Abstract— The duration of the far‐red light‐absorbing form of phytochrome (Pfr) of the photoreceptor pool involved in the control of seed germination was investigated for Datura ferox seeds. These seeds require both Pfr and alternating temperatures (20/30°C) to germinate. After 24 h imbibition (25°C), the seeds received pretreatment‐light pulses providing different phytochrome photoequilibria (Pfr/P), followed by a 24 h dark incubation (25°C), and test‐light pulses providing different Pfr/P immediately prior to transfer to alternating temperatures. Germination increased with increasing Pfr/P provided by the test‐light pulses, but was unaffected by the pretreatment‐light pulses. This suggests that phytochrome synthesis, phytochrome degradation and phytochrome‐mediated changes in response to phytochrome were negligible. In other experiments, red light‐pretreatment pulses were followed by dark incubations (25°C) of different duration before transfer to alternating temperatures. The proportion of Pfr remaining after the 25°C incubation period was estimated by comparing germination rates with those of seeds that received test‐light pulses of known calculated Pfr/P immediately prior to the start of the cycles of alternating temperatures. More than 80% of the Pfr established by a Pfr/P= 0.87 light pulse was present and active even after 48 h dark incubation at 25°C. Surprisingly, when a pretreatmentlight pulse providing a Pfr/P= 0.70 was given, the reduction in [Pfr] was significantly faster. Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light‐absorbing form, the times to reach half the original Pfr pool being > 96 h or <14 h after light pulses providing Pfr/P= 0.87 or 0.70, respectively.

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Although Azospirillum spp. are considered to be important plant growth promoting bacteria, their possible effects on germination and vigor of aged lettuce seeds has not been previously evaluated. In fact, there is a paucity of published data about inoculation effects on seed germination. The aim of this work was to evaluate seed quality of one-year aged lettuce (Lactuca sativa L. cv. Crimor INTA) seeds after Azospirillum brasilense Sp245 inoculation with or without an osmopriming pretreatment. Fresh lettuce seeds were stored in the dark in a dry chamber for one year and then subjected to inoculation with A. brasilense Sp245, to osmotic priming with 0.37 mol/L MgSO4, or to both combined treatments. Seed germination, seed vigor, and seedlings emergence percentages were determined, and the abnormal seedling fraction was characterized. Azospirillum brasilense inoculation without a previous osmopriming enhanced seed vigor and seedling emergence percentages and decreased the fraction of abnormal seedlings. There was an additional effect of osmopriming as a previous treatment on germination percentage. We concluded that the use of A. brasilense inoculation alone or after an osmopriming treatment would contribute to overcome the negative effects of ageing on lettuce cv. Crimor INTA seeds.

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Plant propagators must take two technological critical decisions: the plug-cell size and the growing medium, both of which have been mentioned as abiotic stress sources for bedding pot plants. However, only a few recent reports on bedding pot plants have simultaneously included limiting and non-limiting plug cell volumes and growing medium during nursery. The aim of this work was to assess the nursery performance of Impatiens walleriana seedlings grown in four plug cell volumes and four growing media with significant differences in both physical and chemical properties. Plants were sprayed or not with an early and single benzyl aminopurine (BAP) dose, aiming to understand how they interact on determining biomass accumulation at the pot transplant stage. The hypothesis tested was that, both plug cell volume and growing medium, must be seen as additive abiotic stress sources, which can be partially overridden by exogenous cytokinin supply. The main result was that, in I. walleriana seedlings, the abiotic stress imposed by the growing medium quality during nursery had a higher effect on biomass accumulation (on both fresh and dry base), leaf area expansion and photo assimilates partitioning than plug cell volume and constitute an interactive process associated with cytokinin synthesis. From a grower´s point of view, one expensive option to avoid root restriction is to use high quality growing media and increase the plug cell volume. In contrast, a single 100 mg L-1 BAP spray can partially override the root restriction symptoms related to abiotic stresses. The novelty of this work is related to the fact that growing media quality would be a more limited factor than plug cell volume for I. walleriana seedlings during nursery.