Sporoderm development in Populus L. (Salicaceae)

Populus is the most genetically studied woody plant. Its pollen is highly distinctive. The sporoderm development from meiosis to mature pollen grains has not been studied for any Populus species. Here we present a detailed ontogenetic study with dates and durations of all stages of sporoderm development for Populus species growing in the Moscow region. They are one native species (Populus tremula), one introduced species (Populus alba), and two widespread cultivars (Populus × nevensis and Populus × berolinensis). Pollen development of P. alba, P. × nevensis, and P. × berolinensis is two weeks earlier than that of P. tremula. Simultaneous meiosis in Populus takes place in late February–early March. It lasts for about a week. In one month the pollen grains mature. The most rapid are the stages of the tetrad period. Orbicules and a peritapetal membrane have been found in the plasmodial tapetum. A thin and often intermittent two-layered exine is formed along the entire circumference of the pollen grain. A tectum (with nanoechini and perforations), columellae and a thin foot layer form the ectexine. The endexine consists of short, thick lamellae. The formation and maturation of all five layers takes one to two weeks. The two-layered intine is the thickest sporoderm layer. Its development takes two to three weeks. Tubular elements are found in the outer intine, making the intine similar to the aperture membrane. Morphologically, Populus pollen grains are inaperturate in the exine and omniaperturate in the intine.