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.
