The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem 11 (PSII) was PSll,(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSIIcenters (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyllprotein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.
Abstract. Light transmission through several sands, soils and size classes of glass beads was investigated. The glass beads served as a highly reflective model system for the sands. Light penetration was significantly better through sands than through either sandy or silty loams. For sandy soils, there was a shift in the spectrum leading to a change in the red‐to‐far‐red photon ratio with increase in depth: this ratio dropped by more than 30% in the first few millimetres, and in some cases increased again with further increases in depth. The depths at which these changes occurred in the soil profile were specific for a given soil type. Similar spectral changes were obtained when sample depth for a sandy soil was held constant, and particle (ped) size of that soil was decreased. No such changes, either with sample depth or ped size, were observed with loams. Per cent transmittance in the blue (442 nm) was more than three orders of magnitude lower than in the red (632.8 nm) for sandy soils. When dry sandy soils were water‐saturated, their transmittance increased by up to three orders of magnitude, and the red far‐red ratios in the transmitted light increased in all cases. By contrast, water‐saturated loams were essentially opaque. The possible significance of these spectral properties of soils in seed germination and seedling photomorphogenesis is discussed.
The work addressed the adjustment of the photosystem ratio in the green algaChlamydomonas reinhardtii. It is shown that green algae, much like cyanophytes and higher plants, adjust and optimize the ratio of the two photosystems in chloroplasts in response to the quality of irradiance during growth. Such adjustments are compensation reactions and helpC. reinhardtii to retain a quantum efficiency of oxygen evolution near the theoretical maximum. Results show variable amounts of PS I and a fairly constant amount of PS II in chloroplasts and suggest that photosystem stoichiometry adjustments, occurring in response to the quality of irradiance during plant growth, are mainly an adjustment in the concentration of PS I. The work delineates chromatic effects on chlorophyll accumulation in the chloroplast ofC. reinhardtii from those pertaining to the regulation of the PS I/PS II ratio. The detection of the operation of a molecular feedback mechanism for the PS I/PS II ratio adjustment in green algae strengthens the notion of the highly conserved nature of this mechanism among probably all oxygen evolving photosynthetic organisms. Findings in this work are expected to serve as the basis of future biochemical and mutagenesis experiments for the elucidation of the photosystem ratio adjustment in oxygenic photosynthesis.
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