Heat Priming Impacts on Root Morphology, Productivity and Photosynthesis of Temperate Vegetable Crops Grown in the Tropics Jie He, Cheng-Hsiang Lai, Yong Jie Lim, and Lin Qin National Institute of Education, Nanyang Technological University, Singapore Email: {jie.he, chenghsiang.lai, lin.qin}@nie.edu.sg, illuslyj@gmail.com Abstract—Many plants have an inherent basal thermotolerance and they are able to survive temperatures over the optimum for growth and development. This project aimed to investigate if heat stress priming at the root-zone (RZ) could be used to induce thermotolerance through the studies of root morphology, productivity and photosynthesis of temperate vegetables aerponically grown in the tropics. Two high valued temperate vegetable crops were used for this study: Lactuca sativa (cv. Canasta) and Eruca sativa (cv. Arugula). Heat stress at the RZ priming was studied by exposing the plants to three different root-zone temperature (RZT) treatments: (1) 25 o C-RZT, (2) 25 o C-RZT→42 o C-RZT (defined as non-hardening) and (3) 25 o C -RZT →38 o C- RZT→42 o C–RZT (defined as hardening). It was found that RZ heat stress priming did not have any negative effects on the root morphology for both vegetable crops. Compared to Arugula, Canasta had heat hardening effects which increased productivity at high RZT of 42 o C with better photosynthetic performance. Since RZ heat hardening stimulated shoot growth of Canasta, this finding has practical significance on using RZ heat stress priming to induce thermotolerance of certain temperate vegetable crops grown in the tropics that could enhance productivity at low production cost. Index Terms—heat priming, photosynthesis, productivity, root morphology, temperate vegetable, tropics I. INTRODUCTION Heat stress due to global warming is of major concern worldwide, causing a loss of crop yield including vegetable crops [1]. Plants are sessile organisms that cannot escape heat [2]. In nature, adverse conditions are frequently interminable or repeating. Therefore, plants have evolved a variety of strategies to cope with extreme temperatures to minimise the damage [3]. One of such strategy is known as priming. Plants can be primed by heat stress, which enable them to survive temperatures that are lethal to a plant [4]. It was reported that heat priming could effectively improve heat tolerance to the later recurred heat stress in winter wheat [5]. Singapore is hot, humid and relatively uniform throughout the year with average daily temperature ranges from 25-34C. The highest ambient temperature in Manuscript received July 20, 2018; revised November 21, 2018. the greenhouse can reach up to 42C for 4 to 5 hours on sunny days. Furthermore, Singapore is high population density country with limited agricultural land. Thus, Singapore has to import all her needs in temperate crops. Temperate vegetables such as lettuce (Lactuca sativa L.) has been grown successfully in Singapore using our innovate aeroponic systems by cooling their root zoon (RZ) while shoots were exposed to hot fluctuating ambient temperatures ranging from 26°C to 42°C [6]-[8] in a tropical greenhouse. Today, in Singapore, all kinds of subtropical and temperate vegetables are grown all year- round by simply cooling their RZ while their aerial portions are subjected to hot fluctuating temperatures [9], [10]. High energy costs when cooling the RZ of the vegetable crops, however, is now much of a concern. Identifying thermotolerant temperate crops would be one of the means to minimize the cost of production. On the other hand, plants have the ability to acquire thermotolerance, induced by subjecting them to a sublethal high temperature (“hardening”). The acquisition of high level of thermotolerance protects plants from a subsequent lethal heat stress [11]. Moderate heat stress primes a plant to withstand high temperatures that are deadly to non-acclimatized plants [12]. The ability of plants to tolerate high temperatures, without hardening is referred to as basal thermotolerance, whereas acquired thermotolerance refers to the adaptive capacity to survive lethal high temperatures, after priming [11]. Heat stress adversely affects seed geminating, plant growth and development, photosynthesis, respiration, water relations and membrane stability [13], [14]. Although priming of the entire plants, seeds and cell lines have been reported, there is very little information available on the effects of RZ heat hardening and its effect on root morphology and photosynthetic performances are relatively unexplored. Thus, the objectives of this project were (1) to determine if a sub- lethal heat priming in the RZ could induce thermotolerance in temperate crops grown in the tropics and, (2) to evaluate the effects of heat priming on root morphology and photosynthesis. In this study, during the RZ heat hardening period, that was, from 11 to 20 days after transplanting (DAT), root morphology such as total root length, total root surface area, average root diameter and total number of root tip were first studied to 14 Journal of Advanced Agricultural Technologies Vol. 6, No. 1, March 2019 ©2019 Journal of Advanced Agricultural Technologies doi: 10.18178/joaat.6.1.14-19