CELL NUMBER AND ORGAN SIZE IN DEVELOPING SUNFLOWER HYPOCOTYLS

Hypocotyl elongation in etiolated sunflower seedlings (Helianthus annu us L.) is inhibited by white light (WL). It has been suggested that th is response is largely due to an inhibition of cell elongation with li ttle effect on cell numbers. During growth in darkness, a positive cor relation between the increase in hypocotyl length and cell numbers per organ was observed. In WL, stem elongation and the increment in the n umber of cells were both inhibited to the same extent (correlation coe fficients r > 0.98 for both etiolated and irradiated seedlings). We co nclude that WL inhibits stem elongation via two separate effects: reta rdation of cell expansion and a concomitant suppression of cell divisi on in the meristematic region of the hypocotyl.