Growing Different Lactuca Genotypes Aeroponically within a Tropical Greenhouse—Cool Rootzone Temperatures Decreased Rootzone Ethylene Concentrations and Increased Shoot Growth

Abstract

Temperate crops cannot grow well in the tropics without rootzone cooling. As cooling increased production costs, this experiment aimed to study the growth of various Lactuca genotypes and propose possible ways of reducing these costs, without compromising productivity. A recombinant inbred line (RIL) of lettuce and its parental lines (L. serriola and L. sativa “Salinas”) were grown aeroponically in a tropical greenhouse under 24°C cool (C) or warm fluctuating 30–36°C ambient (A) rootzone temperature (RZT). Their roots were misted with Netherlands standard nutrient solution for 1 min, at intervals of either 5 min (A5, C5) or 10 min (A10, C10) in attempting to reduce electricity consumption and production costs. Lower mortality and higher productivity were observed in all genotypes when grown in C-RZT. Higher shoot fresh weight was observed under C5 than C10, for the RIL and L. serriola. Since “Salinas” had similar shoot fresh weight at both C-RZ treatments, this may indicate it is more sensitive to RZT than water availability. Under A-RZ treatments, higher carotenoid content, with correspondingly higher nonphotochemical quenching, was observed in A10 for the RIL and “Salinas.” Further, total chlorophyll content was also highest at this RZ treatment for the RIL though photochemical quenching was contrastingly the lowest. Cumulatively, productivity was compromised at A10 as the RIL seemed to prioritize photoprotection over efficiency in photosynthesis, under conditions of higher RZT and lower water availability. Generally, higher RZ ethylene concentrations accumulated in A10 and C10 than A5 and C5, respectively—probably due to spray frequency exerting a greater effect on RZ ethylene accumulation than RZT. In the C5 RZ treatment, lowest RZ ethylene concentration corresponded with highest shoot fresh weight. As such, further research on ethylene (in)sensitivity and water use efficiency could be conducted to identify Lactuca cultivars that are better suited for growth in the tropics, so as to allay production costs with reduced cooling and spray intervals.

Getting to the roots of aeroponic indoor farming

“…In vertical farms, high root zone temperatures inhibit root growth and cause nutrient deficiency, reducing photosynthetic efficiency (Tan et al, 2002;He et al, 2007He et al, , 2010He et al, , 2013Choong et al, 2016). This inhibition can be reversed in aeroponic horticulture by root zone cooling or CO 2 supplementation (Tan et al, 2002;He et al, 2010He et al, , 2013.…”

“…This inhibition can be reversed in aeroponic horticulture by root zone cooling or CO 2 supplementation (Tan et al ., 2002; He et al ., 2010, 2013). For example, cooling the root zone of aeroponic‐cultivated lettuce to 20°C increased root surface area and root/shoot mineral content compared with plants grown at tropical temperatures (23–38°C) (Tan et al ., 2002), and similar root‐zone cooling in tropical greenhouses increased lettuce shoot yields (Choong et al ., 2016). Furthermore, root zone CO 2 supplementation of aeroponically grown lettuce, with root zone temperatures of 20–38°C, increased the Rubisco concentration and protected plants against photoinhibition, potentially due to increased NO 3 − uptake (He et al ., 2013).…”

Development and experiment of the intelligent control system for rhizosphere temperature of aeroponic lettuce via the Internet of Things

“…Lettuce production was possible, even at particularly high air temperatures of up to 41°C, as long as their roots were cooled [18] . In this study, all plants were exposed to fluctuating tropical air temperatures while their root zones were maintained within two different ranges: 28.7°C-29.2°C (ACRz), and 29.5°C-31.5°C (NCRz).…”

“…The diameter of the spray circle of the foggers was 0.5 m, 8 L/h flow rate, and 2 bar operating pressure. The spraying time of 1 min and spraying interval of 5 min was kept constant throughout the experiment [18] . The components of the system are shown in Figure 1.…”

Enhancing Productivity and Improving Nutritional Quality of Subtropical and Temperate Leafy Vegetables in Tropical Greenhouses and Indoor Farming Systems

“…Subtropical and temperate vegetable crops cultivated in cool RZTs exhibited longer total root lengths, higher numbers of root tips, expanded root surface areas, and smaller root diameters in contrast to those grown in hot ambient RZTs [62][63][64]. Abscisic acid (ABA) and ethylene synthesized in the roots play major roles in regulating root growth and development [65][66][67][68][69]. Cool RZT decreased ABA concentration and alleviated ABA-induced stomatal closure [65,66,69].…”