ABSTRACT

Jon Anders Stavang

Interest in controlling plant morphology in greenhouse production of flower plants in an environmental friendly way, has initiated extensive research on how temperature can be used to control plant morphology (reviewed by Moe and Heins, 2000). Diurnal temperature alternations affect plant morphology in many species, and the growth hormone gibberellin (GA) has been suggested as the mediator of temperature effects on plant growth. The objective of the work presented in this thesis was to establish the physiological basis for temperature and light regulated plant growth, with emphasis on stem elongation and GA metabolism.

We chose to use pea (Pisum sativum L. cv Torsdag) as a model plant based on the accumulated knowledge of thermoperiodic growth in this species (Grindal, 1997). Furthermore, GA metabolism has been widely studied in pea and many GA mutants were available. The methods used included real time RT-PCR in gene expression studies, GC-MS-SIM for quantitative analysis’s of hormones, triangular transducers for stem elongation rate studies and a cuvette gas exchange system to investigate net gas exchange and photosynthetic capacity of plants subjected to different temperature treatments.

This thesis consists of three individual papers. In the first paper, we investigated the influence of different day and night temperature combinations (DT, NT) on GA levels, and related this to diurnal steady state expression of genes involved in GA-biosynthesis and GA-deactivation and diurnal stem elongation. The second paper aimed to investigate the mechanisms behind temperature mediated changes in stem elongation rate as related to a temperature drop in either light or darkness, by investigating the steady state expression of GA metabolism genes in response to the temperature drops. The third paper aimed to get a better understanding of how temperature regulated changes in GA1 levels affected growth, photosynthesis and respiration using wild type pea and a GA saturated mutant, la crys. The air temperature range has been from 13oC to 21oC in all the papers.

The stem length of wild type was in general reduced by 30% under a DT13oC/NT21oC temperature regime as compared to a DT21oC/NT13oC temperature regime. Diurnal stem elongation rhythms were highly influenced by ambient growth temperature and whether light was on or not. Light reduced stem elongation under constant temperature conditions. Stem elongation was stimulated at 21oC and reduced at 13oC. Low temperature and light reduced stem elongation the most. GA levels control daily elongation at the same average temperature and also final internode length. Reduced levels of GAs under negative DIF were mediated by transcriptional stimulation of a specific GA deactivation gene, PsGA2ox2. A temperature drop stimulated expression of PsGA2ox2 in light, but had no effect in darkness. However, a temperature drop in darkness stimulated the biosynthesis genes NA, PsGA20ox1, and PsGA3ox1. GA levels do not determine photosynthetic capacity. A saturated GA response eliminates a morphological response to diurnal temperature alternations. Reduced dry weight of wild type under a DT13oC/NT21oC temperature regime as compared to a DT21oC/NT13oC temperature regime is explained by reduced diurnal net assimilation of CO2.

In conclusion, we present evidence of an important role of GA-catabolism in thermoperiodic stem elongation. Furthermore, a temperature drop in light is qualitatively different from a temperature drop in the night regarding the response on GA-metabolism gene expression. These results suggest that regulation of GA-deactivation is important in temperature induced light stress responses or low day temperature acclimation in pea. GA levels determine the allocation pattern of assimilated CO2 only, and higher GA levels or a saturated GA response are not major determinants of a plants photosynthetic capacity on a leaf area basis.

Keywords: day temperature, gibberellin metabolism, night temperature, photosynthetic capacity, Pisum sativum, plant morphology, stem elongation, temperature drop, thermoperiodism.

Stavang JA 2005. The involvement of gibberellin in thermoperiodic growt of pea (Pisum sativum L. Cv Torsdag). Doctor Scientiarum Theses 2005:18. Norwegian University of Life Sciences, Ås.



Oppdatert: 12.07.05
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