Production and development of seed and pollen in grafts of Picea sitchensis (Bong.) Carr. treated with gibberellin A4/7 to induce coning and the effect of forcing treatments



Coning was induced on container-grown grafts of Picea sitchensis (Bong.) Carr. by stem injection of gibberellin A4/7 in combination with heat and drought in a polythene house. After overwintering outdoors pollen and seed development in the following year were studied. Control grafts remained outdoors, except for a 16 d period (late April-early May) in the polythene house to facilitate pollen shedding and pollinations. Pollen was at the 2- to 3-cell stage when these grafts were placed in the polythene house and the grafts produced good quantities of high quality pollen and seed which could be used in tree improvement programmes. Placing grafts in warm conditions in January or March, when pollen was predominantly at a pre-meiotic stage, advanced cone-bud development but reduced the yield of pollen and seed, though forcing in March was less detrimental. Cytological study showed no meiotic irregularities in pollen mother cells forced in January or March, but abnormalities in later stages of pollen development included failure of tetrads to separate, retardation of development and degeneration of immature pollen. Poor seed development in January-forced cones may be attributed to retarded development of the distal portion of the cones. Nucelli in the distal ovules had not become secretory at pollination and hence little or no pollen was taken into the micropyle, also there was retarded development of the megagametophyte which could adversely affect fertilization. More proximal ovules of forced cones, and ovules of control cones, had well developed nucelli which were actively secreting pollination droplets, pollen was commonly found in the micropyle and the megagametophyte was developing normally.