How chloroplasts sense different fluence rates

How do chloroplasts know the difference in brightness between two areas irradiated with different fluence rates? To analyze this question we irradiated two adjacent parts of a protonema 30 ^m each in length with different fluence rates of red or blue light (Yatsuhashi et al., 1987b). Sets of different light fluence microbeams were prepared and adjacent parts of protonemata were irradiated simultaneously and continuously for 2 hours and then observed. One group of light conditions was prepared so that the fluence ratio of the two microbeams was constant but the fluence levels of the two beams differed. A second set of conditions established different fluence ratios but held the fluence

Figure 1.15 Chloroplasts detect the ratio of two fluence rates of light irradiated at two adjacent areas, rather than the absolute difference between the two, to determine the direction of movement. Two adjacent areas of a single protonema were irradiated with different sets of fluence rates of light (left panel), such that the ratios were the same, for example 10 or 2, but the differences were one tenth, and one hundredth. After 2 hours of continuous irradiation, chloroplast movement was detected to assess whether they moved in response to a higher fluence rate or not. The slightly shaded combinations show positive movement from weak light to strong light, but the strongly shaded combinations show no movement. Chloroplasts can detect very small ratios between two different fluence rates.

Figure 1.15 Chloroplasts detect the ratio of two fluence rates of light irradiated at two adjacent areas, rather than the absolute difference between the two, to determine the direction of movement. Two adjacent areas of a single protonema were irradiated with different sets of fluence rates of light (left panel), such that the ratios were the same, for example 10 or 2, but the differences were one tenth, and one hundredth. After 2 hours of continuous irradiation, chloroplast movement was detected to assess whether they moved in response to a higher fluence rate or not. The slightly shaded combinations show positive movement from weak light to strong light, but the strongly shaded combinations show no movement. Chloroplasts can detect very small ratios between two different fluence rates.

levels of the two beams constant (Figure 1.15). Each protonema was observed under a microbeam irradiator to determine in which microbeam irradiated areas chloroplasts moved. The results were very clear. Chloroplasts moved from weak light to strong light depending on their ratio but not on the difference in fluence rates. Chloroplasts can detect a small difference in ratio at a threshold of around 1.2-1.5 for blue light and 1.5-2.0 for red light (Yatsuhashi et al., 1987b).

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