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.
Highlights
IntroductionAs roots are more thermosensitive than shoots (Tachibana, 1982; Thompson et al, 1998; Sakamoto and Suzuki, 2015), temperate and subtropical crops have been successfully grown in a tropical greenhouse by cooling only their roots (Lee et al, 1994; Choong, 1998; He and Lee, 1998; He et al, 2001)
As cultivated lettuce is a temperate plant, growing it under tropical conditions decreased head biomass and quality (He and Lee, 1998; He et al, 2001; He, J. et al, 2009; Choong et al, 2013). He and Lee (1998) obtained lettuce that was at least four times heavier when grown in C-rootzone temperature (RZT) of 15–25◦C than A-RZT (26–41◦C), in a tropical greenhouse where aerial temperatures reached a maximum of 41◦C at midday
Plants that were growing in A5 and A10 were exposed to very similar RZT ranges (Figure 1A), throughout the 28-day growth period
Summary
As roots are more thermosensitive than shoots (Tachibana, 1982; Thompson et al, 1998; Sakamoto and Suzuki, 2015), temperate and subtropical crops have been successfully grown in a tropical greenhouse by cooling only their roots (Lee et al, 1994; Choong, 1998; He and Lee, 1998; He et al, 2001). Taking advantage of the innately high specific heat capacity of water, aeroponic systems use small volumes of chilled nutrient solution to lower RZTs to conditions that are ideal for the proliferation of these temperate and subtropical crops (Lee, 1993). Decreased root fresh weight correlated with root morphological traits including decreased total root length, root surface area, and number of root tips (i.e., branching) in A-RZT (Choong et al, 2013) These poorly developed root systems affect shoot growth as root-sourced signals that regulate shoot growth are transmitted via the xylem (Freundl et al, 1998; Dodd, 2005). Greater temperature tolerance of some accessions of L. serriola (Argyris et al, 2008) offers the possibility of minimizing shoot growth inhibition caused by A-RZT
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