Relative Elemental Elongation Rates in the Etiolated Hypocotyl of Sunflower (Helianthus annuus L.)-A Comparison of Straight Growth and Gravitropic Growth

Growth rates were determined throughout etiolated seedling hypocotyls of sunflower (Helianthus annuus L.) during straight growth and after gravitropic stimulus in terms of relative elemental rates of elongation (RELELs). In the context of an absolute elongation rate of ca. 1 mm/h, maximum RELELs of 0.12-0.13 h-1 (120-130 μm/mm/h) occurred along the apical third of the axis and tapered to zero in the basal third. RELELs did not change on either side of the hypocotyl for some minutes after it had been turned to horizontal ("zero-time"). Between 20 and 30 min after zero-time there was a rapid change in growth rates along the acropetal two-thirds of the elongating region. On the upper side, growth stopped; on the lower side, RELELs in the rapidly growing regions increased to 0.23-0.25 h-1. Differential growth then progressed basipetally into the slower growing zones. Still later, growth was initiated in basal nonelongating regions. In all regions, growth stimulation on the lower side was a demonstrated feature of differential growth leading to curvature.