The Physiology of Flowering Plants - KHAM PHA MOI

252 PHOTOMORPHOGENESISIf dark-grown seedlings of most dicotyledonous plants are exposedto continuous far-red light there is a strong inhibition of seedlingelongation. Typically the maximal inhibition results at far-red wavelengths(FR-HIR, Fig. 10.6) but this is species-dependent, with someplants exhibiting maxima in the red (R-HIR). As seedlings green thesemaxima often shift, indicating the operation of more than one inhibitorymechanism. Current theories favour cycling between P R and P FRas central to the operation of the HIR, but the mechanisms underlyingthis remain unclear. The peaks in the UV/blue of the action spectrumshown in Fig. 10.6 are thought to result from the action of specificUV-blue light photoreceptors (Section 10.4) which interact with phytochrometo coordinate plant development.10.3.6 What is the significance of far-red light?Low ratios of red : far-red light are a signal of vegetation shadeFull sunlight contains approximately equal amounts of all visiblewavelengths of light, including red and far-red components(Fig. 10.1). However, plant leaves are rich in chlorophyll, whichstrongly absorbs blue and red light, green light to a lesser extent,but is relatively inefficient at absorbing far-red light. Fig. 10.7 showsthe spectral composition of sunlight after it has passed through oneor two leaves. Absorption of light by the canopy reduces the totalirradiance markedly and has greatly reduced the amount of red lightrelative to far-red light. This shift in red : far-red ratio from valuesclose to 1 in full sunlight to values nearer 0.1 is a strong signalindicating vegetation shade. This signal is not restricted to light fallingdirectly from above. Light reflected from neighbouring vegetationwill also be enriched in the far-red and has the potential to act asa signal of future competition for resources.Fig: 10:6 Action spectrum for theinhibition of hypocotyl growth inLactuca sativa. Hypocotyl elongationis inhibited by far-red light in a highirradiance response (HIR). Inaddition, blue light is also effective:the action spectrum in the 400–500nm region shows the typical threepeaks of many blue-light responses.From Hartmann (1967).Relative quantum efficiency1.751.501.251.000.750.500.250320 400 500 600 700 800 1000Wavelength (nm)