In horticultural practice, accelerated plant development, and particularly earlier flowering, has been reported with microalgae applications. Therefore, the objective of this work was to study the effects of foliar spraying with Scenedesmus sp. and Arthrospira platensis hydrolysates on Petunia x hybrida plant development and leaf nutrient status. Three treatments were tested: T1 (foliar application with water, the control), T2 (foliar application with Arthrospira) and T3 (foliar application with Scenedesmus). Foliar spraying was applied 5 times (0, 14, 28, 35 and 42 days after transplanting). The concentration of both microalgae was 10 g L-1. At the end of the trial, biometric parameters and nutrient concentration in photosynthetic organs (the leaves) were measured. The results of this assay show that foliar application of Scenedesmus accelerated plant development in terms of higher rates of root growth, leaf and shoot development and earliness of flowering. Arthrospira enhanced the root dry matter, the number of flowers per plant and the water content. Nevertheless, a reduction was found in the conductive tissue (stem+petiole) dry weight with Arthrospira compared with Scenedesmus and the control. The results also show that microalgae hydrolysate supply can improve the plant nutrient status. Based on these results, it is advisable to use Scenedesmus hydrolysates in foliar applications to increase the blooming of Petunia x hybrida. 1.Introduction There is a growing need to develop environmentally friendly strategies for sustainable culturing of horticultural crops with high yields involving less environmental damage (Alves-Dias et al., 2016). This consists of reducing or substituting the use of chemical inputs, especially fertilizers and pesticides, with natural or

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Salinity is one of the environmental factors inhibiting productivity in crop plants. Available strategies to mitigate the abiotic stresses are limited. Microalgae and cyanobacteria can mitigate the adverse effects of abiotic stress due to their biostimulant properties. The aim of this work was to study the effect of hydrolysate of the ciyanobacterium Arthrospira platensis on the growth of Petunia plants under salt stress conditions over two growing seasons (winter and spring). Plants were exposed to 2.0, 2.5, and 3.0 dS m −1 EC, with and without application of A. platensis hydrolysate. At the end of the trial, biometric parameters and plant analysis were determined. Petunias had a negative response when the salinity level rose from 2 to 3 dS m −1 and exhibited moderate tolerance to high internal Na and Cl concentrations. Results show that a high salinity reduces the Ca, Mg, K, and S contents in the leaves and provokes a sharp reduction in the K + /Na + relationship. Applying A. platensis hydrolysate alleviates the effects of NaCl stress and stimulates shoot and bud formation in the petunia mother plant while inducing flowering in commercial Petunia plants. The A. platensis hydrolysate application increased the K + /Na + relationship in treatments with an EC of 3.0 dS m −1 .

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