Weakening of muskmelon perisperm envelope tissue 4

Abstract

Muskmelon (Cucumis melo L.) embryos are enclosed in an envelope of tissue consisting of a layer of endosperm and a multi-cell-layered perisperm that the radicle must penetrate for germination to occur. The force and energy required to penetrate the perisperm envelope tissue were measured using an Instron universal testing machine at a crosshead speed of 5 mm min−1 after 0, 10, 15, 22, 23, and 25 h of imbibition at 25°C. The cellular structure of perisperm envelope tissue surrounding the radicle was observed after 10, 15, 20, 25, and 48 h of imbibition using scanning electron microscopy. The force required to puncture 5-mm-long, micropylar seed pieces declined steadily from 1.65 N in dried seeds to 0.65 N after 21 h of imbibition. The penetration energy declined from 3.0 N mm in dry seeds to 1.1 N mm at 21 h after the start of imbibition when the first seeds germinated. The force and energy required to penetrate germinated seed pieces were 0.55 N and 0.9 N mm, respectively, so the net puncture force and energy needed to rupture the micropylar region of the perisperm envelope was roughly 0.10 N and 0.2 N mm at radicle emergence, respectively. Instron measurements of penetration force and energy decreased dramatically at crosshead speeds less than the 5 mm min−1. Crosshead speeds greater than 5 mm min−1 may overestimate the pressure needed to rupture perisperm and endosperm tissues. Intracellular cracks were first observed in SEM images 15 h after the start of imbibition, and after 20 h cracking was apparent throughout the micropylar region of the perisperm envelope. The perisperm envelope ruptured in one of two ways, coincident with radicle emergence. In approximately 85% of muskmelon seeds, a large crack formed in the perisperm envelope adjacent to the radicle, while in roughly 15 % a circular area of the perisperm envelope detached during radicle emergence. In dead seeds, the penetration force remained constant from 10–24 h after the start of imbibition, and there were no visible signs of tissue degradation. Cellular degradation and weakening of the perisperm envelope tissue precedes radicle emergence in muskmelon seeds.