Effects of Root-Zone Temperature on Photosynthesis, Productivity and Nutritional Quality of Aeroponically Grown Salad Rocket (<i>Eruca sativa</i>) Vegetable

Abstract

Although tropical high ambient temperature and humidity severely reduced the productivity of temperate plants, temperate vegetable crops such as lettuce have been successfully grown in Singapore by only cooling its root-zone. In this paper, a cool Meditteranean vegetable, Eruca sativa , was studied to understand how different RZTs can impact its shoot productivity, photosynthesis and nutritional quality . All plants were cultivated using aeroponic systems in a tropical greenhouse under hot ambient conditions where roots were subjected to four different root-zone temperatures (RZTs) of 20&deg;C-RZT, 25&deg;C-RZT, 30&deg;C-RZT and fluctuating ambient temperatures ranged from 25&deg; C to 38&deg;C [25&deg;C/38&deg;C (ambient)]-RZT. Parameters studied include shoot fresh weight (FW), photosynthetic gas exchange, midday chlorophyll (Chl) fluorescence F v /F m ratio, Chl fluorescence photochemical quenching (qP), non-photochemical quenching (qN) and electron transport rate (ETR), total phenolic compounds and mineral content such as potassium (K), calcium (Ca), magnesium (Mg) and iron (Fe). Among the 4 different RZT treatments, E. sativa plants grown under ambient-RZT (25/38&deg;C-RZT) had the lowest shoot and root FW while those plants grown under 20 &deg; C-RZT had highest productivity of shoot and root. However, there were no significant differences in shoot and root FW in plants grown at 25&deg;C- and 30&deg;C-RZT. Compared to plants grown under 25&deg;C/38&deg;C (ambient-RZT), light-saturated photosynthetic CO 2 assimilation rate ( A sat ) and stomatal conductance ( g ssat ) were similarly higher in 20&deg;C-, 25&deg;C- and 30&deg;C-RZT. All plants had midday Chl fluorescence F v /F m ratio lower than <0.8 ranged from 0.785 to 0.606 with the highest and lowest ratios recorded in 20&deg;C-RZT and ambient-RZT plants, respectively. These results indicate that cooling the RZ of E. sativa plants protected their PS II from photoinactivation during midday in the greenhouse. There were no significant differences observed in photochemical quenching (qP), non-photochemical quenching (qN) and electron transport rate among plants grown under 20&deg;C-, 25&deg;C- and 30&deg;C-RZT. However, plants grown under ambient-RZT had lower qP, qN and ETR compared to all other plants. E. sativa at 20&deg;C-RZT with the best developed roots had the highest dietary mineral (K, Mg, Ca and Fe) contents but lower total phenolics content. In contrast, ambient-RZT, plants with poorly developed roots had the lowest mineral content but highest total phenolic content. The results of this study suggest that cooling of roots is a feasible method for the cultivation of E. sativa in the tropic, which enhances the content of dietary minerals in shoots.