Anatomical Study of Salak (Salacca wallichiana C. Mart.): Post-pollination High Temperature Accelerates Pollen-tube Elongation but Inhibits Subsequent Fertilization

The salak fruit (snake fruit) contains one to three seeds covered with an aril. The size of the fruit primarily depends on the number of seeds. Fruits with more seeds grow larger and attain higher commercial value in fresh markets owing to their superior appearance. In eastern Thailand, during the hot period in the early rainy season, the fruit set of salak decreases, which is empirically believed to be caused by high night temperatures. In this study, we pollinated and incubated salak spadices at various temperatures (18-36 degrees C) to determine the optimal post-pollination temperature range needed to produce valuable fruits. Chronological pollen-tube elongation in the pistil and development of the early embryo and endosperm were then observed anatomically, followed by fruit-set estimation. At <= 21 degrees C, pollen-tube elongation was limited. At 24-27 degrees C, although elongation was slow, pollen tubes attained embryo sacs in > 60% of florets within 36 h after pollination. Pollen tubes elongated fastest at 30-33 degrees C and attained embryo sacs within 12 h after pollination. At 36 degrees C, the difference in the elongation speed tended to be higher among the pistils. The percentages of ovules with developing embryos were the highest at 27 degrees C at 48 h after pollination; zygote and early embryos were observed in 10.8% and 55.9% of the ovules, respectively, and the primary endosperm nucleus, dispersing endosperm nucleoplasm, and free endosperm nuclei were observed in 8.6%, 38.7%, and 19.4%, respectively. The second highest percentage was observed at 24 degrees C. The percentages tended to decrease at 30 degrees C and decreased significantly at >= 33 degrees C. At >= 30 degrees C, shriveled embryo sacs were observed. The estimated fruit-set percentage based on embryo development as the consequent fruit set was the highest (approximate to 80%) at 27 degrees C, while the second highest percentage (approximate to 75%) was at 24 degrees C. At >= 30 degrees C, the estimated percentage decreased to less than half that at 27 degrees C. Fruits containing three seeds were expected to grow in the range of 17.6-28.0% at <= 27 degrees C. One-or two-seeded fruits were expected to grow at >= 30 degrees C. Limited fruit set was expected at 36 degrees C. Our results indicate that salak prefers relatively cool temperatures of approximately 25 degrees C for the fertilization and set of valuable fruits.