Micromorphology of lettuce (Lactuca sativa L.) pollen grains

Abstract

In order to contribute to palynological studies of cultivated plants in Serbia, micromorphological features of pollen grains of Lactuca sativa L. (Asteraceae) were examined by both, light microscopy (LM) and scanning electron microscopy (SEM). Morphological parameters, such as grain size and shape, apertures and exine ornamentation were described. The morphometric data such as length of polar axis (P), length of equatorial axis (E), P/E ratio, lacunae diameter, echinae size, and thickness of exine excluding echinae were noted. Pollen grain were found to be isopolar, radially symmetrical, medium-sized, oblate-spheroidal in shape (P/E = 0.94), sub-circular in equatorial view, and hexagonal in polar view with straight sides in outline. The length of the polar axis (P) is 32.6 ± 3.7 μm and of the equatorial diameter (E) 34.8 ± 2.7 μm. The pollen grain is 3-zonocolporate. Each compound aperture consists of ectoaperture which is a meridionally elongated colpus with rounded ends, and pore-shaped endoaperture. Exine ornamentation is echinolophate. Lophate (fenestrate) pollen grains, typical for Asteraceae, are characterized by number of lacunae that appear as large window-like spaces, a depressed area lacking ectexine. Each lacuna is surrounded by a system of echinate ridges. Lettuce pollen grains have 15 lacunae, 3 poral, 6 abporal and 6 paraporal (polar regions are without lacunae, small, reduced to triradiate ridge). The poral lacuna communicating with adjacent abporal lacunae via interlacunar gaps is observed rounded in shape. Paraporal lacunae, lying in the mesocolpial region adjacent to one side of an equatorial ridge, are pentagonal. Abporal lacunae are rounded or angular, broad towards the pole. Average lacuna diameter is 10.8 ± 2.5 μm. Lophae (ridges) are with one row of echines that are 1.9 ± 0.3 μm long, pointed and some are curved. The еctexine thickness excluding echines, measured under LM, averaged 5.7 ± 0.7 µm.