Factors associated with the yield variation among citron watermelon (citrullus lanatus var. citroides (L. H. Bailey) mansf. ex greb.) genotype in South Africa.

Abstract

Citron watermelon [Citrullus lanatus var. citroides (L. H. Bailey) Mansf. ex Greb.] is a plant species used for food and feed, belonging to the family Cucurbitaceae. Wide yield variation is exhibited among citron watermelon genotypes in South Africa. The objective of this study was to assess morphological and physiological factors associated with yield variation among citron watermelon genotypes in South Africa under drought stress and to recommend genotypes that are drought-tolerant and display desirable fruit traits for future crop production, breeding and conservation. The study also sought to understand, if the use of urine-based fertiliser sources, such as struvite, can improve phosphorus (P) availability in acidic soils, leading to increased root growth, nutrient uptake, and subsequently enhanced yield. The study was designed as a 6 x 2 x 3 factorial experiment with the following factors: citron watermelon genotype - 6 levels (WWM- 1, WWM-16, WWM-30, WWM-31, WWM-35, and WWM-37), drought conditions - 2 levels (non-stress (NS) and (DS) drought-stress condition); and phosphorus source - 3 levels (struvite - a urine-derived fertilizer, single superphosphate (SSP), and no added fertilizer, Gromor® potting mix only). Data on quantitative leaf traits (leaf length, leaf width and leaf area), vine length (the longest vine length), root traits (root mass and root length), fruit mass, nutrient uptake (micro- and macronutrients) and leaf physiological data (gas exchange and chlorophyll fluorescence, indicating photosynthetic activity, stomatal conductance, electron transport rate, transpirational flow, intercellular carbon dioxide (CO2) concentration, ratio of intercellular to atmospheric CO2, and maximum fluorescence) were collected. Qualitative (phenotypic) traits, such as fruit colour, were also recorded and data subjected to analysis of variance using GenStat 20th edition. A wide variety of quality parameters among citron watermelon genotypes that could be genetically improved, were observed. The results from this study showed no significant differences (P>0.05) between treatments with respect to leaf traits (leaf length, leaf width and leaf area) with all levels of P sources (struvite, SSP and no fertilizer), under both NS and DS. Struvite- and SSP-treated genotypes outperformed the control with respect to leaf traits (leaf length and leaf width) under NS, however, non-treated (control) genotypes outperformed both struvite- and SSP-treated genotypes, concerning plant leaf area. Application of struvite significantly improved the chlorophyll content index (CCI) of plants to levels higher than the SSP treatment and the control. The urine-based fertilizer was comparable to the SSP treatment concerning vine length of all six citron watermelon genotypes under both NS and DS. This

suggests that struvite improved vine growth of all selected genotypes. A significant difference (p<0.05) was observed between struvite and SSP treated genotypes in terms of fruit mass, and in terms of fruit nutrient (micro- and macronutrients) composition under both NS and DS. The study revealed that both struvite and SSP effectively increased root development (resulting in greater root dry mass and longer roots) under DS. A significant difference (p<0.05) was observed in all levels of P sources (struvite, SSP, and no fertilizer) in terms of the root mass and root length. Therefore, the application of struvite improved root growth and development. The better uptake of micro- and macronutrients (higher mineral concentration than the control and the SSP treatment) by struvite-treated genotypes could be attributed to the high cation exchange capacity (CEC) of the Gromor. The increase of N uptake by citron watermelon fruit grown in SSP- and struvite-treated soils could be due to mineralization of organic matter in the soil, resulting in the release of initially unavailable N. Results from this study revealed no significant differences (P>0.05) between treatments with respect to leaf gas exchange and chlorophyll fluorescence parameters. Struvite-treated genotypes performed well compared to both SSP-treated and control genotypes concerning the ratio of intercellular and atmospheric CO2 (increased ratio) under NS, meaning struvite treatment improved the photosynthesis process of genotypes because of high intercellular CO2 concentration in leaves. Struvite-treated genotypes performed well compared to SSP-treated genotypes concerning the stomatal conductance (reduced stomatal conductance), this indicates the efficient drought avoidance among the genotypes. Citron watermelon genotypes WWM-30, WWM-31, and WWM-37 recorded the highest maximum chlorophyll fluorescence in both NS and DS, meaning the effect of drought stress was reduced by enhancing P availability. The genotypes WWM-1, WWM-30, and WWM- 35 revealed high maximum quantum efficiency of photosystem II photochemistry and produced higher marketable fruit yields and a higher fruit number per plant than WM-16, WWM-31, and WWM-37. These traits are important as characteristics of parental genotypes in breeding programmes. The present study recommends citron watermelon genotypes such as WWM-1, WWM-35, and WWM-37, for use as leafy vegetables, due to their higher leaf biomass, whereas genotypes including WWM-16, WWM-35, and WWM-37, are recommended for fruit production due to their large fruit size. In conclusion, the study shows that the use of urine-based fertiliser sources, such as struvite, seems to increase P availability in acidic soils and thereby leads to increased

root growth, nutrient uptake, and, ultimately, higher quality fruit. Keywords: Citron watermelon, yield variation, physiological factors, morphological factors, soil phosphorus, nutrient uptake.