Tolerance mechanism in hybrid citrus rootstock progenies against Phytophthora nicotianae Breda de Haan

Phytophthora species is the major threat for world citrus industry in general, and for India, in particular due to commercial use of susceptible rootstocks. The resistant gene possessed by Poncirus genus may be of immense use, if transferred in a well acclimatized citrus species which can have good impact on fruit size of scion varieties. Being a soil borne problem, development of resistant/tolerant rootstock(s) is the most eco-friendly solution to combat with this deadly disease. The present study was conducted during 2016 to understand the tolerance mechanism in the intergeneric hybrids of citrus rootstocks against Phytophthora nicotianae. The materials of study consisted of 30 hybrids, ten each of Pummelo (P) × Troyer (T), Pummelo (P) × Sacaton (S) and Pummelo (P) × Trifoliate orange (TO) were tested against the inoculation of P. nicotianae, taking Pummelo, Troyer and Citrumelo as control treatments. Of the total hybrid progenies, only six hybrids (P × TO-103, P × TO- 112, P × S-117, P × S-119, P × T-125 and P × T-130) expressed resistance against P. nicotianae on the basis of lesion length (nil or <2.5 cm). Of the tested hybrids, P x S-117 had the highest photosynthetic rate (A) (8.12 µmol m-1s-1) followed by P × TO-112 and P × S-119. Excised leaf water loss (ELWL) was lowest in P × S -119 (7.47%) without having significant diference with Troyer citrange, and rest of the resistant hybrids. The highest relative leaf water content (RWC) was registered in P × T-125 (84.47%), which was similar statistically with P × T-119, P × T-103 and P × T-112 (77.59-83.42%) hybrids. Hybrid P × S -117 tended to show the highest total chlorophyll content (12.14 mg g-1FW), followed by P × TO-112 and P × T-127. P × S-117 expressed the lowest level of hydrogen peroxide (100.72 mM mg-1 FW) without having any significant difference with those of P × TO-110, P × T-127 and P × T-130 hybrids. The lipid peroxidation was highest in P × T-132 (25.99 µmol g-1 FW), while its lowest accumulation was in P × S-119 (6.44 µmol g-1 FW) with statistically similar to P × T-130 hybrid. The highest content of glycine betain was noticed in P × TO-103, P × TO-117 and P × TO-130 (0.33 mg g-1 FW in each). Of the total hybrid progenies, highest accumulation of leaf N was found in P × T-125 (2.74%) followed by P × S -119 and P × T-130. All six resistant hybrids excelled over other hybrids in respect of leaf K+ content. The content of Ca+2 was highest in the leaves of P × S-117 (4.90%) having similarity statistically with P × T-125 (4.87%). The resistance of identified hybrids (P × TO-103, P × TO- 112, P × S-117, P × S-119, P × T-125 and P × T-130) against P. nicotianae was also evidenced by low ROS generation and ELWL, with high RWC and leaf nutrient status over other hybrids. Among the various physicochemical characters studied, only A, ELWL and N were found to have significant but inverse relationship with lesion length caused by Phytophthora inoculation