Genetic characterization of citron watermelon (citrullus lanatus var. citroides [L.H. Bailey] mansf. ex greb.) and development of experimental hybrids = Isimo sofuzo sekhabe i-citron (Citrullus lanatus var. citroides [L.H. Bailey] Mansf. ex Greb.) kanye nokuthuthukiswa kwenhlanganisela yokuhlola

Abstract

Citron watermelon (Citrullus lanatus var. citroides [L.H. Bailey] Mansf. ex Greb.) is indigenous to sub-Saharan Africa (SSA) with multiple uses, including human food and animal feed. Its succulent leaves are used as leafy vegetables, while the ripened yellow and orange-fleshed fruits are used to prepare various traditional dishes, and the seeds are roasted and consumed as snack. It is an emerging potential rootstock for producing grafted sweet watermelon (Citrulus lanatus var. lanatus) to improve fruit yield and biotic and abiotic stress tolerance. It is also a source of novel genes for breeding in sweet watermelon to improve fruit yield, quality and disease resistance. Citron watermelon in SSA is mainly cultivated using unimproved landrace varieties. Landraces exhibit marked phenotypic variation for fruit shape, size, skin colour patterns, and seed coat colours. Phenotypic and genetic variation among South African citron watermelon landraces is yet to be systematically assessed for diverse use and cultivar design. The overall goal of this study was to initiate a pre-breeding program for citron watermelon through identification and selection of unique and complementary genotypes for production, value-adding and breeding. The specific objectives of this study were: i. To determine the extent of genetic diversity among South African citron watermelon landrace accessions using selected simple sequence repeat (SSR) markers to identify genetically divergent accessions for trait integration and variety development; ii. To assess the phenotypic diversity of citron watermelon landrace accessions of South Africa and to select desirable genotypes with suitable agronomic and horticultural traits for direct production, breeding and conservation; iii. To estimate variance components, heritability and genetic advance of phenotypic traits in citron watermelon to guide the selection of superior genotypes for direct production and breeding; iv. To determine the combining ability and hybrid performance of citron watermelon genotypes for agronomic traits for breeding. In the first study, 48 citron watermelon landrace collections widely grown in the Limpopo Province of South Africa were genotyped using 11 selected SSR markers.

The SSR markers amplified a total of 24 alleles, with a mean expected heterozygosity value of 0.38, indicating moderate genetic diversity among the studied accessions. Analysis of molecular variance attributed 8%, 75%, and 17% of the molecular variation between populations, among accessions and within accessions, respectively. Three distinctive genetic groups were identified based on cluster analysis. The following distantly related genotypes are recommended as breeding parents namely: WWM03, WWM04, WWM15, WWM16, WWM18, WWM22, WWM23, WWM24, WWM25, WWM26, WWM28, WWM33, WWM34, WWM35, WWM38, WWM39, WWM41, WWM66, WWM76, WWM78, WWM81, WWM84, WWM86 and WWM89 (selections from Cluster I), WWM14, WWM37, WWM42, WWM44, WWM46, WWM50, WWM65, WWM79, WWM85 and WWM87 (Cluster II), and WWM38, WWM47 and WWM48 (Cluster III). These are useful parental lines for pre-breeding to develop and release new varieties with multiple uses. In the second study, 36 selected citron watermelon landrace accessions were evaluated under field conditions across two environments using a 6 × 6 lattice design with three replicates. Data on key qualitative and quantitative traits were collected and subjected to non-parametric and parametric statistical analyses. The accessions showed wide phenotypic variation and unique traits for genetic improvement. Positive and significant correlations (p < 0.001) were recorded between total fruit yield per plant with plant height (r = 0.64), number of harvestable fruits (r = 0.70), number of marketable fruits (r = 0.73) and marketable fruit yield (r = 0.96). Seed yield per plant positively and significantly (p < 0.001) correlated with number of male flowers (r =0.68), plant height (r = 0.61) and total fruit yield (r = 0.79). Principal component analysis identified nine components which accounted for 86.38% of total variation amongst accessions for assessed phenotypic traits. The study recommended citron watermelon accessions such as WWM14, WWM16, WWM39, WWM41, WWM67 and WWM79 for use as leafy vegetables owing to their profuse branching ability and longer vine production. Whereas accessions including WWM03, WWM17, WWM35, WWM40, WWM50, WWM67, WWM79 and WWM85 are selected with larger fruit size. Accessions WWM05 and WWM09 are sour-flesh types which are suitable genetic stocks for breeding sweet-and-sour and sweet dessert watermelons. Orange-fleshed accessions such as WWM03, WWM04, WWM46, WWM64, WWM66 and WWM67 are recommended for fresh consumption, cooking, processing or variety design.

Accessions WWM02, WWM03, WWM08, WWM14, WWM16, WWM23, WWM38, WWM40, WWM41 and WWM67 have red and white seed coat colour which are superior selections to prepare roasted citron watermelon seed snack. In the third study, variance components, heritability and genetic gains of phenotypic traits were estimated involving 36 accessions of citron watermelon grown under field conditions across two test environments using a 6 × 6 lattice design with three replicates. High broad-sense heritability and genetic advance as percent of the mean were recorded for fruit length at 83.86 and 4730.45%, seed length (77.73 and 1731.27%), hundred seed weight (73.73 and 4027.36%), fruit diameter (70.44 and 2949.64%) and fruit weight (70.39 and 8490.05%), respectively. Step-wise regression analysis revealed marketable fruit yield and total number of fruits per plant explaining 89% (R2 = 0.89) of total variation for total fruit yield per plant, whereas number of seed per fruit and hundred seed weight explained 92 (R2 = 0.92) of total variation for seed yield per fruit. Citron watermelon landrace accessions WWM03, WWM14, WWM16, WWM39, WWM65, WWM67 and WWM79 with high total fruit yield and seed yield per fruit were selected for production or breeding programme. In the fourth study, five selected parental genotypes were crossed in a 5 × 5 half-diallel mating design to develop 10 hybrids. The 15 families (five parents and 10 F1 hybrids) were evaluated across two environments using a randomized complete block design (RCBD) with three replications. General combining ability (GCA) and specific combining ability (SCA) effects were significant (p < 0.001) for most traits. Environment × GCA was non-significant, whereas Environment × SCA effects were significant (p < 0.001) for most traits. The ratios of GCA/SCA variances were less than unity for most traits, indicating non-additive gene action of the traits. Broad-sense heritability varied from low to moderate, implying variable selection response of the assessed traits among the F1 hybrids. The parental genotypes WWM16 with positive GCA effects for fruit and seed yield and WWM66, with positive GCA effects for the number of seeds per fruit and seed yield, were identified for hybrid breeding. The following F1 hybrids, namely: WWM04 × WWM16, WWM03 × WWM66 and WWM16 × WWM50 with positive SCA effects on total fruit yield per plant and marketable fruit yield per plant, and WWM04 × WWM50, WWM03 × WWM16 and WWM03 × WWM66 with positive SCA effects for number of seeds per fruit and total seed yield were identified. The study identified novel and best-performing F1 hybrids of citron watermelon for economic traits and are recommended for multi-environmental evaluations, variety registration and commercialization. Overall, the study revealed genetic and phenotypic variation in citron watermelon to select and recommend suitable genotypes for production and for breeding new generation varieties based on market needs and consumer preferences. The study recommends accessions such as WWM14, WWM16, WWM39, WWM64, WWM67, WWM76 and WWM79 with high fruit yield, and WWM03, WWM04, WWM14, WWM15, WWM16, WWM24, WWM28, WWM37, WWM46, WWM66 and WWM68 exhibiting high fruit and seed yield for breeding or direct production. The parents WWM04, WWM03 and WWM16 were identified as good combiners for fruit or seed yield and related-component traits for future breeding. The F1 hybrids derived from these parents, including WWM04 × WWM16, WWM03 × WWM16, WWM03 × WWM66, WWM16 × WWM50, and WWM04 × WWM50 were best performing for economic traits and new breeding population development.

Iqoqa.

Ikhabe le-citron (i-Citrullus lanatus var. citroides [L.H. Bailey] Mansf. ex Greb.) lingelendabuko e-Afrikha eseMazansi, (i-sub-Saharan Africa (SSA)) elinemisebenzi eminingi, okubalwa kuyo ukudla kwabantu kanye nokudla kwezilwane. Amahlamvu alo amnandi asetshenziswa njengemifino emaqabunga, bese kuthi izithelo esezivuthiwe eziphuzi kanye neziwukudla ezisawolintshi zisetshenziswa ukwenza ukudla kwendabuko okwahlukene, bese imbewu iyosiwa bese idliwa njengokudla okulula. Yisiqu sempande esafufusa ukukhiqiza ikhabe elingxube (i-Citrulus lanatus var. lanatus) ukuthuthukisa umkhiqizo wesithelo kanye nokubekezelela ingcindezi yokuphilayo nokungaphili. Kuyimbangela yofuzo olusha ukukhiqiza ikhabe elinoshukela ukukhulisa ekhabeni elinoshukela ukuthuthukisa umkhiqizo wesithelo, izingabunjalo kanye nokulwa nezifo. Ikhabe le-citron e-SSA litshalwa kakhulu kusetshenziswa izinhlobo ezahlukene zohlobo olusangulube engathuthukile. Uhlobo olusangulube olwehlukile ukumisa isithelo, ubungako, umbala wesikhumba kanye nemibala eyemboze imbewu. Uhlobo lokufanisa nofuzo phakathi kwekhabe lwe-citron lwaseNingizimu Afrikha nezingulube kusamele luhlolwe ngendlela ukuveza umsebenzi owehlukile nohlaka lokutshala. Inhloso enkulu yalolu cwaningo kwakungukuqala uhlelo lokuqalela ukwandisa ikhabe le-citron ngokuveza kanye nokukhetha uhlobo lokulekelela olwehlukie ukukhiqiza, ukwengeza ukubaluleka kanye nokwandisa. Izinhloso ezikhethekile zalolu cwaningo kwakuyilezi: i. Ukuthola ukuthi luhamba luze lufike kuphi uhlobo lomehluko phakathi kwekhabe laseNingizimu Afrikha olusangulube olungenisiwe kusetshenziswa ukulandelana okulula okukhethekile kokuphinda izinkomba, (ama-simple sequence repeat (SSR)) ukuveza ukungena okwehlukile kofuzo ukuveza ukuhlanganisa kanye nokuthuthuka okwehlukile; ii. Ukuhlola umehluko wohlobo lwekhabe le-citron olusangulube olungenisiwe lwaseNiningizimu Afrikha kanye nokukhetha ufuzo oludingekayo kanye nezinkomba zomhlabathi olimekayo kanye notshalekayo ukuqondisa umkhiqizo, ukwandisa kanye nokugcineka;

iii. Ukuhlawumbisela umehluko wezinhlaka, ifa kanye nokuqhubeka kofuzo lwezinkomba zohlobo ekhabeni lwe-citron ukuhlola ukukhetha kohlobo lofuzo olukhulu ukuqondisa umkhiqizo kanye nokwandisa; iv. Ukuthola ukukwazi okuhlanganisayo kanye nokusebenza okuyinhlanganisela yohlobo lwekhabe lwe-citron ukulimekela ukwandisa. Ocwaningweni lokuqala, amaqoqa amakhabe ekhabe le-citron olusangulube olutshalwa esiFundazweni saseLimpopo saseNingizimu Afrikha afaniswa kusetshenziswa izinkomba eziyi-11 ezikhethiwe ze-SSR. Izinkomba se-SSR zakhulisa isamba sokuzo olungama-24, ngemini elindelekile ebalwe ngenani lika-0.38, okukhomba umehluko wobuhlobo ophakathi nendawo phakathi kwezingenelelo ezacwaningwa. Ukuhlaziya komehluko wezinhlayiya kwaveza u-8%, u-75%, kanye no-17% womehluko wezinhlayiya phakathi kwamaqoqo, phakathi kwezingenelelo kanye nangaphakathi kwezingenelelo, ngokulandelana kwakho. Amaqoqo ohlobo olwehlukile amathathu avezwa ngokuhlaziya ngendlela yokubeka ngamaqoqo. Izinhlobo ezihlobene ngokuqgagqana ziyaphakanyiswa njengabazali bokwandisa ababizwa: i-WWM03, i-WWM04, i-WWM15, i-WWM16, i-WWM18, i-WWM22, i-WWM23, i-WWM24, i-WWM25, i-WWM26, i-WWM28, i-WWM33, i-WWM34, i-WWM35, i-WWM38, i-WWM39, i-WWM41, i-WWM66, i-WWM76, i-WWM78, i-WWM81,i-WWM84, i-WWM86 kanye ne-WWM89 (ukukhetha eQoqweni I), i-WWM14, i-WWM37, i-WWM42, i-WWM44, i-WWM46, i-WWM50, i-WWM65, i-WWM79, i-WWM85 kanye ne- WWM87 (IQoqo II), kanye ne-WWM38, i-WWM47 kanye ne-WWM48 (IQoqo III). Laba olayini bokuzala abanomsebenzi wokulungiselela ukwandisa ukuthuthukisa kanye nokukhulula izinhlobo ezintsha kanye nemisebenzi eminingi. Ocwaningweni lwesibili, ikhabe le-citron elikhethiwe elingama-36 olusangulube lokungenisiwe lwahlolwa ngaphansi kwezimo zensimi ezimweni ezimbili ezingefani kusetshenziswa uhlobo lohlaka u-6 × 6 nezifaniso ezintathu. Imininingo yezinkomba ezisemqoka zekhwalithethivu kanye nezinombolo yaqoqwa bese yahlotshaniswa nokuhlaziya okunemikhawulo kanye nokungenamkhawulo. Okwangeniswa kwaveza umehluko wohlobo omkhulu kanye nezinkomba ezahlukile zokuthuthukisa ufuzo. Ukuhlobana okuhle futhi okubalulekile (p < 0.001) kwaqoshwa phakathi kwesithelo esikhiqiziwe ngokwesitshalo esinobude baso obu-(r = 0.64), inombolo yezithelo ezinokuvuneka (r = 0.70), inombolo yezithelo ezidayisekayo (r = 0.73) kanye nomkhiqizo wesitho esidayisekayo (r = 0.96). Umkhiqizo wembewu ngokwesitshalo kwahlobana kahle futhi kakhulu (p < 0.001) nenombolo yezimbali zesilisa (r = 0.68), ubude bezitshalo (r = 0.61) kanye nenani lomkhiqizo wesithelo (r = 0.79). Ukuhlaziya kwengxenye enkulu kuveze izinhlaka eziyisishiyagalombili kwachaza u-86.38% yomehluko ohlangene phakathi kokungenisiwe kwahlola izinkomba zezinhlobo ezinofuzo. Ucwaningo luphakamisa ukuthi ikhabe le-citron olufana ne-WWM14, i-WWM16, i-WWM39, i-WWM41, i-WWM67 kanye ne-WWM79 elisetshenziswa njengemifino engamaqabunga liyimbangela yokukwazi ukusabalala okwedlulele kanye nomkhiqizo wesivuno omude. Kanti okungenisiwe okufana ne-WWM03, i-WWM17, i-WWM35, i-WWM40, i-WWM50, i-WWM67, i-WWM79 kanye ne-WWM85 kukhethwa njengesithelo esikhulu ngobungako. Okungenisiwe kwe-WWM05 kanye ne-WWM09 kuyizinhlobo zokudliwayo okumuncu okuwufuzo olulungele ukugcinelwa ukwandiswa kwamakhabe anoshukela nesimuncwana kanye nesidlo sokuphetha. Okungenisiwe okudliwayo okusawolintshi okufana ne-WWM03, WWM04, i-WWM46, i-WWM64, i-WWM66 kanye ne-WWM67 kuphakanyiswa ukudliwa kusekusha, ukuphaka, ukukhiqizwa, noma uhlaka lohlobo. Okungenisiwe kwe-WWM02, i-WWM03, i-WWM08, i-WWM14, i-WWM16, i-WWM23, i-WWM38, i-WWM40, i-WWM41 kanye ne-WWM67 kunembewu ebomvu kanye nemhlophe yemibala yokwembozile okukhetheke kakhulu ukulungisela imbewu eyosiwe yekhabe le-citron lokudla okulula. Ocwaningweni lwesithathu, izinhlaka zoshintsho, ukufuza, imivuzo yofuzo yezinkomba zezinhlobo kwahlawumbiselwa kubalwa okungenisiwe okungama-36 kwekhabe le-citron elitshalwa ngaphansi kwezimo zensimu ngapha nangapha kwezizinda zokulinga ezimbili kusetshenziswa uhlaka lohlelo luka-6 × 6 nezimpinda ezintathu. Ukufuza okusabalele ngokubanzi kanye nokusabalala kohlobo njengephesenti lemini kwaqoshelwa ubude besithelo ku-83.86 kanye no-4730.45%, ubude bembewu (77.73 no-1731.27%), isisindo sembewu esiyikhulu (73.73 no- 4027.36%), ubude bephakathi lesithelo (70.44 no-2949.64%) kanye nesisindo sesithelo (70.39 no-8490.05%), ngokulandelana. Ukuhlaziya ukuphendukela emumva ngokwesigaba ngesigaba kwaveza imiphumela yesithelo edayisekayo kanye nenani lenombolo yezithelo ngesitshalo kuchaza ama-89% (R2 = 0.89) womehluko ophelele wesiphumo sesithelo ngesitshalo, kanti inombolo yembewu ngesitshalo kanye nesisindo sembewu eyikhulu kwachazwa ngo-92% (R2 = 0.92) wenani lomehluko wesiphumo sembewu ngesitshalo. Okungenisiwe kwekhabe le-citron elisangulube i-WWM03, i-WWM14, i-WWM16, i-WWM39, i-WWM65, i-WWM67 ne- WWM79 nenani eliphezulu leziphumo zesithelo kanye nesiphumo sembewu ngesitshalo kwakhethelwa umkhiqizo noma uhlelo lokwandisa. Ocwaningweni lwesine, izinhlobo ezifuzene zokuzalana ezinhlanu zakhethwa ngapha nangapha kuhhafu ka-5 × 5 wohlaka lokuhlanganisa uqondanisa ukuthuthukisa inhlanganisela eyi-10. Imindeni eyi-15 (abazali abahlanu kanye nenhlanganisela ka-10 F1) kwahlolwa ngapha nangapha ezimweni ezimbili kusetshenziswa uhlaka lweqoqo oluphelele olukhethwe ngokungenhloso, (i-randomized complete block design (RCBD)) nokuphindaphinda okuthathu. Ukukwazi kokuhlanganisa okwejwayelekile, (i-General combining ability (GCA)) kanye nemiphumela yokukwazi kokuhlanganisa okuthile, (i-specific combining ability (SCA)) kwakubalulekile (p < 0.001) ezinkombeni eziningi. I-Environment × GCA yayingabalulekile, kanti imiphumela ye-Environment × SCA yayibalulekile (p < 0.001) ezinkombeni eziningi. Ubudlelwane bomehluko we-GCA/SCA babubuncane ngaphansi ngokukodwa ezinkombeni eziningi, okukhomba ukusebenza kofuzo olungengezeleli lwezinkomba. Ufuzo olubanzi ngokusabalele lwehluka kusukela phansi kuya phakathi nendawo, okusho impendulo yokukhetha ivarebuli ezinkombeni ezihloliwe phakathi kwenhlanganisela ye-F1. Uhlobo lofuzo lokuzala i-WWM16 nemiphumela emihle ye-GCA yesithelo kanye nesiphumo sembewu kanye ne-WWM66, nemiphumela emihle ye-GCA enombolweni yembewu ngesithelo kanye nesiphumo sembewu, kwatholakala ukwandisa okuyinhlanganisela. Inhlanganisela elandelayo ye-F1, nokuyi-: WWM04 × WWM16, i-WWM03 × WWM66 kanye ne-WWM16 × WWM50 nemiphumela emihle ye-SCA enanini lesiphumo sesithelo ngesitshalo kanye nesiphumo sesithelo esithengisekayo, kanye ne-WWM04 × WWM50, i-WWM03 × WWM16 kanye ne-WWM03 × WWM66 nemiphumela emihle ye-SCA enombolweni yembewu ngesitshalo kanye nenani lesiphumo lembewu kwatholakala. Ucwaningo lwaveza izinhlanganisela ezintsha futhi ezisebenza kangcono kakhulu ze-F1 zekhabe le-citron ezinkombeni zomnotho kanti ziyanconywa ukuhlolelwa imvelo okuningana, ukubhalisa umehluko kanye nokuthengisa. Sekukonke, ucwaningo luveze ukuthi umehluko wofuzo kanye nowohlobo lofuzo ekhabeni le-citron ukukhetha kanye nokuphakamisa uhlobo lofuzo olulungele imkhiqizo kanye nokwandisa izinhlobo zenzalo entsha kuncike ezidinweni zemakethe kanye nasekukhetheni kwabathengi. Ucwaningo luphakamisa okungeniswayo okufana ne-WWM14, i-WWM16, i-WWM39, i-WWM64, i-WWM67, i-WWM76 kanye ne-WWM79 nesiphumo esiphezulu sesithelo, kanye ne-WWM03, i-WWM04, i-WWM14, i-WWM15, i-WWM16, i-WWM24, i-WWM28, i-WWM37, i-WWM46, i-WWM66 kanye ne-WWM68 kuveza isithelo esiphezulu kanye nesiphumo sembewu ukwandisa umkhiqizo oqondile. Abazali i-WWM04, i-WWM03 kanye ne-WWM16 batholakala njengezihlanganiso zesithelo noma zesiphumo sembewu kanye nezinkomba zezinhlaka ezihlobene ukwandisa kwesikhathi esizayo. Izinhlanganisela ze-F1 ezatholakala kulaba bazali, okubalwa kubo i-WWM04 × WWM16, i-WWM03 × WWM16, i-WWM03 × WWM66, i-WWM16 × WWM50, kanye ne- WWM04 × WWM50 kwakusebenzela kakhulu izinkomba zomnotho kanye nokuthuthukisa inani lokwandisa.