Fructose inhibits pear pollen germination on agar medium without loss of viability

Pollen of Japanese pear (Pyrus pyrifolia Nakai) germinated well on agar medium containing 10% sucrose or glucose, but not on agar containing fructose. The inhibitory effect of fructose was dose-dependent. Sucrose enhanced pollen tube growth much more effectively than glucose. Addition of 5% fructose to 5% or 10% sucrose or glucose media suppressed germination completely. Ungerminated pollen, however, showed similar respiration rate and stainability against acetocarmine dye as germinated pollen. When pollen was transferred onto fructose medium after culturing it on glucose or sucrose medium for 1–2 h, germination was completely impeded. Reversely, pollen transferred to sucrose or glucose medium from fructose medium germinated at almost the same ratio as pollen on sucrose or glucose medium without transfer. Thus, pollen inhibition by fructose is reversible. Compared with uncultured pollen, cultured pollen contained less than half amount of total sugars, even if failed in germination on fructose medium. Germinated pollen on sucrose and glucose media contained sucrose and glucose, but ungerminated pollen on fructose medium contained only trace levels of these sugars, suggests that pollen on fructose medium predominantly uses sucrose and glucose as respiration substrates and cannot maintain the constant levels of these sugars. However, as pollen germination occurred on agar medium without any sugar, fructose may impede a physiological factor that triggers germination, and once the trigger is impeded, many physiological pathways including sugar biosynthesis may be blocked.