Root-Zone Heat Priming Effects on Maximum Quantum Efficiency of PSII, Productivity, Root Morphology and Nutritional Quality of Two Aeroponically Grown Leafy Greens in a Tropical Greenhouse

Abstract

This study investigated the effects of root zone (RZ) heat priming on Eruca sativa (cv. Arugula) and Lactuca sativa (cv. Canasta) in a tropical greenhouse by exposing them to three different RZ temperatures (RZTs). For the first group, plants were grown at 25 °C-RZT for 36 days. The second group of plants was grown at 25 °C-RZT for 10 days before transferring to 38 °C-RZT for 10 days and then to 42 °C-RZT for another 16 days (primed). For the third group, plants were grown at 25 °C-RZT for 20 days and for the last 16 days were transferred to 42 °C-RZT (non-primed). RZ heat priming did not affect the leaf expansion of Arugula while the leaf expansion was slower in RZ heat primed Canasta compared to 25 °C-RZT plants. After transferring to 42 °C-RZT in the later stage, RZ heat primed Arugula had similar productivity but a larger root system with higher nutritional quality at harvest and during postharvest storage compared to those of 25 °C-RZT plants. After subjection to 42 °C-RZT, although RZ heat priming enhanced its nutritional quality, the productivity of Canasta was compromised. The results of this study suggest that it is feasible to enhance productivity and nutritional quality of leafy greens at lower production cost through RZT management.

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

“…In another recent study, we carried out the same RZ heat priming for 10 days for the same species in the same greenhouse, but plants were exposed to a subsequent RZT of 42 • C for 16 days. It was found that RZ heat priming at RZT of 38 • C resulted in the slower leaf expansion of canasta after six days of RZ heat priming compared to those grown under RZT of 25 • C. However, RZ heat priming did not affect the leaf expansion of arugula compared to 25 • C-RZT plants [20]. During RZ heat priming, dynamic photoinhibition measured by lowered midday Fv/Fm ratios in canasta leaves [20] could lead to a decline in overall photosynthetic performance [17,86], which could be partially responsible for the slower leaf growth.…”

“…It was found that RZ heat priming at RZT of 38 • C resulted in the slower leaf expansion of canasta after six days of RZ heat priming compared to those grown under RZT of 25 • C. However, RZ heat priming did not affect the leaf expansion of arugula compared to 25 • C-RZT plants [20]. During RZ heat priming, dynamic photoinhibition measured by lowered midday Fv/Fm ratios in canasta leaves [20] could lead to a decline in overall photosynthetic performance [17,86], which could be partially responsible for the slower leaf growth. Improvements in heat resistance to prevent damage to the photosynthetic apparatus may be coupled with reduced photosynthetic performance during heat priming.…”

“…This could be due to the development and maintenance of the primed effect [84]. Despite its significantly lower midday Fv/Fm ratio, RZ heat priming did not affect the leaf expansion of arugula [20]. Both plant species had predawn Fv/Fm ratios greater than 0.8, indicating that no chronic photoinhibition occurred in any plants during RZ heat priming [20].…”

“…Increases in both the average temperatures and frequency of heat waves are likely to happen severely in the tropics [84]. Thus, RZ heat priming could serve as one of the promising means to further reduce high energy consumption in cultivating vegetable crops in tropical greenhouses [19,20]. In our previous study, two different leafy greens, Eruca sativa (cv.…”

“…For a tropical greenhouse, even when cooling only the RZ of the vegetable crops, the power energy requirement is still a major production cost. Our recent studies show that RZ heat priming could serve as a promising strategy to reduce the energy costs of growing temperate crops in a tropical greenhouse [19,20]. Thus, the first part of this review discusses the mechanisms responsible for the manipulation of RZT on the productivity of aeroponically grown leafy vegetables in the tropical greenhouse.…”