Lignin Accumulation in Three Pumelo Cultivars in Association with Sucrose and Energy Depletion

Abstract

Lignification, which occurs in many horticultural fruit and vegetables, brings about undesirable texture and unfavorable consumer preference. However, this problem has rarely been studied. In this work, three pumelo cultivars cvs “Hongroumiyou” (HR), “Bairoumiyou” (BR), and “Huangroumiyou” (HuR) were stored at 25 °C for 90 days, and juice sacs were sampled to explore the lignin accumulation and its relationship to sucrose and energy depletion were investigated. The results displayed that HuR contained lower sucrose content, lower ATP level, but higher lignin content compared to BR and HR during postharvest storage, indicating that the sequence according to storage resistance on the basis of lignin content is as follows: HuR < BR < HR. Furthermore, sucrose degradation attributed to enhanced activities of neutral invertase (NI), soluble acid invertase (S-AI), cell wall-bound invertase (B-AI), and energy deficit on account of declined ATP level, showed significantly negative correlation with lignin accumulation, suggesting that lignin accumulation occurrence could induce sucrose degradation and energy deficit during postharvest storage. Additionally, higher activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD) could accelerate lignin synthesis and resulted in lignin accumulation during postharvest pumelo storage.

The Involvement of Energy Metabolism and Lipid Peroxidation in Lignin Accumulation of Postharvest Pumelos

“…HR pumelos were harvested according to our previous work [ 6 ] from an orchard in Dapu county, Guangdong, China. Fruits at identical size and absent of mechanical injury were chosen and wrapped up by a 0.03 mm thick polyethylene film bag with one fruit in one bag at 25 °C and 85% relative humidity (RH).…”

“…ATP, ADP, and AMP levels were measured according to the previous study [ 6 ]. EC was computed with the formula: ([ATP] + 0.5 × [ADP])/([ATP] + [ADP] + [AMP]).…”

“…It was well documented that energy deficit highly contributed to lignification of loquat fruit [ 26 ], water bamboo shoot [ 27 ], and mushroom [ 28 ] during postharvest storage. Moreover, the possible link between energy deficit and lignification was reinforced by the observation that declined ATP level accelerated the lignification process of three postharvest pumelo cultivars in our previous work [ 6 ]. Energy charge (EC) could reflect cellular energy status [ 18 ], which is closely influenced by some critical enzymes, such as cytochrome c oxidase (CCO), succinic dehydrogenase (SDH), ATPase, nicotinamide adenine dinucleotide phosphate (NADP), and nicotinamide adenine dinucleotide (NAD).…”

“…However, improper storage could promote the occurrence of lignin accumulation, which consequently result in quality deterioration and commodity loss of postharvest pumelo fruit [ 4 , 5 , 6 ]. Our previous study illustrated that the sucrose metabolism and ATP deficit contributed to the accumulated lignin content in harvested pumelos during storage [ 6 ]. Nevertheless, this is just an iceberg of the complex mechanism of lignification process during postharvest pumelo fruit storage.…”

“…Therefore, it could be inferred that the membrane integrity loss could initiate decompartmentalization of monolignols and polymerization-related enzymes, finally promote the process of lignin biosynthesis. However, previous studies mainly focus on the role of ROS metabolism in the activities of lignin biosynthesis-related enzymes [ 6 , 45 ], while that of membrane destruction caused by lipid peroxidation in the lignin biosynthesis have been scarcely reported to date. Therefore, there is a critical need for further investigation on lignin accumulation in association with energy metabolism and lipid peroxidation in postharvest pumelos.…”