Water Relations of Seed Development and Germination in Muskmelon (Cucumis meloL.)

Abstract

The germinability, vigour, and desiccation tolerance of muskmelon (Cucumis melo L. cv. Top Mark) seeds was studied in relation to changes in seed water content during development within the fleshy fruit. Seed water content (fresh weight basis) declined from 91% to 42% between 10 d and 35 d after anthesis (DAA) (when maximum dry weight was attained), then declined more slowly to a minimum of 35% at 50 DAA before increasing again to 43% at 65 DAA. Fresh intact seeds were first germinable at 25 to 30 DAA and attained maximum germination percentages at 45 DAA. Between 15 and 35 DAA, cotyledons, hypocotyls, radicles and epicotyls of isolated embryos (testa and perisperm envelope removed) sequentially developed the ability to grow when incubated on water. Dehydration to water contents less that those attained within the fleshy fruit is not a requirement for development of germination capacity of muskmelon seeds. Seeds became tolerant of rapid desiccation after 25 DAA, and drying of immature seeds (25 to 40 DAA) increased their germination percentages upon subsequent imbibition. Washing, drying, or washing followed by drying increased seedling vigour (root length) as compared to fresh seeds, which had very poor vigour. Water absorption isotherms were constructed to test whether changes in water-binding components were correlated with the development of desiccation tolerance. Isotherms for seeds older than 25 DAA fit well to the D'Arcy/Watt model, which postulates the existence of high-affinity, low-affinity and multi-molecular water-binding sites. Desiccation-intolerant seeds younger than 25 DAA lacked the component of the absorption isotherm characteristic of the high-affinity water-binding sites which have been hypothesized to confer desiccation tolerance. However, we were unable to determine whether the absence of high-affinity binding characteristics was specifically related to desiccation intolerance or was artifactual due to the loss of volatiles when immature seed samples were dried at high temperatures.