Wall ultrastructure and cytochemistry and the longevity of pollen of threegrass species

The ultrastructures and cytochemistry of pollen grains with different longe vities of three grass species, rice (Oryza sativa L.), maize (Zea mays L.) and pennisetum (Pennisetum alopecuroides Spreng.), were studied by transmis sion electron microscopy. The pollen wall of rice is the thinnest and that of pennisetum the thickest. The exine of rice pollen grains is loose and po rous with communication channels, or microchannels, and thick columellae, b ut that of pennisetum pollen grains is very dense and nearly without microc hannels and columellae. The tectum of pennisetum pollen wall is also the th ickest of the three species. In the pollen walls of rice and maize, the mic rochannels traverse the tectum and foot layer. The exinous microchannels ar e slightly more abundant in rice pollen wall than in pennisetum pollen wall and the intines of the pollen walls of rice and pennisetum have higher ele ctron densities than that of maize. In rice pollen wall glucose-6-phosphate dehydrogenase (G6PD) and succinate dehydrogenase (SDH) are located in the exine, mainly on the surface, along the microchannels and in the spaces bet ween columellae, and in the intine. However, they occur mainly in the intin e of maize pollen wall and on the surface of pennisetum pollen wall. Under natural conditions, rice's thinnest pollen wall with richer exinous microch annels, G6PD and SDH, appears to be related to the short life of the pollen grains, while the opposite characteristics of pennisetum pollen grains are associated with greater longevity.