CONTROL OF STEM GROWTH BY LIGHT 

 AND GIBBERELLIC ACID 



JAMES A. LOCKHART 



KerckhofF Laboratories of Biology, California Institute of Technology, 



Pasadena, California 



It has already been reported that appHcation of gibberellic acid re- 

 verses the low-intensity light inhibition of pea stem growth (Lockhart, 

 1956). Similar results have been obtained by Vlitos and Mendt 

 (1957) with light of higher intensities for the green and blue as well 

 as the red region of the spectrum. Lona and Bocchi (1956) have also 

 shown that application of gibberellic acid reverses the red Hght inhibi- 

 tion of light-grown Cosmos. 



The effect of gibberellic acid on the light inhibition of stem growth 

 has now been examined in several additional species. In all cases (with 

 one important exception), a marked interaction between light and 

 gibberellic acid has been found (Fig. 1). Pisum sativum and Heli- 

 anthus ammiis show an essentially complete reversal of light inhibition 

 at both low (60 ergs • cm"- • sec"^) and relatively high (2000 ergs • 

 cm"- -sec" M light intensities. This is true for both blue and red radia- 

 tion. 



Cucurbit a pepo (Dark Green Zucchini) and Cucumis sativus 

 (National Pickhng) are very sensitive to light. Thus, in the case of 

 Cucurbita, maximum inhibition (ca. 50% ) is induced by even the 

 low-intensity red light. Cucumis is also markedly inhibited by the 

 low-intensity light and even further inhibited by the high-intensity 

 radiation. Application of gibberellic acid usually causes only partial 

 reversal of the light inhibition of stem growth in these species. It is 

 interesting to note that in those species in which the gibberellic acid is 

 fully effective, the optimum dose is approximately 0.04-0.4 jUg /plant, 

 whereas in Cucurbita and Cucumis a dose of 4.0-10.0 /tg /plant is 

 required for maximum response. In the latter species gibbereUic acid 



217 



