EFFECT OF VARIOUS COMPOUNDS 1921 



stringent nitrogen deficiency, addition of nitrate caused only a very slight 

 increase in A02/Af, and a considerable decline in — AC02/A^, so that the 

 photosynthetic ratio rose as high as 1.8. The earlier assertion (p. 329), 

 that inhibition of photosynthesis by nitrogen deficiency is immediately 

 relieved by nitrate supply, thus needs qualification: in case of extreme 

 deficiency, "nitrate respiration" (eq. 19.1) may be the main or only effect, 

 at least for several hours. No "nitrate photosynthesis" (eq. 19.2) — which 

 would cause an increase in AO2/A/ — becomes apparent in this period. 



It is mentioned in Pirson and Wilhelmi's paper that, according to an 

 earlier study by Pirson (1938), the effect of nitrogen deficiency on photo- 

 synthesis cannot be rapidly relieved by a supply of ammonium salt. 



(e) Varioiis Organic Compounds 



Dam (1944) found vitamin K in Chlorella, after having previously 

 noted its presence in chlorophyll-bearing tissues, and relative rarity in 

 nonchlorophyllous tissues of the higher plants. This caused him to specu- 

 late on the possible relation of this compound to photosynthesis. Of two 

 synthetic vitamin K substitutes, menadione inhibited photosynthesis 

 within 24 hrs., while dicumarol had no effect. 



Freeland (1949) sprayed bean plants with indoleacetic acid, and other 

 synthetic auxins. All — except /3-naphthoxyacetic acid— diminished the 

 (net) photosynthesis; all but indoleacetic acid temporarily stimulated 

 respiration. The effect on net photosynthesis was in part^ — but not en- 

 tirely- — attributable to the increase in respiration. 



Skvorzov (1950) reported that photosynthesis is stimulated by naph- 

 thylacetic acid in weak as well as in strong light. 



Brebion (1948) and Gavaudan and Brebion (1951) compared the ef- 

 fects of benzene, chloroform, different alcohols, colchicine, etc., on photosyn- 

 thesis, chlorophyll synthesis and certain physiological processes in animal 

 tissues. 



(/) Ultraviolet Light 



The effect of ultraviolet light on photosynthesis was discussed in chapter 

 13 (p. 344). It was tentatively suggested there that 253.7 m^i photons 

 attack an enzymatic compound — perhaps the CO2 acceptor — rather than 

 chlorophyll (since the latter appears intact). However, since the relative 

 inhibition was found to be the same in steady and in flashing light (c/. 

 chapter 34, p. 1465), and could not be offset, in flashing light, by longer 

 dark periods, it appears that ultraviolet light must affect also (or exclu- 

 sively) the enzyme (Eb) that we have assumed to limit the rate in saturating 

 continuous light, as well as in saturating light flashes. (An alternative is 

 to assume that each molecule Eb can operate only in conjunction with a 



