Abstract
The prevalence of rough lemon rootstock in India is presently threatened by the spread of Phytophthora root rot. Developing an alternative Phytophthora-resistant rootstock that also withstands other abiotic constraints is a priority for a sustainable citrus industry in India. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Citrus and Poncirus, a closely related genus. The production of hybrids accumulating the desired dominant traits, irrespective of the heterozygosity level of the parents, is an effective way to develop new candidate rootstocks. Rough lemon was hybridized with Swingle citrumelo, X-639 citrandarin, and GouTou Cheng. A reliable method was developed for greater recovery of hybrid seedlings from these crosses on the basis of morphology and simple sequence repeat (SSR) markers. The greatest proportion of hybrid seedlings resulted from embryo rescue at 95 days after pollination (DAP) (48.5% in rough lemon × X-639 citrandarin and 56.7% in rough lemon × Swingle citrumelo) on the basis of multifoliate leaves. The proportion of hybrid seedlings decreased with increase in embryo development stage after 95 DAP until the last stage of observation (125 DAP). Significantly higher proportion of multifoliate seedlings was recovered by in vitro culture compared with in vivo germination from the mature seeds (26.9% vs. 5.2%, respectively). Swingle and X-639 citrandarin are not homozygous for the multifoliate allele from Poncirus, and GouTou Cheng lacks the multifoliate allele; thus, unifoliate hybrids were selected from all crosses on the basis of SSR marker genotypes.
Highlights
Citrus fruits are grown over an area of 8.9 million ha with 137.8 million tons of production.China, with 34.9 million tons of production, is the major producer of citrus fruits, followed by Brazil (19.1 million tons), India (11.15 million tons), the USA (8.5 million tons), and Mexico (7.8 million tons) [1]
Rough lemon (Citrus jambhiri Lush.) flowers at popcorn stage were pollinated with the pollen of X-639 citrandarin (X; Cleopatra mandarin × trifoliate orange), Swingle citrumelo (SC; Duncan grapefruit × trifoliate orange), and GouTou Cheng sour orange (GT)
The minimum numbers of embryos were recorded at 65 days after pollination (DAP) (2.0, 2.6, and 1.7 for Rough lemon (RL) × X, RL × SC, and RL × GT, respectively), which reached the maximum values of 5.6, 6.7, and 8.1 for RL × GT, RL × SC, and RL × X, respectively, at 95 DAP
Summary
Citrus fruits are grown over an area of 8.9 million ha with 137.8 million tons of production. With 34.9 million tons of production, is the major producer of citrus fruits, followed by Brazil (19.1 million tons), India (11.15 million tons), the USA (8.5 million tons), and Mexico (7.8 million tons) [1]. Owing to tolerance of citrus tristeza virus (CTV), suitability to high pH soils and high scion yields, rough lemon is a major citrus rootstock in India. Rough lemon rootstock is highly susceptible to Phytophthora root rot, which affects tree performance and longevity and imparts poor fruit quality to the scion varieties. Raf), namely, citranges (sweet orange × trifoliate orange), X-639 citrandarin (Cleopatra mandarin × trifoliate orange), Agronomy 2020, 10, 1940; doi:10.3390/agronomy10121940 www.mdpi.com/journal/agronomy
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