Abstract: The authors have studied enzyme cytochemistry, electron probe and ultrastructures in leaf cells of rice seedlings treated under horizontal clinostatic conditions simulating microgravity for 14 days. After treatment, the activity of Ca2+-ATPase in the leaf cell membrane disappeared; the density of total Ca2+ in plasm increased as that density of the cytoskeleton in the leaf cells became sparse; the cell wall was irregular being either or convex in appearance; the structure of chloroplast granas and mitochondria cristae were changed. The authors have proposed the following sequence of events: firstly, the activity of Ca2+-ATPase in leaf cell membrane disappeared; then the calcium-influx in the membrane ceased functioning thereby abolished the cross-membrane Ca2+ gradient; this weakened the assembly of the microtubule and microfilaments, loosened the cytoskeleton, disoriented the movement of the secretory vacuoles, and lead to cell wall thinning and irregularity.

Key words: horizontal clinostatic condition: Ca2+-ATPase: cytoskeleton, Cell wall