HORTSCIENCE 53(9):1314–1318. 2018. https://doi.org/10.21273/HORTSCI13226-18 Photosynthetic Response and Antioxidant Content of Hydroponic Bitter Gourd as Influenced by Organic Substrates and Nutrient Solution Md. Jahedur Rahman 1 Department of Horticulture, Sher-e-Bangla Agricultural University, Dhaka 1207, Bangladesh Md. Quamruzzaman Department of Agronomy, Sher-e-Bangla Agricultural University, Dhaka 1207, Bangladesh Jasim Uddain, Md. Dulal Sarkar, Md. Zahidul Islam, and Most. Zannat Zakia Department of Horticulture, Sher-e-Bangla Agricultural University, Dhaka 1207, Bangladesh Sreeramanan Subramaniam 1 School of Biological Sciences, Universiti Sains Malaysia, Penang, Malaysia Additional index words. stomatal conductance, photosynthesis, chlorophyll content, growth and yield, antioxidant content, protein, bitter gourd Abstract. Crop-specific nutrient solution and suitable growing media mixtures are necessary in hydroponic system to improve the yield of vegetables. Therefore, an experiment was conducted to specify the required amount of nutrient solution and ecofriendly mixtures of growing substrates of a hydroponic bitter gourd. Treatments consisted two factors, viz., four different types of growing substrate (M 1 = 60% rice husk + 30% coconut coir + 10% vermicompost, M 2 = 60% coconut coir + 30% khoa + 10% vermicompost, M 3 = 60% sawdust + 30% khoa + 10% vermicompost, and M 4 = 60% ash + 30% khoa + 10% vermicompost) and three different composition of nutrient solutions [N 1 = full strength Rahman and Inden (2012), N 2 = full strength Hoagland and Arnon (1940), and N 3 = ½ strength Rahman and Inden (2012)]. Photosynthetic responses and its related parameters, namely, stomatal conductance (g s ), transpiration (E), and photo- synthetic rate (P N ) were significantly affected by nutrient solution composition and growing substrate mixtures. Results revealed that the leaf gas exchange parameters, yield contributing characters, and biochemical parameters showed that full strength of Rahman and Inden (2012) nutrient solution application was better in a growing mixture of 60% rice husk + 30% coconut coir + 10% vermicompost in soilless culture for obtaining high yield and high quality of bitter gourd. Use of suitable growing substrate is essential for production of high-value horti- cultural crops. It directly affects the develop- ment and later maintenance of the functional rooting system. A suitable growing medium provides a sufficient support to the plant, serves as reservoir for nutrients and water, allows oxygen diffusion to the plant roots, and permits gaseous exchange between the roots and atmosphere (Abad et al., 2002). Many organic materials are widely available in the tropics, e.g., rice hull, coco peat, carbonized rice husk, and sawdust. These materials are mainly agricultural byproducts obtained after the extraction of fiber from the coconut husk, paddy, and saw mills and may be used as horticultural growing substrates. As a growing medium, coco peat can be used to produce several crop species with acceptable quality (Yahya et al., 1997). Coco peat is considered as a good growing substrate component with acceptable pH, electrical conductivity (EC), and other chemical attributes (Abad et al., 2002). However, coco peat has been recog- nized to have high water-holding capacity which causes poor air–water relationship, leading to low aeration within the medium, and it affects the oxygen diffusion to the roots. Carbonized rice husk may be used as a horti- cultural growing substrate, although it has the problem of air–water relationship. Rice hull is often incinerated to form fine charcoal-like dust. When it is used as a component for growing substrate, it might behave like fine sand. However, it is lighter and sterile and may contain some nutritional elements. Carbonized rice husk induced faster cell division and differentiation for root formation (Moe, 1988) and it was the best growing substrate for growing horticultural crops (Budiartoa et al., 2006) and sweet pepper production. Sawdust is used as growing substrate and is available in almost all over the world and it could be renewable. Wood residues (i.e., sawdust and bark) have been used in containers for growing ornamentals (Klett et al., 1972). But, microor- ganisms involved in decomposition of raw wood residues are more efficient than higher plants in nitrogen absorption and assimilation (Alexander, 1961). Large amount of nitrogen must, therefore, be added to wood residues used as media to grow plants. This problem can be solved, however, by composting residues before using them for growing substrates (Still et al., 1974). Furthermore, decomposition of old sawdust is not necessary for using as growing substrates. However, desirable physi- cal and chemical properties of these substrate components are highly dependent on their processing technique and handling. It is desir- able to improve physical and chemical proper- ties of them before using as growing substrates. Incorporation of coarser materials into the substrate components could improve the aera- tion and drainage status of the substrate mix- tures (Sambo et al., 2008). Perlite (Islam, 2008; Sambo et al., 2008), bora (volcanic soil), but rice hull may be used as the alternative of perlite, brick broken, gravel, etc. may be the possible coarser materials. These materials can be used to improve the air–water relationship of the substrate components. Furthermore, a suit- able combination of different growing substrate components influences the growth and yield of horticultural crop production like bitter gourd as a test crop. The nutrient solution is directly involved in producing higher yield and quality of horticultural crops (Rahman and Inden, 2012a). Nutrient solution formulation and its nutritional constituents may have the effect on quality and antioxidant components of horticultural crop production. Nowak (1980) stated that all nutrient combinations had the most beneficial effect on hydropon- ically grown plants. Monforte-Gonz alez et al. (2010) stated that nitrate accumulation in placenta depended on its availability in the nutrient solution and the pod presenting placenta with low nitrate content accumu- lated, which may be related to limited nitro- gen availability in this tissue of crops, such as pepper. Thus, improving the quality and yield contributing characteristics in bitter gourd are important factors in soilless culture tech- nique. These may be improved by managing external proper nutrient availability in the growing substrates. Proper nutrient combina- tions in the solution may improve the yield and yield contributing characters in the crop. Consequently, the present experiment was aimed to find out a suitable and sustain- able growing media mixture and a nutrient strength for producing high yield and quality of soilless bitter gourd producing in green- house in the tropics. Received for publication 21 May 2018. Accepted for publication 17 June 2018. 1 Corresponding authors. E-mail: jrahman04@ yahoo.com or sreeramanan@gmail.com. 1314 HORTSCIENCE VOL. 53(9) SEPTEMBER 2018