Mechanical aspects of rapid flower opening in Asiatic lily

Several processes contribute to the rapid opening of Asiatic lily flowers. Opening occurs after the tepals stop increasing in length, and have reached maximum thickness. Up to day -0.5 (where day 0 is time that flowers are half-open), the sepals entirely encase the petals, and their edges are tightly clasped in deep grooves beside the petal midribs. At day -0.5 to -0.3, the petal midrib reduces in width, opening the groove and allowing the tepals to pull apart (bud 'cracking'). Unlocking is not complete until about day -0.15: it is not the key event directing opening, but helps to sharpen its onset. The midribs control subsequent opening, pulling the laminae behind them. In the first 3-4 h of opening ('dark phase'), the main process is an increase in the angles the midribs make where they join the pedicels. During this time, the petals and sepals are only slightly curved. Subsequently through to day 0.5 (the 'light phase'), an increasingly marked curvature of the midribs pulls the tepals further apart and causes them to progressively reflex. By day 1 the flowers are mature and the laminae are no longer being pulled by the midribs. In studies of isolated tissue segments, we found that in the later stages of opening there is greater cell expansion in the epidermis of the inner surfaces. contributing to the recurving. Petal expansion is accompanied by cell separation and osmotic changes, and inhibitors of starch hydrolysis inhibit expansion of petal segments. Of the various processes operating during lily opening, we believe that the most influential is the change in angle between midribs and pedicel, suggesting a pulvinus-like mechanism. (C) 2000 Annals of Botany Company.