Water relations of seed development and germination in muskmelon (Cucumis melo L.). VIII. Development of osmotically distended seeds

Dead seeds that expand to nearly twice their normal volume when fully hydrated are called osmotically distended (OD). These seeds swell osmotically in response to a water potential (psi) gradient created by solutes trapped in the free space between the embryo and the surrounding endosperm or perisperm tissues. The formation of OD seeds in planta is poorly understood, although they often occur in newly harvested muskmelon (Cucumis melo L. Reticulatus group) seed lots. Muskmelon fruit senescence and seed germinability were contrasted with Armenian cucumber (Cucumis melo L. Flexuosus group) from 50 d after anthesis (DAA) to when seeds were released from the fruit. Fifty DAA muskmelon seeds were incubated in the laboratory for 30 d at 15, 25, and 35 degrees C in factorial combinations of ethanol, acetic acid, and P to simulate conditions in decaying fruits. Seed release from Armenian cucumber occurred 20 d earlier than muskmelon. In both years of the study, less than 25% of the muskmelon seeds released from the fruit were viable, and 52% and 24% of the dead seeds were OD in year one and two, respectively. All Armenian cucumber seeds were viable or had germinated precociously at seed release. From 50 to 60 DAA, soluble solids in muskmelon fruit pericarp tissue declined from 11.4 to 7.8 degrees Brix, pH declined from 6.2 to 5.1, water potential increased from -1.76 to -1.36 MPa, acetic acid increased to 61 mol m-3 and ethanol content rose from 0 pressures in the seed cavities of 40 to 55 DAA fruits were generally 12 and 8 kPa, respectively, at midday. All 50 DAA muskmelon seeds incubated in acetic acid and ethanol germinated, because these chemicals could not penetrate the perisperm tissue. Incubating 50 DAA muskmelon seeds in the laboratory for 30 d at 15 or 25 C had little effect on germinability, regardless of water potential. Germination percentages of muskmelon seeds incubated at 35 degrees C and water potential's < -1.28 MPa were less than 50%. Muskmelon seeds died and became OD inside decaying fruits in the field because of the combined effects of low water potential, high temperature, and low O2 partial pressures. Fruits of muskmelon cultivars bred to resist decomposition and to have high sugar content showed decreased reproductive capacity compared to Armenian cucumber which decomposed more rapidly.