The effects of nitrogen starvation on photosynthetic efficiency were examined in three unicellular algae by measuring changes in the quantum yield of fluorescence with a pump-and-probe method and thermal efficiency (i.e. the percentage of trapped energy stored photochemically) with a pulsed photoacoustic method together with the inhibitor 3-(3,4-dichlorophenyl)-l,l-dimethylurea to distinguish photosystems I and II (PSI and PSII). Measured at 620 nm, maximum thermal efficiency for both photosystems was 32% for the diatom Thalassiosira weissflogii (PSII:PSI ratio of 2:1), 39% for the green alga Dunaliella tertiolecta (PSI1:PSI ratio of 1 :1), and 29% for the cyanobacterium Synechococcus sp. PCC 7002 (PSII:PSI ratio of 1 :2). Nitrogen starvation decreased total thermal efficiency by 56% for T. weissflogii and by 26% for D. tertiolecta but caused no change i n Synechococcus. Decreases in the number of active PSll reaction centers (inferred from changes in variable fluorescence)were larger: 86% (T. weissflogia, 65% (D. fertiolecta), and 65% (Synechococcus). The selective inactivation of PSll under nitrogen starvation was confirmed by independent measurements of active PSll using oxygen flash yields and active PSI using P700 reduction. Relatively high thermal efficiencies were measured i n all three species in the presence of the PSll inhibitor 3-(3,4-dichlorophenyI)-1,l -dimethylurea, suggesting the potential for significant cyclic electron flow around PSI. Fluorescence or photoacoustic data agreed well; in r. weissflogii, the functional cross-sectional area of PSll at 620 nm was estimated to be the same using both methods (approximately 1.8 x 10' A'). The effects of nitrogen starvation occur mainly in PSll and are well represented by variable fluorescence measurements.In marine phytoplankton, nitrogen limitation affects photosynthesis by reducing the efficiency of energy collection due to loss of Chl a and increases in nonphotochemically active carotenoid pigments Geider et al., 1993). It also directly affects photochemical energy conversion because of decreases in protein synthesis that appear to affect chloroplastic proteins (and thus the proteins of PSI and PSII reaction centers) more strongly than cytoplasmic proteins (Rhiel et al., 1986;

Kanadese vlier sabugueiro do Canada sauco del Canada Sambucus canadensis American elder(berry), bourtree, Canada elder, eastern elderberry, elderberry, common elder, sweet elder, pie elder, elder-blow, blackberry elder, Mexican elder sureau blanc, sureau du Canada, sirop blanc D. CHARLEBOIS, P. 1. BYERS, C. E. FINN, AND A. 1. THOMAS almindelig hyld Flieder, memeine Holunder, Holunder, schwarzer Flieder, schwarzer Holunder fekete bodza sambuco, sambuco nero, sambuco nostrale, zambuco, zambuco arboreo bez czarny sabugueiro, sabugueiro negro, sabugueiro preto 4epeH 6b3

The effect of preharvest UV-C on the levels of bioactive compounds in strawberry fruits appears to be cultivar- dependent, with season or growing conditions having a significant impact.

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