Abstract
Biostimulants are substances able to improve water and nutrient use efficiency and counteract stress factors by enhancing primary and secondary metabolism. Premise of the work was to exploit raw extracts from leaves (LE) or flowers (FE) of Borago officinalis L., to enhance yield and quality of Lactuca sativa ‘Longifolia,’ and to set up a protocol to assess their effects. To this aim, an integrated study on agronomic, physiological and biochemical aspects, including also a phenomic approach, has been adopted. Extracts were diluted to 1 or 10 mL L–1, sprayed onto lettuce plants at the middle of the growing cycle and 1 day before harvest. Control plants were treated with water. Non-destructive analyses were conducted to assess the effect of extracts on biomass with an innovative imaging technique, and on leaf photosynthetic efficiency (chlorophyll a fluorescence and leaf gas exchanges). At harvest, the levels of ethylene, photosynthetic pigments, nitrate, and primary (sucrose and total sugars) and secondary (total phenols and flavonoids) metabolites, including the activity and levels of phenylalanine ammonia lyase (PAL) were assessed. Moreover, a preliminary study of the effects during postharvest was performed. Borage extracts enhanced the primary metabolism by increasing leaf pigments and photosynthetic activity. Plant fresh weight increased upon treatments with 10 mL L–1 doses, as correctly estimated by multi-view angles images. Chlorophyll a fluorescence data showed that FEs were able to increase the number of active reaction centers per cross section; a similar trend was observed for the performance index. Ethylene was three-fold lower in FEs treatments. Nitrate and sugar levels did not change in response to the different treatments. Total flavonoids and phenols, as well as the total protein levels, the in vitro PAL specific activity, and the levels of PAL-like polypeptides were increased by all borage extracts, with particular regard to FEs. FEs also proved efficient in preventing degradation and inducing an increase in photosynthetic pigments during storage. In conclusion, borage extracts, with particular regard to the flower ones, appear to indeed exert biostimulant effects on lettuce; future work will be required to further investigate on their efficacy in different conditions and/or species.
Highlights
In the last years, the use of biostimulants has been constantly increasing for sustainable agriculture, because these substances enhance nutrient use efficiency, reduce fertilizers consumption, stimulate plant development and growth (Kunicki et al, 2010; Calvo et al, 2014; Halpern et al, 2015; Le Mire et al, 2016), and counteract stress factors, eventually enhancing crop quality and yield (Ziosi et al, 2013; Van Oosten et al, 2017)
I.e., 4 days after the first treatment, the estimated average head weight (34 g) of plants treated with 1 mL L−1 of both LE and FE did not deviate from the control average (35 g), whereas some difference in the growth rate appeared in the groups treated with both LE and FE at the highest concentration (10 mL L−1)
The biostimulant Actiwave R applied as an additional component to the nutrient solution increases yield of rocket grown in a floating system, even with reduced nutrient concentrations (Vernieri et al, 2005); its effect was confirmed when administered as a spray on baby leaf lettuce grown in plastic tunnel (Amanda et al, 2009)
Summary
The use of biostimulants has been constantly increasing for sustainable agriculture, because these substances enhance nutrient use efficiency, reduce fertilizers consumption, stimulate plant development and growth (Kunicki et al, 2010; Calvo et al, 2014; Halpern et al, 2015; Le Mire et al, 2016), and counteract stress factors, eventually enhancing crop quality and yield (Ziosi et al, 2013; Van Oosten et al, 2017). Biostimulants are generally made of raw organic materials containing bioactive compounds Their nature is heterogeneous, since they include humic acids, protein hydrolysates, plant growth-promoting Rhizobacteria and fungi, and extracts from seaweeds and higher plant species (Ertani et al, 2013, 2016; du Jardin, 2015). Their chemical complexity and the wide range of molecules present make very difficult to understand which are the most active compounds (Brown and Saa, 2015; Bulgari et al, 2015; du Jardin, 2015; Posmyk and Szafranska, 2016; Yakhin et al, 2017)
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