JXB Advance Access published online on May 8, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm068
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RESEARCH PAPER |
Uncoupling light quality from light irradiance effects in Helianthus annuus shoots: putative roles for plant hormones in leaf and internode growth
1Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4
2Biology Department, Trent University, Peterborough, Ontario, Canada K9J 7B8
* To whom correspondence should be addressed. E-mail: leon{at}phytophys.com
An attempt has been made to uncouple the effects of the two primary components of shade light, a reduced red to far-red (R/FR) ratio and low photosynthetically active radiation (PAR), on the elongation of the youngest internode of sunflower (Helianthus annuus) seedlings. Maximal internode growth (length and biomass) was induced by a shade light having a reduced R/FR ratio (0.85) under the low PAR of 157 µmol m–2 s–1. Reducing the R/FR ratio under normal PAR (421 µmol m–2 s–1) gave similar growth trends, albeit with a reduced magnitude of the response. Leaf area growth showed a rather different pattern, with maximal growth occurring at the higher (normal) PAR of 421 µmol m–2 s–1), but with variable effects being seen with changes in light quality. Reducing the R/FR ratio (by enrichment with FR) gave significant increases in gibberellin A1 (GA1) and indole-3-acetic acid (IAA) contents in both internodes and leaves. By contrast, a lower PAR irradiance had no significant effect on GA1 and IAA levels in internodes or leaves, but did increase the levels of other GAs, including two precursors of GA1. Interestingly, both leaf and internode hormone content (GAs, IAA) are positively and significantly correlated with growth of the internode, as are leaf levels of abscisic acid (ABA). However, changes in these three hormones bear little relationship to leaf growth. By implication, then, the leaf may be the major source of GAs and IAA, at least, for the rapidly elongating internode. Several other hormones were also assessed in leaves for plants grown under varying R/FR ratios and PARs. Leaf ethylene production was not influenced by changes in R/FR ratio, but was significantly reduced under the normal (higher) PAR, the irradiance treatment which increased leaf growth. Levels of the growth-active free base and riboside cytokinins were significantly increased in leaves under a reduced R/FR ratio, but only at the higher (normal) PAR irradiance; other light quality treatments evoked no significant changes. Taken in toto, these results indicate that both components of shade light can influence the levels of a wide range of endogenous hormones in internodes and leaves while evoking increased internode elongation and biomass accumulation. However, it is light quality changes (FR enrichment) which are most closely tied to increased hormone content, and especially with increased GA and IAA levels. Finally, the increases seen in internode and leaf GA content with a reduced R/FR ratio are consistent with FR enrichment inducing an overall increase in sunflower seedling GA biosynthesis.
Key words: Internode, leaf, light irradiance, light quality, PAR, phytohormone, R/FR ratio
Received 15 August 2006; Revised 6 March 2007 Accepted 8 March 2007
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