Abstract
Photo-selective shading screens are emerging practices that aim to combine crop physical protection with different solar radiation filtration to achieve desired physiological responses. The objective of the present study was to evaluate the effect of photo-selective shading screens on the growth and physiology of purple lettuce for two transplanting seasons in tropical climate in Brazil. A 2 × 4 factorial arrangement was used, being considered the first factor, the transplanting season (spring and fall), and the second factor is the three 35% shading photo-selective shading screens (red, black and silver) and full sun control. The experimental design was a randomized complete block with four replications. The variables studied were: total fresh matter, total dry matter, leaf number, stem diameter, stem length, leaf area, SPAD chlorophyll index, nitrogen balance indexes, chlorophyll, flavonoids and anthocyanins. The photo-selective shading screens influenced the microclimate and the growth variables of purple lettuce, mainly when these plants were sown in the fall. Planting lettuce during spring may result in lower yields due to the higher investment of plants in secondary metabolites to defend against abiotic stress. According to the results, photo-selective shading screens are an appropriate agronomic technique to reduce phenolic compounds and improve lettuce cultivation conditions and can be implemented within protected cultivation practices to improve crop performance.
Highlights
Cultivation in protected environments has revolutionized the production of vegetables, making it possible to condition the microclimate to the needs of plants and extending the production period to times of the year and regions previously unfit for certain crops[5]
The graphical analysis showed that the microclimatic conditions of each environment, in which the lettuce was developed, apparently did not undergo major changes, judging by the variability of the average daily temperature (Adt) inside the structures of photo-selective screens and in the open field, indicating that there is no statistical difference in Adt between the treatments analyzed (Fig. 1)
Considering that the availability of light was reduced by photo-selective screens, these results suggest that, in our study, the amount of light appeared to be more important than the quality of light for changes in growth characteristics, how suggested by Bastías et al.[35]
Summary
World vegetable production has increased considerably over the past five decades, in which population growth plays a key role in the growing demand for these foods[1] Among these vegetables, lettuce (Lactuca sativa L.) has been gaining more and more market share, where in Brazil it is among the most important cultivated vegetables. The main cultivation techniques for vegetables in Brazil are those of open field cultivation, protected environment and hydroponic cultivation, being used according to the technical level of the producer and the need to maximize production. The use of pigmented screens, or technically photo-selective screens, is recent in Brazil and its effects go beyond isolated temperature interference They can increase the relative proportion of diffused light and absorb several spectral bands, affecting the quality of light[7,8].
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