An ultrastructural study of seed reserves in triploid watermelons
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Poor germination, inconsistent emergence and relatively high seed costs limit the interest of potential growers in triploid watermelons. The major problems in triploid watermelon production are considered morphological, physiological and environmental factors, such as hard thick seed coats, small and weak embryos, dense endotesta layers, large seed cavities, moisture control, planting depth, and temperature control. Previously, we found that the starch content in triploids (11.6-27.3%) was lower than that in diploids (42.2%). Linoleic acids (C18:2) dramatically increased in triploids (54-87%) compared with diploids (35%), whereas other fatty acids were lower in comparison to the fatty acid composition of diploids. Also, higher fatty acid content (high-germination lots vs. low-germination lots = 72-128 mg/g and 157-320 mg/g dry seed weight) was detected in low-germination lots of triploids. At the light microscope level (800), cotyledon cells were characterized by abundant protein bodies that were the most discernible organelles. At the electron microsc ope level (5000), the size of the starch granules in triploids was significantly less than in the diploid reflecting the lower starch content found in triploids. There was, however, no significant difference of starch granule size between the high and low germinating triploid lots. Lipid bodies in triploids with high germination were significantly smaller than those with low germination. Irregularly shaped lipid bodies were also observed in triploids. In conclusion, better germination and vigor of watermelon seeds would be expected from seeds in which the cotyledons have higher storage reserves. The differences found in the cotyledon cells of triploids may indicate more storage of lipid reserves and less availability of starch reserves in triploids in comparison to diploids during seed germination.
