Physiological characteristic analysis of a leaf-yellowing mutant in watermelon

Abstract：【Objective】Leaf color mutant, also known as chlorophyll deficiency mutation, directly or indirectly affects the synthesis and degradation of chlorophyll. In basic research, leaf color mutants are ideal materials for studying on a series of physiological metabolic processes such as photosynthesis, photomorphogenesis, hormone physiology and disease resistance mechanism of plants; in breeding, leaf color variation can be used as a marker characteristic to simplify the breeding and cross production of improved varieties. This experiment material originates from a natural mutation. The leaf turns yellow from cotyledon stage and can bear fruit normally. Therefore, the study on its leaf structure, photosynthesis and physiological characteristics can provide theoretical reference for releasing the mutation mechanism of leaf color mutation and its application in leaf color marker breeding.【Methods】The physiological characteristics of YL were studied with a leaf-yellow mutant of watermelon. The leaf color during the whole growing period was yellow. ZK is the parent material of a cultivated species with stable characteristics. The physiological indexes were measured according to three key stages: extending stage, flowering stage and mature stage. The determination time of Extending stage, Flowering stage and mature stage were 15 days, 25 days and 50 days after planting, respectively. The leaf ultrastructure, photosynthetic pigment content, photosynthetic parameters, antioxidant enzyme activity and MDA content were analyzed. The ultrastructure of leaves was observed and photographed by means of sectioning under a transmission electron microscope. The photosynthetic parameters were measured by CIRAS- 3(USA), and the parameters were adjusted according to the measurement environment. The measurement was carried out at 9:00-11:00 a.m. when the weather was fine. The contents of photosynthetic pigment and MDA as well as antioxidant enzyme activity were completed according to the guidance of plant physiological experiment.【Results】Compared with ZK, the chloroplast of YL developed incompletely, which showed that the number of grana thylakoids in the chloroplast was few, the number of grana lamellae in the grana thylakoid was less, the arrangement was disordered and the cell metabolism was weak. The cells of ZK were rich in chloroplasts, with clear grana structure, dense matrix, and developed thylakoid membrane system with large number of grana and orderly arrangement. The time, position and stage of the materials were the same. Starch grains accumulated more in chloroplast, but not in the yellow leaf mutant. The contents of chlorophyll a, chlorophyll b and carotenoid in the leaves of YL were 80.95%, 83.87% and 69.87% lower than those of ZK on average. The total chlorophyll content in the three periods of ZK was 7.35, 4.26 and 5.45 times higher than that of YL, respectively. According to the difference in photosynthetic pigment ratio of leaf color mutants, predecessors can be divided into different mutation types. One was that the mutant contained no chlorophyll b or the chlorophyll a/b value was close to or more than 20, the other was between 6-10. In this study, the chlorophyll a/b was 6.03, 6.38 and 3.18, respectively, which changed with the growth period. So the mutant belonged to the second type of mutation, probably being the total chlorophyll deletion type. The assimilation rate, stomatal conductance and transpiration rate of the leaves near the top and middle of YL were higher than those of the leaves near the base, and each index of the leaves in the middle was the highest, among which the assimilation rate was 1.04 times and 1.28 times of the leaves near the top and the base, respectively. The stomatal conductance and transpiration rate of the leaves near the top and middle of ZK were higher than those of the leaves near the base. In addition to the CAT activity of the two kinds of materials at mature stage, the SOD, POD and CAT activities of the mutants were lower than that of the green plants， among which there was a big difference in the extending stage. At the flowering stage, the average SOD content of YL mutant was 29.61 U · g- 1 · min- 1 . The POD activity of YL mutant was lower than that of ZK, about 81.3% of ZK. The change of antioxidant enzyme content indicated that the YL had certain adaptability to the environment, but its overall growth, resistance to adversity and diseases, insects and pests were weaker than the normal plants, and its metabolism was disordered in the mature stage. The content of MDA was 0.56 μmol· L- 1 , which was significantly higher than that of ZK, but there was no obvious trend in the other two stages.【Conclusion】The decrease of photosynthetic pigment was the main cause of yellow leaf color. This experiment material may belong to the mutation type of total chlorophyll and chlorophyll b synthesis reduction. However, the incomplete development of chloroplast affected the light harvesting ability of plants, resulting in the decrease of net photosynthetic rate and metabolism ability of plants. These results could provide basic information for elucidating the mechanism of watermelon leaf yellowing.