Improving transplant success: algae extracts and mycorrhiza

Two experiments were performed to evaluate if inoculation in the nursery with an algae extract or mycorrhiza can result in sturdy plants better able to tolerate transplant stress. Experiment 1 was conducted at the Ersaf forest nursery (Curno, BG, Italy). To test the hypothesis that algae extract can improve root growth and morphology, seeds of 5 woody species were seeded in containers using a growing media enriched with 0x, 1x, 2x and 3x the label dose (1 kg/m3) of a pure Ascophyllum nodosum extract. After germination, 6400 seedlings were arranged according to a randomized complete block design with 10 blocks and grown under nursery conditions for 2 years. Plant, root, stem, and leaf dry weights were monitored by destructive measurements. Total plant leaf area was measured by scanning all leaves with an A3 scanner and performing image analysis using ImageJ software. Total root length was measured using the root line intersect method. Specific root length was calculated as root length/root dry biomass. Leaf gas exchange and chlorophyll content were measured using an infra-red gas analyser and a spad-meter, respectively. All measurements were conducted on 2, 6, 12, 24, 48, and 72 weeks after germination. During the first growing season, biostimulants improved plant growth, root length, root density, net photosynthesis, and chlorophyll content only when applied at 3x label dose. Minor effects, instead, were observed on root-shoot ratio and stomatal conductance. Growth-improvements, however, were not confirmed in the second growing season. Results suggest that substrate amendment with Ascophyllum extracts may have short-term positive effects on plant and root growth, likely due to a nutritional boost. However, it failed to trigger structural changes in plant traits that can enhance transplant tolerance in the long run, as also observed in other studies conducted with commercial biostimulants or mycorrhiza inocula. Experiment 2 was conducted at Fondazione Minoprio (Vertemate con Minoprio, CO) to test the hypothesis that species-specific mycorrhizal inocula can improve transplant success in Acer campestre and Tilia cordata. Two-year seedlings were obtained from a local nursery into 0.4L containers and re-potted into 3L containers. At repotting, half of the plants were inoculated with species-specific mycorrhizal fungi, obtained through a controlled mycorrhization approach, while the remaining half was not inoculated. The inocula contained endo- (Acer) or both endo- and ecto- (Tilia) mycorrhizal fungi. Plants were grown under nursery conditions for two years (avoiding any fungicide treatment). Then, 144 plants (36 per species and treatment) were transplanted in an experimental field characterized by a loam soil according to a randomized complete block design with 6 blocks. Leaf gas exchange, water relations, and growth were measured for 34 months after transplant. Inoculated A. campestre showed higher survival and growth, and more favourable water relations and leaf gas exchange than non-inoculated plants after transplanting. By contrast, survival, growth and physiology of Tilia cordata were little affected by inoculation, because the inoculum failed to improve root mycorrhizal colonization after transplanting. The overall results suggest that nursery mycorrhizal inoculation can affect post-transplant performances, although its effectiveness depends on species.