Identifying suitable citron melon (citrullus lanatus var citroides) genetic resources for improved cultivation under low-nitrogen growing conditions.

Abstract

Low-nitrogen production environments, an example of abiotic stress in agricultural crop production, affect productivity and breeding in citron melon (Citrullus lanatus var. citroides). Citron melon is an underutilised and nutritious crop possessing natural adaptation to stressprone environmental growing conditions. Despite being naturally adapted to environmental stress, the crop still requires optimisation for performance under poor growing conditions. Variable performance of citron melon under differential growing conditions affects breeding progress for important traits in breeding programs. Furthermore, citron melon is a significant genetic resource for aiding breeding and enhancing environmental stress tolerance in related cucurbits, such as sweet watermelon. Soil nitrogen deficiency is among the key limiting factors affecting crop productivity, influencing plant growth, photosynthetic capacity, yield, and compositional attributes. Additionally, there is limited information and research output reporting nitrogen requirements for optimum citron melon productivity. These make the nitrogen use efficiency important objective for inclusion in citron melon breeding, to enhance the crop’s productivity under limited growing conditions and to improve response to selection in breeding activities. This study aimed to improve citron melon for enhanced growth and accumulation of compositional attributes under low-nitrogen production/ growing conditions. The specific study objectives were (1) to identify suitable citron melon genotypes possessing enhanced adaptive attributes for cultivation under low-nitrogen availability for production and breeding, and (2) to determine accumulated metabolites in citron melon genotypes cultivated under low and optimal nitrogen applications to aid selection of suitable genotypes for cultivation. The first study assessed morpho-physiological traits of four selected citron melon genotypes (WWM-Ranoto, WWM-16, WWM-58, and WWM-65) under low nitrogen (LN) and optimum nitrogen (ON) conditions in greenhouse (GH) and shadehouse (SH) production sites. The following morpho-physiological traits were evaluated: leaf Area (LA), vine length (VL), nitrogen use efficiency (NUE), fruit weight (FW), fruit length (FL), fruit diameter (FD), rind thickness (RT), seed per fruit (SPF), root length (RL) and root weight (RW), photosynthetic rate (A), stomatal conductance (gs), transpiration rate (E), intercellular CO2 concentration (Ci), maximum quantum yield of photosystem II (PSII) in photosynthesis (Fv’/Fm’), the effective quantum efficiency of PSII photochemistry (ΦPSII), partial pressure of CO2 (PhiCO2), photochemical quenching (qP), electron transport rate (ETR), amount of light absorbed by the leaf (leafAbs). Analysis of variance revealed a significant genotype effect (p ≤ 0.01) for morpho-physiological attributes LA, VL, FL, RT, SPF, gs, E, and Ci across all study environments. The nitrogen level effect was significant (p ≤ 0.05) for morpho-physiological traits VL, SPF, An, E, Ci and PhiCO2, suggesting differential genotype performance under varied levels of nitrogen availability. Morpho-physiological traits LA, VL, NUE, FW, FL, FD, RT, RL, A, gs, E, Ci, Fv’/Fm’, ΦCO2, qP, and LeafAbs deferred significantly (p ≤ 0.05) based on production site. Pearson’s correlation analysis revealed strong positive associations between morpho-physiological traits LA, VL, NUE, FW, FD, RT, A, E and qP under LN production conditions across all sites. Principal components analysis (PCA) revealed a strong positive interrelationship between genotype WWM-58 with morpho-physiological traits NUE, FW, FL, RT, RW, RL, PhiCO2 and qP under LN conditions in GH. Principal components analysis (PCA) revealed a strong positive interrelationship between genotype WWM-16 with morphophysiological traits RW, FD, RT, LA, VL, ETR, ΦPSII, Fv’/Fm’ and PhiCO2 under LN conditions in SH. Also under SH, genotype WWM- 58 associated positively with morphophysiological traits VL, LA, RW, FL and NUE. This suggests genotype WWM-58 is a selected candidate for further breeding. The second study assessed the biochemical responses of the citron melon genotypes under LN and ON conditions in GH and SH production sites. The following biochemical traits were sampled: Total soluble solids (TSS), titratable acid (TA), sugar-acid ratio (TSS/TA), total carotenoids (TC), sucrose (S), glucose (G), fructose (F), ascorbic acid/ vitamin C (AA), and water content (WC). The analysis of variance revealed a significant (p ≤ 0.01) genotype effect for TSS, TA, TSS/TA, TC, G, AA, and WC. The contents of all evaluated metabolites differed significantly based on nitrogen application. The test site significantly affected (p ≤ 0.01) most of the studied traits, except TSS/TA, TC, and %WC. Pearson’s correlation revealed a positive correlation between AA and TC in LN conditions. Only negative bi-variate correlations were observed for TSS/TA with TA, and for AA with G. Principal components analysis (PCA) associated with citron melon genotypes revealed that these citron melon genotypes, WWM-65 and WWM-58, exhibited high TA, TSS, TSS/TA, TC, and AA, highlighting their potential for commercialisation and selection for fruit quality improvement.