Evaluation by optical and soil sensors of forest seedlings grown in controlled conditions under low-energy lighting (LED)

Introduction LED lights have a lower environmental impact than traditional lights due to a series of factors such as their wavelength specificity and narrow bandwidth, high energy-conversion efficiency, small volume, longer life, low thermal energy output. Concerning plant growth, the uses of LED lights provide specific wavelength as well as the possibility to adjust light intensity/quality. The increasingly need to reduce energy consumption worldwide, raised the necessity to improve LED lights use. The present study aims to 1) examine the effect of different LED light spectra on forest seedlings growth of different species, in order to define a species-specific cultivation protocols under optimal plant growth spectrum to enhance plant growth performance 2) compare direct measurements with indirect method by optical sensors system for automatic plant phenotyping 3) develop soil sensors system for automatic measurements of optimal soil water content. The plant species analysed, widely used in protective and productive planted forests, were: Scots pine Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.), Beech (Fagus sylvatica L.), Holm oak (Quercus ilex L.), Pomegranate (Punica granatum L.), Strawberry-tree (Arbutus unedo L.), Firetree (Morella faya (Ait.) Wilbur), Sanguinho (Frangula azorica Tutin) and Azores laurel cherry Prunus azorica (Hort. ex Mouillef.). Materials and methods Seeds of almost all investigated species were pre-treated to removal dormancy before sowing in mini-plug container. Trays were placed in growth chambers (12-14-16 h photoperiod, 110 ±10 μmol m-2 s-1 PAR, temperature 20-30°C, humidity 55-80%). Seedlings were left to grow for 4-8 weeks under G2, AP67, AP67-3L, NS1, AP67tube LED light (Valoya) and Fluorescent light (FL). The LED lamps used in this study emitted a continuous spectrum thanks to a mixture of blue, green, red and far-red LEDs. Direct measurements of plant height, root length, shoot and root biomass were carried. In order to estimate total root length, roots were scanned and analyzed by WinRHIZO Pro V. 2007d. Non-destructive analysis was carried by measuring Greenness (percentage of shoot cover projected on tray ground) and Plant height by optical sensors. Greenness data were obtained by a series of images acquired by specific developed Optical sensors system (ACREO) and analysed by uEyeDualcam software. After that HeightMap software recalculate greenness using uEyeDualCam settings output and create a plant height map (cm) of the tray conferring a value to the pixel of the selected images. Plant height data, manually taken during the growth period, were used to find a relationship with plant biomass and with data from images analysed by uEyeDualcam HeightMap software (ACREO). Electronic soil sensors tested in the trays with and without seedlings helding watering for two weeks in order to evaluate the soil water content measurements efficiency of the soil. Soil water content measurement obtained by the software “Zephyr logger” (Acreo) was compared with the SWC calculated by gravimetric measurements. Results and discussion The best results recorded for all studied species were under AP67, AP67-3L and G2 LED light type for all morphological parameters analysed. The lowest values among all LED light type were obtained under NS1 LED type for almost all morphological parameters. Results showed a linear increment of seedling height in time for six species (Pinus sylvestris, Picea abies, Quercus ilex, Fagus sylvatica and Punica granatum) and for all different light types. P. sylvestris, P. abies and A. unedo showed interesting results in root length mainly under G2 LED type. A. unedo showed slightly higher biomass values for seedlings growth under G2 LED light type, in particular for root and leaves biomass. The lowest values among all LED light type were obtained under NS1 LED type for almost all studied morphological parameters. The low seeds germination of Morella faya species was detected under all different LED light types. In particular, Morella faya seeds did not show any germination under AP67-3L LED light type. Analysis of the total dry mass increment (g; shoot + root) showed the highest values for seedlings growth under AP67 light type (bar) among all LED light types and control light. The lowest values of total dry mass increment were measured for seedlings growth under AP67 tube. A high heterogeneity in seed germination and plant growth was observed. Concerning the optical analysis results, relation between greenness and seedling biomass showed good correlation for all species until the tray was fully covered. Instead, the relation between seedling height and biomass showed good results with the two broad-leaved species but no relation was found for the two needle-leaved species. Indeed, the constant height of P. abies (L.) and P. sylvestris (L.) because internodes elongation did not occur during the consecutive emissions of new leaves, did not relate to the continuous increment of seedling biomass. This is probably due to the specie-specific characteristic. Thus, the best regression model to explain the relationship between direct biomass data and indirect measurements was based on parameters such as plant height, for broad-leaved species, and plant greenness for needle-leaved species. The main result of our study is that the relevance of relations between non-destructive parameters and forest seedlings growth is species specific. Conclusion - Plant growth performance with LED light is specie-specific. - LED lights represent an efficient and valid alternative to the fluorescent light. - The best performance for all studied species are observed with AP67, AP67-3L and G2 LED light type whereas NS1 LED type seems to not be suitable for this use. - G2 gave some good results but due to its higher percentage of far-red/red, it can cause operators’ eyes fatigue and could interfere with optical measurements such as greenness. - Finally, AP67 and AP67-3L LED type could represent the best option for a standard cultivation protocol. Data collected confirm that optical system (sensors and software) could represent a robust method to measure plant phenotype as alternative to the traditionally used destructive methods. Protocol of seed germination developed during the present study and applied to Morella faya seeds, showed good results for the ex-situ plant species conservation objective. Electronic soil sensors represent a good system to monitor the water content in the soil and when they are used in combination with LED light, and optical sensors the result is a complex system characterized by high level of cost-effectiveness coupled with a good possibility to save energy consumption and reduce pollution