In recent years, the automation of genotyping has significantly enhanced the efficiency of genome-wide association studies. Due to this development, phenotyping is now usually the rate-limiting step, especially in the field. Efforts are now focused on further automating in-field phenotyping. Here we present a GWAS study on 194 field-grown accessions of lettuce (Lactuca sativa). These accessions were non-destructively phenotyped at two time points 15 days apart using an unmanned aerial vehicle. Our high throughput phenotyping approach integrates an RGB camera, a multispectral camera to measure the reflectance at 5 wavelengths (blue, green, red, red edge, near-infrared), and precise height estimation. We used the mean and other descriptives such as median, quantiles, minimum and maximum to quantify different aspects of color and height variation in lettuce from the drone images. Using this approach, we confirm several previously described QTLs, now in populations grown under field conditions and identify several new QTLs for plant-height and color. ### Competing Interest Statement The authors have declared no competing interest.

For a horticultural crop such as tomato (Solanum lycopersicum), the initial growth phase of young plants can take place in multi-layer systems to reduce space. Here LEDs form the ideal lighting system, as they decouple light intensity from heating and can thus be placed in close proximity to the plants. Moreover, the spectral quality control of LEDs may be utilized to steer the plants towards a desired compact and sturdy phenotype. To achieve this, we must understand how light quality affects plant elongation and stem thickness during early plant development. Therefore, we assessed apical and radial growth of tomato and Arabidopsis thaliana (Arabidopsis) plants grown in white, red, or blue LED conditions. Our analysis revealed that in both species the red LED condition increased cell elongation in hypocotyls and stems, whereas the blue LED condition decreased cell elongation, compared to the white light condition. In seedlings, hypocotyls were thinner in the red LED condition, and thicker in the blue LED condition, compared to white light. However, in flowering plants, Arabidopsis showed sensitivity of primary radial growth to light quality, while tomato plants appeared indifferent. Finally, analysis of Arabidopsis photoreceptor mutants suggested that cryptochromes and type II phytochromes are the main regulators of light-mediated apical and primary radial growth. To summarize, LEDs can be used to regulate both apical and primary radial plant growth, but the resulting phenotypes may be plant age- or species-specific.

In nature, plant shoots are exposed to light whereas the roots grow in relative darkness. Surprisingly, many root studies rely on in vitro systems that leave the roots exposed to light whilst ignoring the possible effects of this light on root development. Here, we investigated how direct root illumination affects root growth and development in Arabidopsis and tomato. Our results show that in light-grown Arabidopsis roots, activation of local phytochrome A and B by far-red or red light inhibits respectively PHYTOCHROME INTERACTING FACTORS 1 or 4, resulting in decreased YUCCA4 and YUCCA6 expression. As a result, auxin levels in the root apex become suboptimal, ultimately resulting in reduced growth of light-grown roots. These findings highlight once more the importance of using in vitro systems where roots are grown in darkness for studies that focus on root system architecture. Moreover, we show that the response and components of this mechanism are conserved in tomato roots, thus indicating its importance for horticulture as well. Our findings open up new research possibilities to investigate the importance of light-induced root growth inhibition for plant development, possibly by exploring putative correlations with responses to other abiotic signals, such as temperature, gravity, touch, or salt stress.

In vertical farming, plants are grown in multi-layered growth chambers supplied with energy-efficient LEDs that produce less heat and can thus be placed in close proximity to the plants. The spectral quality control allowed by LED lighting potentially enables steering plant development toward desired phenotypes. However, this requires detailed knowledge on how light quality affects different developmental processes per plant species or even cultivar, and how well information from model plants translates to horticultural crops. Here we have grown the model dicot Arabidopsis thaliana (Arabidopsis) and the crop plant Solanum lycopersicum (tomato) under white or monochromatic red or blue LED conditions. In addition, seedlings were grown in vitro in either light-grown roots (LGR) or dark-grown roots (DGR) LED conditions. Our results present an overview of phenotypic traits that are sensitive to red or blue light, which may be used as a basis for application by tomato nurseries. Our comparative analysis showed that young tomato plants were remarkably indifferent to the LED conditions, with red and blue light effects on primary growth, but not on organ formation or flowering. In contrast, Arabidopsis appeared to be highly sensitive to light quality, as dramatic differences in shoot and root elongation, organ formation, and developmental phase transitions were observed between red, blue, and white LED conditions. Our results highlight once more that growth responses to environmental conditions can differ significantly between model and crop species. Understanding the molecular basis for this difference will be important for designing lighting systems tailored for specific crops.