The Physiology of Flowering Plants - KHAM PHA MOI

PHYTOCHROME AND PHOTOMORPHOGENESIS 249Germination (%)100755025Inhibition PromotionControlFig: 10:3 The action spectrum ofgermination of Grand Rapidslettuce (Lactuca sativa).Approximately 50% of untreated,control seeds placed in darknessgerminate. This increases to 100%when seeds are illuminated with redlight, and falls close to zero if theyare illuminated with far-red light.0500 600 700 800Wavelength (nm)maxima at the same peaks as required for the physiological response(Fig. 10.5). At its heart is a chromophore composed of a linear tetrapyrrolemolecule (Section 10.7.1). With specially designed equipmentit is possible to detect the change in the phytochrome absorptionspectrum on illumination in an intact etiolated seedling, althoughsimilar changes are masked in mature tissues which contain chlorophyllwith its strong absorption peak in the red.10.3.2 Low-fluence responsesThe control of the germination of lettuce seeds by phytochrome wastermed a low-fluence response (LFR) because it is governed byexposures to low fluences of red, or far-red, light. Many aspects ofplant development exhibit low-fluence phytochrome responsesincluding seed germination, seedling development, internode elongationand flowering in short day plants. Other key features of the LFRwere determined using such experimental systems. As alreadydescribed, low-fluence phytochrome responses exhibit photoreversibility.However, far-red light can reverse the effect of red light onlyif given within a certain period of time known as the escape time(i.e. escape from phytochrome control). Likewise there is a delay – theP RRed (660 nm) lightFar-Red (730 nm) lightP FRPromotes germinationFig: 10:4 Phytochrome exists intwo photointerconvertible states.Phytochrome is synthesized in thedark in the P R form whichpredominantly absorbs red light.Exposure to red light results inconversion to the P FR form whichtriggers many phytochromeregulatedresponses. The P FR formcan be converted back into the P Rform by exposure to far-red light.