Abstract
Lettuce (Lactuca sativa L.) is a leafy vegetable cultivated widely for its fast and year-round production and its beneficial phytochemical content, which may be boosted further by plant biostimulants that are considered eco-sustainable means for enhancing horticultural crop production. A greenhouse experiment was carried out to evaluate the yield and qualitative parameters of two differently pigmented lettuce cultivars grown in a floating raft system either untreated or treated (leaf, root or leaf/root application) with vegetal protein hydrolysates (PHs). For foliar application (F), lettuce plants were sprayed at a dose of 3 mL L−1, whereas for root application, 0.15 (T1) or 0.3 (T2) mL L−1 was applied to the nutrient solution alone or in combination with foliar spray (T1 + F and T2 + F) with the same foliar concentration. Bio-morphometric and production data were collected after harvest. Physiological and plant nutrition assays included leaf gas exchange, leaf fluorescence, SPAD index, mineral content, carotenoids, total phenols, total ascorbic acid content and antioxidant activities. Cultivar-specific reactions to biostimulant application were noted: whilst the green pigmented cultivar thrived under nutrient solution applications and recorded higher yield by 82.7% (T1) or (T1 + F) and 71.7% (T2), the red cultivar thrived under combined treatments, yielding 55.4% (T2 + F) higher than control and providing the most concentrated phytochemical content. These latter treatments also engendered the highest SPAD index, Fv/Fm ratio, CO2 assimilation, stomatal conductance and transpiration. In addition, the T2 + F treatment boosted ‘Canasta’ hydrophilic antioxidant activity (21.9%) and total ascorbic acid (5.6-fold). Nutrient solution treatments alone proved advantageous when compared to foliar treatments, while mixed treatments proved genotype-specific. New research on genotype specificity of biostimulant effects is warranted for future use, in order to rationalize biostimulant application modes and dosages.
Highlights
Such increases can be explained by the modulation of yield parameters by the biostimulant, as they are consistent with leaf area, leaf fresh weight and stem fresh weights increases, which in turn provided for higher dry weights figures
The very different growing systems have an impact on this parameter: a consensus can be found in the available literature of leafy vegetables and in lettuce in particular grown in hydroponics having—other than the already mentioned advantages—higher leaf numbers compared to traditional soil and substrate-based systems [39,40,41], which is due to a variety of factors that are inherent to soil cultivation, such as suboptimal oxygen and moisture contents, competition from soil organisms and biotic and/or abiotic stresses [42,43,44,45] that are the prime culprits of yield losses
The results obtained in this trial suggest that the application of a legume-derived protein hydrolysates biostimulant on L. sativa has positive effects on crop performance, seen as elevated yield, physiology and quality parameters
Summary
Lettuce (Lactuca sativa L.) is one of the most grown vegetable crops at over 29 million tons harvested in 2019 [1] and in particular it embodies Italy’s most cultivated leafy green [2]. Whilst commonly grown in soilbased systems, concerns over land, fertilizers abuse and specialized soil-borne pathogens due to intensive cropping [6,7,8] has favored the introduction of soilless farming, of which the floating raft system represent a notable example. Advantages of growing Lettuce in floating systems include low maintenance, higher yield, nutrient and water efficiency and continuous cropping throughout the year [9,10]. By virtue of soil absence, floating systems can be employed in urban agriculture projects that provide better food availability, local social and economic development and reduced environmental impact [11]
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