion of pachytene was positively correlated with the average rate of triploid production in each bud developmental stage. Thus, among the phases of meiosis, the pachytene stage may be most susceptible to heat treatments aiming to diploidise the megaspores in P. pseudo-simonii. The megasporocytes in these treated flower buds were mostly in the first meiotic division stages, suggesting that the first meiotic division restitution (FDR) type 2n megaspores may have been induced by our experimental procedures. These triploid hybrids derived from FDR type 2n megaspores will probably contribute significantly to programmes aimed at improving commercial properties of Populus congeners in the section Tacamahaca.