InvitroGrowth of Vegetative Tomato Shoot Apices

Explants from the shoot apex of the tomato, comprising the apical dome and youngest primordium together with small amounts of sub-apical tissue were cultured for periods of 1 to 4 plastochrons. By the use of a simple parameter, the axillary distance, the growth-rate could be accurately monitored throughout each plastochron. Gibberellic acid, coconut milk, and kinetin, in addition to sucrose and inorganic salts, all promoted growth of the apex; a combination of gibberellic acid and coconut milk gave the fastest growth. Temperature had a large effect on the growth-rate with an in vitro Q10 of 2.1 contrasted with an in vivo Q10 of 1.2 over the range of 15 to 25 °C. On gibberellic acid and coconut milk at 15 °C two-thirds of the in vivo growth--rate was sustained in culture for two plastochrons after which the growth-rate gradually declined; at 20 and 25 °C growth-rates slightly higher than in vivo rates were sustained for 1 plastochron before a more rapid decline. The anatomy of these in vitro apices was normal for 1½ plastochrons after which there were small increases in cell volume in the developing primordium. Reducing the amount of sub-apical tissue drastically reduced the growth rate but had little effect on the responses to gibberellic acid and coconut milk. Explants are considered to be useful material for studying the changes that take place in the apex during the course of 1 or 2 plastochrons, but inadequate on the media tested for experiments involving longer periods of growth. Explants also provide a sensitive assay system for the effects of growth factors on the rate of shoot apical growth.