Photosynthetic apparatus in primary leaves of barley seedlings grown underblue or red light of very low photon flux densities

Chlorophyll (Chl) a and Chl b contents, rate of CO2 gas exchange, quenching coefficients of chlorophyll fluorescence, and endogenous phytohormones hav e been studied in primary leaves of barley seedlings cultivated under blue (BL) or red (RL) light. Photon flux densities (PFD) were between 0.3 and 12 mu mol m(-2) s(-1). Plants grown at PFD of 0.3 mu mol m(-2) s(-1) demonstr ated in BL tenfold and in RL threefold decreased Chl content compared to pl ants grown at 12 mu mol m(-2) s(-1). Chl a/b ratio increased from 2.3-2.5 t o 4.4-4.5 in BL, not in RL, following the decrease in PFD at plant cultivat ion from 12 to 0.3 mu mol m(-2) s(-1). Plants cultivated at weak BL demonst rated severalfold decreased rate of photosynthetic CO2 uptake, whereas decr ease in PFD of RL from 12 to 0.3 mu mol m(-2) s(-1) caused only 20% decline in the rate of photosynthesis. Decrease in PFD during a plant cultivation reduced the maximum quantum yield of photosynthesis in BL, not in RL leaves . Light response curves of non-photochemical and photochemical quenching of chlorophyll fluorescence calculated on the basis of absorbed quanta were n ot affected by PFD of RL during plant cultivation. On the contrary, both no n-photochemical quenching and accumulation of Q(A)(-), reduced primary acce ptor of Photosystem II, occurred at lower amounts of absorbed quanta in lea ves of BL plants grown at 0.3 than at 12 mu mol m(-2) s(-1). Two photoregul atory reactions were suggested to exert the light control of the developmen t of photosynthetic apparatus in the range of low PFDs. The photoregulatory reaction saturating by very low PFDs of RL was supposed to be mediated by phytochrome. Phytochrome was proposed to enhance (as related to other pigme nt-protein complexes of thylakoids) the accumulation of chlorophyll- b-bind ing light-harvesting complex of Photosystem II (LHC II). It acts independen tly of the pigment mediating the second photoregulatory reaction, as eviden ced by the results of experiments with plant growth under mixed blue plus r ed light. The contents of cytokinins and indole-3-acetic acid in a leaf wer e not significantly affected by either light quality and PFD thus indicatin g those phytohormones not to be involved into photoregulatory processes.