Altering solar light with high tunnel coverings to improve health-promoting phytochemicals of lettuce and tomato

Abstract

High tunnels (HTs) have shown to increase marketability and yield of numerous crops compared to open field production. These structures alter light by utilizing specific polyethylene (poly) films and/or shade cloth, which may negatively affect phytochemical accumulation. This study evaluated the effect of HT coverings on the microclimate, yield, sensory attributes, and phytochemical concentration of tomato (Solanum lycopersicum cv. BHN 589) and lettuce (Lactuca sativa cv. Two Star, New Red Fire). Field experiments were carried out at the Kansas State University Olathe Horticulture Center in consecutive years from fall 2017 to spring 2019. An adjacent, open field (open) bed was used during the descriptive sensory study. The six HT coverings included standard, UV-stabilized poly (standard); diffuse poly (diffuse); full-spectrum clear poly (clear); UV-A/B blocking poly (block); standard + 55% shade cloth (shade); and removal of standard poly 2 to 3 weeks prior to initial harvest to simulate a movable tunnel (movable). The microclimate measurements of photosynthetic active radiation (PAR) were highest under the movable covering, and exceeded the diffuse and shade covering in the spring and summer seasons. During the fall, a decrease in available PAR, net photosynthesis (Pn), soil and canopy temperatures were observed under all tested coverings, minimizing the effect of the coverings. The coverings with higher soil temperatures in the spring and summer resulted in greater yields of lettuce and tomato. Phytochemical concentration of ascorbic acid (AsA), flavonoids, and phenolic acids were measured in breaker and light red mature fruit at harvest and throughout ripening in tomato and at harvest and after storage in lettuce. For tomato, the fruit under the clear and standard covering increased in AsA compared to the shade and movable coverings. The flavonoids, quercetin and rutin, increased with on-plant ripening, i.e. light red harvested fruit once reaching mature red was higher in concentration compared to breaker harvested fruit once reaching mature red. In the spring red and green lettuce, the flavonoid concentration of isoquercetin and rutin increased under the clear and movable covering. During the fall, a decrease in individual phenolic compounds was observed for both red and green lettuce. Sensory attributes of fall red lettuce were evaluated with a descriptive study at the Center for Sensory Analysis and Consumer Behavior in Manhattan, KS. The lettuce under the open, clear, and movable coverings had redder pigmented leaves relative to the other coverings as determined by both the panelists and instrumental analysis. Results from PCA analysis show a cluster was formed with the movable, clear, and open coverings, and 73% of the perceived variability between the HT coverings was due mostly to color intensity. The results of this work indicate that HTs can significantly alter solar light and temperature, thus altering yield and health-promoting phytochemical concentration of lettuce and tomato and perceived sensory attributes of red lettuce. As growers continue to adopt controlled production systems for increased yields, phytochemical accumulation should be considered in the system design, as it is important for human health.