588 T. Hayashi 



unit fresh weight of leaves was not increased by gibberellin treat- 

 ment (Plant Physiol. 32: 152. 1957). 



Dr. Larsen: I should like to mention an additional point, namely 

 the possible effect of gibberellin on the rate of respiration, an item 

 that also enters the balance sheet for the production of dry matter. 

 Does Dr. Hayashi have data on the output of carbon dioxide in the 

 dark? It would be interesting to know whether leaf sheaths of rice 

 can have a positive net rate of photosynthesis or whether their maxi- 

 mum rate of photosynthesis is just capable of counterbalancing the 

 output of carbon dioxide by respiration. If there are stomata on the 

 leaf sheaths of rice (as there are on those of Avena), the sheaths may 

 contribute to the net increase in dry weight. The effect of gibberellin, 

 not only on the leaf area but also on the sheath area, would thereby 

 become important. 



Dr. Hayashi: I haven't discussed the respiration rate, but I have 

 determined it in these experiments. The respiration rate is from 10 to 

 20 per cent of the photosynthesis rate at saturated-light conditions, 

 based upon COo exhaustion and fixation, respectively. 



Dr. Larsen: Would gibberellin influence the rate of respiration? 



Dr. Hayashi: Yes, gibberellin increases the rate per plant, but the 

 increase is small. 



Professor Blackman: May I make a comment and a suggestion in 

 relation to the analysis of the effects of gibberellin on growth? The 

 value of growth analysis is admittedly a hobby horse of mine, but it 

 seems to me that these concepts can help in elucidating how these 

 changes in growth are brought about in the field. It can be shown 

 that the relative growth rate (the rate of gain in dry matter per day) 

 is the product of the net assimilation rate (rate of gain in dry matter 

 per unit area of leaf) and the leaf area ratio (total leaf area to total 

 plant weight). It follows that if gibberellin increases the growth rate 

 but does not bring about much change in the rate of assimilation, then 

 it must increase the leaf area ratio. I suggest that it would be worth- 

 while seeing how far gibberellin has altered this ratio. There is an- 

 other point which needs to be taken into account, namely, that there 

 may be self-shading of one leaf by another and in consequence the 

 relationship between light intensity and photosynthesis \\\\\ not be 

 the same for individual leaves and whole plants. We have been work- 

 ing with Salvinia natans where there is no trouble of self-shading 

 since all the leaves are flat on the surface of the water. Gibberellin 

 has little effect on the net assimilation rate, but it depresses the rela- 

 tive growth rate by decreasing the leaf area ratio. 



