Light irradiance differentially regulates endogenous levels of cytokinins and auxin in alpine and prairie genotypes of Stellaria longipes.

The growth patterns of plants from alpine (sun) and prairie (shade) ecotypes of Stellaria longipes in response to change in light irradiance was investigated and involvement of cytokinins (CKs), auxin (IAA) and abscisic acid (ABA) was studied to examine the mechanism behind phenotypic plasticity of these plants in response to light signalling. Low light irradiance induced shoot growth in plants of both ecotypes, but IAA levels were higher in plants from alpine, but not prairie ecotype. Dynamics of CK profiles in response to changing photosynthetically active radiation were quite different between ecotypes and changes were more pronounced in the plants of alpine ecotype, where opposite patterns in CK accumulation between low and normal light irradiances were observed. The plants of both ecotypes showed similar trends in ABA levels under low light irradiance. Thus, the highly plastic plants of prairie ecotype may have evolved mechanisms to control the growth in response to reduced light irradiance without major alterations in the levels of CKs or IAA. These results demonstrate that within species, plants from open habitats show less growth response to reduced light irradiance than plants from shaded habitats.

Growth and ethylene evolution by shade and sun ecotypes of Stellaria longipes in response to varied light quality and irradiance.

Plants growing in the shade receive both low light irradiance and light enriched in far red (FR) (i.e., light with a low red (R) to FR ratio). In an attempt to uncouple the R/FR ratio effects from light irradiance effects, we utilized Stellaria longipes because this species has two distinct natural population ecotypes, alpine (dwarf) and prairie (tall). The alpine population occupies the open, sun habitat. By contrast, the prairie population grows in the shade of other plants. Both 'sun' and 'shade' ecotypes responded with increased stem elongation responses under low irradiance, relative to growth under 'normal' irradiance, and this increased growth was proportionally similar. However, only the shade ecotype had increased shoot elongation in response to a low R/FR ratio. By contrast, the sun ecotype showed increased stem elongation in response to increasing R/FR ratio. Varying the R/FR ratios had no significant effect on ethylene evolution in either sun or shade ecotype. Under low irradiance, only the sun ecotype showed a significantly changed (decreased) ethylene evolution. We conclude that R/FR ratio and irradiance both regulate growth, and that irradiance can also influence ethylene evolution of the sun ecotype. By contrast, R/FR ratio and irradiance, while having profound influences on growth of the shade ecotype, do not appear to regulate these growth changes via effects on ethylene production.

Involvement of gibberellins in the stem elongation of sun and shade ecotypes of Stellaria longipes that is induced by low light irradiance.

Plants from two ecotypes of Stellaria longipes, alpine (an open, sunny habitat) and prairie (where adjacent plants provide a shaded habitat), were grown under normal and reduced levels of photosynthetically active radiation (PAR). Growth under low PAR is significantly promoted in both ecotypes. When quantified by the stable isotope dilution method, endogenous gibberellins (GAs) (GA1, GA8, GA20, GA19) were significantly elevated under low PAR in both 'sun' and 'shade' ecotypes, as was GA53 in the shade ecotype. Changes in endogenous GA1 levels were significantly correlated with stem growth during a 28 d growth cycle and with relative growth rate (RGR) for height under low PAR for both ecotypes. Interestingly, under low irradiance PAR, changes (both increases and decreases) in GA8, the 2beta-hydroxylated 'inactive' catabolite of GA1, closely parallel bidaily stem growth changes for both ecotypes. Because the significantly greater stem elongation of both ecotypes in response to low irradiance PAR is associated with significant increases in the endogenous levels of five GAs (GA53, GA19, GA1, GA8) in the early 13-hydroxylation GA biosynthesis pathway (measured at days 7,14 and 21), we conclude that the low irradiance PAR has very likely induced an overall increase in GA biosynthesis.

Phytochrome-mediated long-term memory of seeds.

The question is how long phytochrome, stored within the cytoplasm of plant diaspores, may stimulate their germination. This question arose from the observation that soil cultivations in darkness for weed control gave inconsistent results. Namely, after a single nighttime or daytime cultivation during spring and summer, differences in weed emergence became hardly detectable after a period of six weeks. However, after nighttime and daytime cultivations in late autumn, emergence differences persisted for up to nine months. To examine whether this differing memory effect is phytochrome-mediated, seeds of Chenopodium album and Stellaria media were sown in pots with wet peat, either in daylight or after sunset. In the latter, seeds were irradiated with far-red light for one day prior to being covered and buried. For more than two years the far-red irradiated seeds produced significantly reduced emergence, indicating that germination and emergence of weeds in the field may be supported by maternal far-red absorbing seed phytochrome B(fr) over several months or even years. This conclusion allows refining of the strategy of lightless tillage.