Daily intake of vegetables is known to be beneficial to human health, but the availability of vegetables is at risk because of the current trend of global warming. Vegetable production is greatly influenced by environmental factors, such as light intensity, temperature, and ambient CO2 concentration. Furthermore, past research has revealed that different crop species may respond differently to elevated temperatures. For example, Lara and Andreo (2011) indicated that C4 plants show higher photosynthesis and growth rates under high light intensity and high temperature conditions. Although the effects of elevated temperatures on warm-season C4 cereals and weeds are widely studied (Crafts-Brandner and Salvucci, 2002; Du et al., 2009; Pompeiano et al., 2013; Wang et al., 2016), the effects of elevated temperatures on C4 vegetables such as amaranth remain largely unknown. There is an urgent need to study the physiological responses of C4 vegetables under elevated temperature conditions to fill these knowledge gaps.
Summer vegetable production in Taiwan is quite vulnerable to typhoons, pests, and diseases. After a natural disaster that causes a shortage of fresh vegetables, leafy vegetables are normally the first to return to markets because of their short growth from seeding to harvest. Because C3 and C4 plants have been reported to respond differently to elevated temperatures, we selected one widely grown C3 summer leafy vegetable (pak choi, aka nonheading Chinese cabbage) and one important C4 summer leafy vegetable (edible amaranth) for this study. A recent study on the effects of sudden increase in temperature on photosynthesis showed that net photosynthesis of Chinese cabbage is decreased when leaf temperature is greater than around 25 °C (Oh et al., 2015). Furthermore, the effect of long-term elevated temperatures and CO2 on Chinese cabbage was cultivar dependent (Choi et al., 2011). The effects of elevated temperatures on Chinese cabbage root growth and leaf nutrient status were not documented in the above-mentioned studies. However, a recent study reported that some common foods including vegetables could serve as natural sources of antioxidants if they possess a high phenolic content (Kamath et al., 2015). It is of interest to know how elevated temperatures may influence the total phenolic compounds (TPCs) in leafy vegetables. In addition, nitrate content in leafy vegetables is a major concern for vegetable consumers, especially in countries such as Taiwan where consumers prefer eating leafy vegetables. Previous reports indicated that nitrate may turn into nitrite that may then react with some amines or amides to form nitrosamines, which are known to be carcinogenic (Bruning-Fann and Kaneene, 1993; Magkos et al., 2006). Thus, it is important to investigate how elevated temperatures may influence the levels of nutrients, TPCs, and nitrate in leafy vegetables such as pak choi and amaranth.
Leafy vegetables are cultivated extensively in central Taiwan where the climate is subtropical. Results from this study allow us to compare the root growth and the physiological responses of leaves of pak choi (a C3 vegetable) and edible amaranth (a C4 vegetable) under current (28 °C) and elevated (32 °C) temperatures and to further provide relevant information that can lead to the development of strategies to cope with the effect of elevated temperatures on leafy vegetable production in Taiwan and other subtropical regions.
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