Study On Low-Temperature Tolerant Mechanism Of Bitter Gourd (Momordica Charantia L.)

Bitter gourd (Momordica charantia L.) is a member of Cucurbit family. Its immature fruits are edible and have medicinal value. Present studies mainly focus on its medicinal functions, seldom on the aspects of physiology and breeding. Bitter gourd mainly distributes in tropic and subtropic areas. Low temperature limits the geographical locations and the planting season suitable for bitter gourd growth. Chilling temperature always accounts for significant losses in crop production. So this experiment aimed to study on low temperature physiology and low-temperature-resistance mechanism on this special vegetable crop. This work would help to shelter cultivation and low-temperature-resistancebreeding of bitter gourd.In this paper, in order to understand the low temperature tolerance and the mechanism of tolerance of bitter gourd, the effect of different low night temperature on the physiological and biochemical characteristics in two bitter gourd inbred lines was studied; then low-temperature-induced methylation pattern changes in CCGG sites of genomic DNA were analyzed by methylation-sensitive amplified polymorphism (MSAP) technique to explore the possible molecular mechanism in low temperature adaptation. The effects of exogenous salicylic acid (SA) application on the low-temperature tolerance of bitter gourd were investigated with a view to further study the physiological mechanism under low temperature and give some useful advice in cultivation and management when bitter gourd suffered hypothermia. Finally, based on half-diallel design, 21 F₁ hybrids were confected by 7 inbred lines of bitter gourd. Additive-dominant model was used to process genetic analysis of low-temperature tolerance. At the same time the character of low-temperature-tolerance of all F₁ materials was identified. In combination with other agronomic traits, excellent hybrid combinations were selected. The main research results were as follows:1. A 7-d 12℃low night temperature treatment had little impact on the physiological and biochemical indicators in bitter gourd leaves and caused little damage to bitter gourd seedlings. A 7-d 8℃low night temperature treatment reduced the stability of the membrane, PSâ…¡potential activity (Fv/Fo) , the coefficient of photochemical quenching (qP) and PSâ…¡quantum yield (ΦPSâ…¡) of the bitter gourd seedlings, and increased the part of light energy that dissipated as fluorescence (E) , resulting a certain injury. And the injury can be partially or fully restored after growing at room temperature for 7 days. A 7-d 5℃low night temperature treatment did the most serious harm to the bitter gourd seedlings. It further reduced the membrane stability, Fv/Fo, and qP andΦPSâ…¡. It also led decreases in chlorophyll content, the minimal fluorescence (Fo), the maximal fluorescence (Fm) and the variable fluorescence (Fv). Increases in chlorophyll a/b ratio and E were observed. After being growed at room temperature for 7 days, all the indicators restored to a certain level. The extent of the restoration showed difference between lines. As for the two lines we tested there, Z-1-4 showed better low-temperature tolerance than Y-106-5.Under low night temperature,ΦPSâ…¡was primarily droved by the composition state of the reaction center, rather than the efficiency of the absorbed light energy to reach the reaction center. The maximal photochemical efficiency of the PS II (Fv/Fm) was affected slightly and Fv/Fo was affected largely by the low night temperature. In the studies about effect of low night temperature on plants, Fv/Fo was a more sensitive and useful indicator than Fv/Fm.2. The total DNA methylation content in bitter gourd is about 22.92% -25.86%, the average internal cytosine methylation is 19.63 % and the average hemi-cytosine methylation is 4.66%. The order for the total methylation content in the four inbred lines is Y-106-5＞Y-81-3＞Z-1-4＞Y-70-1.Low temperature induced methylation pattern changes at the CCGG sites in genomic DNA. Nine fragments which were observed variation in at least two inbred lines were cloned and sequenced successfully. BLASTn search revealed that fragments Y1, Y2 and Y5 showed homology to certain sequences in NCBI database. Y1 matched withβ-1, 3-glucanase mRNA in Sesbania rostrata to a certain extent. Fragment Y2 showed a certain similarity with the GCN5-related N-acetyltransferase (GNAT). Fragment Y5 exhibited high similarity (98%) with psaA which encoding chloroplast PS1A subunit in cucumber.3. 0.5mmol/L SA pretreated seedlings had high peroxidase (POD), catalase (CAT) and Superoxide dismutase (SOD) activity and (POD + CAT)/ SOD ratio, showed strong antioxidant activity, maintained relative higher levels of photosynthetic pigments. Fv/Fm andΦPSâ…¡were impacted slightly, and lipid peroxidation degree was low, indicating 0.5mmol/L can reduce the injurys caused by low temperature. Higher concentration of SA (1.0 and 1.5 mmol/L) and lower concentration of SA (0.5 mmol/L) had different effect on bitter gourd seedlings under low temperature stress.4. Genetic parameters indicated that the MDA content, electrolytic leakage of intact leaves (Eb) and electrolytic leakage of detached leaves (Ea) after low temperature stress were mainly dominant effect. All the traits showed some additive effect. Environmental conditions also have a significant impact on them. These indicators showed high broad heritability (H_B²) and relative low narrow heritability (H_N²), so that the corresponding traits should be selected at a higher generation.Under low temperature stress, the inbred lines P1, P2, P3 and P6 showed negative additive effect for content of MDA, Ea and Eb. They displayed relative good low temperature tolerance and could be used as good breeding parents for low temperature tolerant breeding of bitter gourd. Hybrids P1×P5, P1×P6, P1×P7, P2×P3, P2×P5, P3×P5, P4×P6 showed negative dominant effect in all the three indicators. Combining low temperature tolerance and other agronomic traits, bitter gourd variety that suitable for greenhouse cultivation can be selected. The trial cultivation of 4 hybrids had already been started. Among them 2 hybrids could be used in greenhouse cultivation.