Identification of Phytophthora nicotianae tolerant citrus hybrids using subordinate function analysis of disease and morpho-physio-biochemical responses

Phytophthora nicotianae is a devastating fungus, which causes significant losses to citrus. To find new Phytophthora tolerant citrus rootstock hybrids, 13 Rangpur lime (RL; susceptible parent) × sour orange (SO; tolerant parent) hybrids were screened against P. nicotianae using artificial soil inoculation method. Their response was judged on the basis of feeder root rot (FRR) ratings and the alterations of morphology (shoot length, shoot diameter, leaf number and root weight), physiology [net photosynthesis rate (Pn), transpiration rate (E), stomatal conductance (gs)] and activity of defence enzymes [Leaf and root activity of peroxidase (POX), lipoxygenase (LOX), and phenyl alanine ammonia lyase (PAL)] under Phytophthora infection. In FRR ratings (1–5), the donor parent, SO and six hybrids scored rating of one, delineating to the tolerant reaction. From rest, four, two and one hybrids were rated as moderately tolerant (MT), moderately susceptible (MS) and susceptible (S), respectively. Phytophthora infection adversely impacted the morphological and physiological parameters in most of the hybrids. A significant (P < 0.05 or 0.01) negative correlation of feeder root rot ratings (FRRR) with morphological (r = −0.53 for shoot length to r = −0.87 for shoot diameter) and physiological traits (r = −0.76 for leaf chlorophyll content to r = −0.95 for transpiration rate) verified this adverse impact. The relative reduction in these parameters was maximum for S or MS-individuals, while the T-individuals were least affected. The transpiration rate among physiological parameters witnessed significant (P < 0.01) reductions only for S or MS category hybrids, indicating its importance in screening programs. Among the enzymes, the leaf POX activity showed moderate negative relationship with FRR score, reflecting its role in host defence. The six tolerant hybrids harboured high leaf POX activity (≥1000 ΔA min⁻¹ g⁻¹ fresh weight), while it was lower in S or MS category hybrids. To find overall response of the hybrids to pathogen infection, principal component analysis (PCA) and subordinate function analysis were performed on all the parameters, and comprehensive value (D-value) was calculated. SO attained the D value of 0.716 while RL scored 0.060. Four of the six tolerant hybrids shared D value in the range of 0.700–0.905. The four tolerant hybrids identified through this comprehensive analysis should prove useful to the citrus industry provided that the tolerance to other biotic and abiotic stresses is evaluated.