The Physiology of Flowering Plants - KHAM PHA MOI

PHYTOCHROME AND PHOTOMORPHOGENESIS 253Relative energy units87654321st leaf2nd leafFig: 10:7 The calculated spectraldistribution of midday daylighttransmitted through one or twoleaves of sugar beet (Beta vulgaris).Data from Smith (1973).10400 500 600 700 800Wavelength (nm)Shade tolerance vs. shade avoidanceThe response of plants to the red : far-red ratio is both species-specificand highly dependent upon developmental state. Consider the exampleof a small-seeded plant such as Arabidopsis. Seeds of Arabidopsis havelimited nutrient reserves, which will support growth for only a fewdays. Once these reserves are exhausted the seedling must photosynthesizeif it is to continue to grow. The low-fluence response ofthese seeds stimulates germination if at, or near, the soil surfaceunless the sunlight has been filtered through a vegetation canopyand is thus enriched in far-red light. If the seed does germinate,etiolation must be considered a ‘last chance’ response – the hypocotylelongates rapidly until the cotyledons are above the soil. TheHIR is then triggered even if the seedling is shaded. De-etiolation,resulting from exposure to far-red light, is thus a shade-toleranceresponse.Some plants, e.g. dog’s mercury (Mercurialis perennis), can tolerateshade throughout their lives and do not respond greatly to alterationsin the ratio of red : far-red light. However, mature plants of manyspecies exhibit a shade-avoidance response if exposed to elevatedfar-red light. Typically, the shade-avoidance response results in anincrease in apical dominance, an increase in stem growth as a resultof internode elongation, increased petiole elongation and acceleratedflowering. The morphological changes may enable the plantto grow out of the shade of other plants. These responses can betriggered by a decrease in the ratio of red : far-red light falling fromabove, but can also be triggered by far-red reflected from surroundingvegetation. A single, brief exposure to far-red light can also triggerthe shade-avoidance response if given at the end of the photoperiod(the ‘end-of-day far-red effect’, EOD-FR). Mature Arabidopsis plants exhibita clear shade-avoidance response with increased petiole extensionand accelerated flowering, although,asthisplantgrowsasarosette,the internodes do not elongate. Clearly, the shade-tolerance responseof seedlings and shade-avoidance response of mature plants are