Leafy vegetables are an important component in the farming systems as well as in the diets of people in Asia and Africa. Various leafy vegetables such as leafy brassicas (Chinese cabbage, pak-choi, kale, mustard, etc.), amaranth, water spinach, Malabar spinach, jute mallow, chayote, spider plant and African nightshade are grown in different parts of Asia and Africa, and they are mostly considered as “traditional” vegetables, since they are a part of alimentary traditions and cultural identity (Towns and Shackleton, 2018). These vegetables play an important role among the smallholder farmers, since they are a source of food and nutritional security, besides serving as income generating high value crops. For instance, Cambodian farmers were able to generate a revenue of US$ 4,776/ha from Chinese kale cultivation, whereas Vietnamese farmers earned US$ 5,070/ha from water spinach cultivation (Genova et al., 2010). Vegetable production has led to 3–14 times higher profits per hectare than in rice farming in Cambodia and Vietnam, while profits per labor-day are double (Joosten et al., 2015). The leafy vegetables are repeat-cycle crops, and the average length of growing period is 7–8 weeks, though farmers often re-sow their field after harvest or use staggered sowing to extend the harvest period (Schreinemachers et al., 2017). Thus, leafy vegetables are an important source of income for smallholder farmers.

Leafy vegetables are also an important source of nutrients. The leafy vegetables including amaranth and Chinese kale supply vitamins (especially vitamins A, C, folate), minerals such as calcium, potassium, iron, phosphorus, zinc, copper and manganese and dietary fiber to the human diet (Makobo et al., 2010; Ebert et al., 2011; Fowler, 2011). Water spinach is rich in protein, calcium, pro-vitamin A and vitamin C (Westphal, 1994). In addition, water spinach is a key contributor of lutein/zeaxanthin (Pan et al., 2018). However, the production of leafy vegetables is highly seasonal. For instance, amaranth is the predominant leafy vegetable in the summer months in Taiwan. Although water spinach is also grown during the summer months, leafy brassicas are mostly produced during the cooler months (Wang and Ebert, 2012).

Production of leafy vegetables is constrained by abiotic and biotic factors. Typhoons, for example, which regularly hit Taiwan during summer months, are often associated with heavy rainfall of up to 3,000 mm and subsequent flooding. Such extreme weather conditions not only lead to heavy yield losses in leafy vegetables, but also escalates their prices in the market (Wang and Ebert, 2012). Prices may soar to 3–5 times the normal level (Lee and Yang, 1999), which reduce their consumption during the summer months. Biotic factors such as pests and diseases also adversely affect the productivity of leafy vegetables. The leaf webber, Spoladea recurvalis F. (Lepidoptera: Crambidae) causes significant yield losses on amaranth in Asia (Hsu and Srinivasan, 2012) and Africa (Smith et al., 2018). White rust of amaranth, caused by Albugo bliti (Biv.) Kuntze, is a serious problem during the hot and humid conditions in Taiwan as well as in Southeast Asia (Grubben and Van Sloten, 1981; National Research Council, 1984), which causes almost 100% infection, considerably reducing the commercial value of the crop. Spotted tortoise beetle, Aspidomorpha miliaris F. (Coleoptera: Chrysomelidae), Convolvulus hawk-moth, Agrius convolvuli L. (Lepidoptera: Sphingidae), sweet potato stem borer, Omphisa anastomosalis Guenée (Lepidoptera: Crambidae) and common armyworm, Spodoptera litura F. (Lepidoptera: Noctuidae) are the major pests of water spinach (Muniappan et al., 2012).

Most leafy vegetables can be successfully grown under protective structures year-round. The leafy vegetables grown under protective structures using plug seedlings have been demonstrated to be grown faster, and harvested earlier with fewer pest problems compared to conventional production using direct seeding (Lee and Yang, 1999). Production under protective structures not only increases the total annual crop yield per unit area, but also improves the quality, besides extending their production period (Nordey et al., 2017). In recent years, use of color shade nets was found to protect the crops from adverse environmental conditions, improve the quality of vegetables and maintain post-harvest quality for an extended period (Ilić et al., 2018). But there is no evidence yet on how these shades affect yield or nutritional content of leafy greens. Hence, the objective of the current study is to determine the effects of different protective structures on the horticultural and nutritional yield of amaranth and water spinach across the seasons in Taiwan.

In this study, we investigated the effect of different colored poly-net house compared to open field conditions in terms of yield, dry matter content, iron, carotenoids and anti-oxidant activity for three leafy vegetables, viz., white amaranth, red amaranth, and water spinach, over three seasons (fall, winter and spring). In addition, the overall effect of pests and diseases was also evaluated under the above-mentioned conditions. Finally, the number of people who could receive selected micro-nutrients from the crops harvested under different production conditions was estimated based on the RDI values. This is quite important to understand if sufficient quantity of nutrient-dense vegetables can be grown in a relatively small area, considering the context of fast shrinking farmlands. To our knowledge, there are not many scientific information available on this perspective.

Generally, the amaranth yield was higher in the fall season, which could be due to the higher prevailing temperature during this season, ranging between 25.7 and 27.2°C (Table 11). An increased temperature (from 28 to 32°C) was found to promote both root and leaf growth in edible amaranth (A. tricolor cv. White leaf) in Taiwan (Hwang et al., 2018). Amaranth is a C4 plant, and an earlier study had demonstrated that photorespiration losses in C4 plants were limited and hence C4 plants had higher net photosynthetic rates at higher temperatures compared to C3 plants (Long, 1999). Hence, the higher temperature during the fall season was found to have a positive effect on the growth of this C4 crop. Besides temperature, the light conditions inside the poly-net houses were believed to have provided the optimum microclimate for growth and development of amaranth crops. For instance, the blue irradiance (400–500 nm) inside the pink net house was about 40%, whereas it was about 25% in the white poly-net house and open field conditions. It was already demonstrated that the blue (400–450 nm) polyethylene shade condition produced taller plant height, a greater number of leaves, biomass yield and bioactive compounds in red amaranth, compared with other conditions of films (Khandaker et al., 2010). This might be due to the fact that some plant species are more sensitive to blue light (Casal, 1994). Thus, the combination of higher temperature and blue light in the poly-net houses could have contributed for the better yield. Although both white and red amaranths are C4 plants, the yield of red amaranth was slightly higher in the winter season compared to the fall season, whereas it was not the case for white amaranth. Hence, besides temperature and light conditions, other factors may also contribute to these yield differences across the seasons. For instance, the photosynthetic pigments of higher plants include chlorophyll and carotenoids. The carotenoids content of the red amaranth were higher than the white amaranth, and most of the carotenoids in the white amaranth were lower in the winter season than in the fall. Hence, future studies should elucidate the role of photosynthetic pigments on yield under different production systems across the growing seasons. In addition, comparatively lower yield in the spring season, despite the warmer temperature could have been attributed to the higher spider mite infestation during this season. The growth and performance of water spinach was relatively better in the poly-net houses in all the seasons compared to the open field conditions, in which the lowest yield was recorded in the winter. Water spinach has originated from tropical regions, with high tolerance to heat and wet (Liou, 1981). The optimal growth conditions are temperatures between 20 and 27°C and the humidity of above 75% (Pinker et al., 2007). Since the winter temperature was quite low (Table 11), especially in the open field conditions, the yield might have declined significantly.

It is not surprising to record the spider mite infestation during the fall and spring season, in which the temperature is relatively warmer than the cold winter in Taiwan (Table 11). Most spider mites prefer warm and dry conditions. Although spider mite populations are found on crops in winter months in Taiwan (Ho, 2000), we did not find mite infestation on both the amaranth varieties and water spinach during the winter season. This is also due to the fact that aphids emerged as a major pest during the winter months and hence they outcompete spider mites. Aphids have been recorded as a serious pest of amaranth (Ebert et al., 2011). In fact, aphids occurred highly inside the poly-net houses. Several earlier studies have documented the aphid outbreaks inside the net house conditions (Talekar et al., 2003; Majumdar and Powell, 2011). A warm weather inside the poly-net houses during the winter months and the absence of natural enemies inside the protective structures could have led to the proliferation of aphid infestation in the current study. The major entry point for aphids and mites is through the nylon net. Most net houses were built with 40-mesh netting. The 40-mesh or coarser mesh nets failed to exclude the thrips, whitefly and aphid in several countries (Talekar et al., 2003; Harmanto, 2006). The poly-net houses used in the current study have been built with 32-mesh netting only. Hence, they were not able to prevent the entry of aphids and spider mites. The most promising way to prevent the entry of small-sized insects and mites in net houses is to use nets with finer mesh. Nets with 50-mesh or 60-mesh size to exclude thrips, whitefly, and aphids have been suggested (Polston and Lapidot, 2007; Shahak et al., 2008; Palada and Wu, 2009). However, finer mesh size reduces the ventilation rate and increases the relative humidity inside the net houses (Harmanto et al., 2006), which could favor the incidence of diseases, although it was not the case in the current study. It is interesting to note that the crop yields in both the poly-net houses were similar or higher than the open field production conditions despite the higher incidences of spider mite or aphids inside the poly-net houses. Hence, proper control options to manage aphids and spider mite on leafy vegetables in poly-net house conditions should be considered so that the crop productivity can be further increased.

Among the three production conditions compared in the current study, most of the carotenoids consistently occurred in higher quantities in the white amaranth crop produced under pink poly-net houses. Neoxanthin, lutein, and β-carotene were slightly higher in the red amaranth grown under the poly-net houses. Similarly, α-carotene was also higher in the amaranth crops grown under the poly-net houses. A recent study conducted under open field conditions in the same location documented higher amounts of violaxanthin, neoxanthin and lutein (Nordey et al., 2021) than the white amaranth in the current study, but the α- and β-carotenes were similar in both the studies. However, all the carotenoids in the previous study were lower than the quantities in the red amaranth in our study. It should be noted that the previous study involved amaranth accessions from different species (Amaranthus hypochondriacus, A. cruentus, A. dubius and A. blitum), whereas the varieties in the current study belonged to A. tricolor. In line with this, other papers on amaranth studying the carotenoid profiles corroborated our results, where higher carotenoids in the red amaranth have been observed compared to other genotypes (Khanam and Oba, 2013; Sarker and Oba, 2020). Hence, the quantity of carotenoids not only differed among the species of amaranth, but also varied among the varieties within a species. In addition, these variations could be altered by the production conditions, as we observed in the case of red amaranth under poly-net house conditions compared to the open field conditions. Hence, protected cultivation conditions not only lead to more marketable yields but also result in higher quantities of health promoting nutrients.

The quantities of the carotenoids such as lutein and β-carotene observed in water spinach in the current study were similar to the earlier findings (Khoo et al., 2011; Chandra-Hioe et al., 2017), and the α-carotene was 10–25-fold higher than an earlier report (Khoo et al., 2011). As observed in the case of amaranth crops, α-carotene in water spinach was also two to three-fold higher in the crops harvested from the poly-net houses than the open field condition. The iron content of the amaranth varieties produced from the open field conditions was comparable with the previous study (Nordey et al., 2021), but surprisingly it was lower in both the varieties harvested from the poly-net houses. However, the iron supply from the pink poly-net house was comparable with the open field in the winter for both the amaranth crops but pink poly-net house was the highest iron supplier from water spinach in the winter. The poly-net houses were the sole supplier of iron through amaranth crops in the spring, although the niche was shared by open field with the pink poly-net house for the water spinach. It is important to note that only poly-net houses enabled the production of amaranth crops in the spring season, and the water spinach in the winter season.

While comparing pink poly-net house with the white poly-net house, the supply of carotenoids and iron was comparatively better from the pink poly-net house in most of the seasons for all the three crops. A previous study from Japan also demonstrated that red amaranth grown under blue shade polyethylene produced more biomass yield with health beneficiary bioactive compounds betacyanins, polyphenol and antioxidant activity during the low temperature regime in spring season (Khandaker et al., 2010). A recent study had assessed the effects of various ratios of combined red, blue, and amber light-emitting diodes on the expression of carotenoid biosynthetic genes and carotenoid accumulation in brassicas (Alrifai et al., 2021). It found that total and individual carotenoids were increased significantly under dose-dependent increasing amber–blue light and decreasing red in most brassica microgreens. According to this study, lipophilic 2,2-diphenyl-1-picrylhydrazyl and ferric reducing antioxidant power antioxidant activities were significantly increased under higher amber and blue light fractions, while oxygen radical absorbance capacity was generally decreased. Hence, the higher blue irradiance inside the pink net house than in the white poly-net house and open field conditions could have involved in the regulatory mechanism of carotenoid biosynthesis in the current study, which requires further validation in future studies.

Based on the RDI values of iron and β-carotene, the number of people who could receive these nutrients from the crops harvested under different production conditions was estimated in the current study. During the off-season (winter for water spinach and spring for amaranths), pink poly-net house was estimated to supply more iron to people than the other production conditions. In all the seasons, the poly-net houses provided more β-carotene than the open field produced crops. Hence, sufficient quantities of iron and β-carotene can be obtained from the leafy vegetables, particularly during the off-season using less cultivation area under protective structures. This is quite important in countries like Taiwan, where the overall share of agricultural land decreased during the last two decades (Chen et al., 2019). However, it should be noted that we made these theoretical estimates based on the RDI values, without considering the bioavailability. Hence, these results should be interpreted in terms of nutrient supply from different crop production conditions, but the actual contribution of these nutrients based on their bioavailability to improving health conditions should be investigated in further studies.

Thus, protected cultivation has demonstrated the supply of nutritious vegetables, especially during the off-season. Since the investment in the construction and maintenance of both the white and pink poly-net houses is similar, use of pink poly-net houses may be considered to produce more nutritious vegetables. Supply of vegetables during the off-season is quite important to bridge the gaps in the seasonal variations in vegetable consumption. In addition, off-season vegetable production can provide better income opportunities to the smallholder farmers. If properly constructed and maintained, protective structures will last longer and reduce the incidence of pests and diseases thus reducing the use of harmful chemical pesticides in vegetable production systems. Thus, protected cultivation is expected to build economic and environmental resilience to the smallholder vegetable producers, while supplying nutritious vegetables to the consumers, especially in the off-season. However, the impacts of different types of protected cultivation on the horticultural and nutritional yield are location- and crop-specific, which can be piloted in new locations before scaling out among smallholder farmers in Asia and Africa for better nutrition and incomes.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

SR and PS-C: conceptualization and formal analysis. SR: funding acquisition and writing–original draft. SR, M-YL, W-JW, H-IW, and PS-C: methodology and investigation. M-YL, W-JW, H-IW, and PS-C: writing–review and editing. All authors have read and agreed to the published version of the manuscript.

Funding for this research was provided by the World Vegetable Center and by long-term strategic donors to the World Vegetable Center: Taiwan, the Foreign, the Commonwealth & Development Office (FCDO) from the UK government, United States Agency for International Development (USAID), Australian Centre for International Agricultural Research (ACIAR), Germany, Thailand, Philippines, Korea, and Japan.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.