ISHS Acta Horticulturae 711: V International Symposium on Artificial Lighting in Horticulture
LIGHT-REGULATED PLANT GROWTH AND FLOWERING; FROM PHOTORECEPTORS TO GENES, HORMONES AND SIGNALS |
| Author: | R. King |
| Keywords: | Gibberellins, sucrose, light quality/intensity |
| Abstract:
Plant function, architecture and reproduction are affected in major ways when plants are grown in protected cultivation or in artificial lighting. Removal of UV-B by glass has significant effects and reduced visible light transmission may limit photosynthesis and delay flowering. The converse, enhanced flowering in higher light intensities is associated with growth but also with a specific florigenic effect of increasing shoot apex sucrose content. One outcome is that synchronized flower induction is possible by growing plants at a limiting light intensity then shifting them to higher intensity. Controlled shading in commercial production systems could achieve this result but using artificial lighting to boost flowering would be inefficient. On the other hand, for long day (LD) responsive species, flowering can also be induced by overnight exposures to a low irradiance from incandescent lamps while short day (SD) responsive species can be kept vegetative in such LD. While effective for flowering, such far-red (FR)-rich light also increases stem/petiole elongation, probably because of increases in the content of the gibberellin (GA) class of plant hormones. The flowering and elongation of Arabidopsis thaliana in response to LD provides an ideal system for the study of the action of FR light on GA synthesis and, particularly, because extensive molecular and genetic information is available for this species. Its petiole elongation almost doubles in FR-rich light compared to red (R) whether given as a 10 min end-of-day (EOD) FR exposure or as a LD (16 h light). That brief and prolonged low intensity FR exposures are effective shows that photosynthesis is not limiting. More cogently, genetic evidence using a phyA mutant along with studies of R/FR photoreversibility affirms a role for the “B” class of phytochromes. Applied GA mimics the effect of a LD extended by FR both by increasing petiole elongation and by causing flowering in SD. Furthermore, after a LD the endogenous content of GAs in growing petioles can increase 3-fold and, within 4 h of starting the LD, expression of an important biosynthetic gene, a GA 20-oxidase, has increased 5- to 10-fold. Conversely, in a 20-oxidase gene-silencing line the flowering response to LD is reduced considerably. Similar findings on GA biosynthesis have been obtained for the grass Lolium temulentum where LD leads to increases in both 20-oxidase expression and in leaf GA5 content. The timing of the GA5 increase at the shoot apex is delayed relative to that in the leaf but this fits with the expected speed and distance of its transport. Thus, GA5 may be a floral signal transmitted from the leaf to the apex. Overall, these findings highlight important responses of both growth and flowering to light intensity and spectral quality. While the findings on the regulation of GA biosynthesis cannot be applied directly to commercial enterprises, the understanding of GA metabolism provides a focus for future manipulation of plant growth. | |
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