Crossing Eastern Cottonwood

Ea~,tern cottonwood (Populus deltoides Bartr.) is the subject of breeding programs designed to develop planting stock with potential for rapid growth, desirable wood properties, and pest resistance (4). Techniques for making controlled crosses within the species are essential to breeding, and their development constitutes an early phase of genetics research. This paper will describe techniques that are being used in the lower Mississippi Valley. Controlled crossing techniques for the dioecious Populus species were developed in Europe and have been used there for several decades. The aspens (Section Leuce) may be crossed by placing bases of dormant branches in water after chilling requirements are met, forcing these branches in greenhouses, and pollinating the female flowers. Fresh pollen is obtained by similarly forcing male flowers. Fertilized catkins are matured on the branches in three to four weeks. This procedure, originally developed for Salix, was first used for Populus by W ettstein-W estersheim (11) and has been adapted for the American aspens by Einspahr and Benson (1), Johnson (7), and others. While black poplars (Section Aigeiros) can be crossed by these means, their larger catkins with sometimes longer maturation time require more nutrition than is supplied by cut branches. Consequently, branches bearing female flowers are bottle-grafted to supporting rootstocks. The technique, as used in Europe, has been generally described by Muhle Larsen (8) and Schreiner (10). We have modified it for cottonwood. In brief, female scions are bottle-grafted to potted juv.enile stock in mid-autumn. After chilling, flower buds on these grafted scions are forced and pollinated in the greenhouse. This procedure obviates climbing sometimes remotely situated trees.