Cold-induced sweetening in potato Storage at 4–8°C is desirable for extending potato shelf life, but cold-induced sweetening , that is, the degradation of starch to hexoses, occurs. These sugars are protective, however, sugared tissues will form carcinogenic acrylamide under high-temperature processing, blacken, and become bitter, leading to PLW. Manipulating the pathways that influence hexose levels, that is, starch biosynthesis and degradation, sucrose hydrolysis to hexoses, and glycolysis, has provided some protection against tuber CIS, and thus has the potential to reduce PLW. Postharvest chilling injury This describes the loss of quality and accelerated spoilage when tropical commodities are stored below 13°C. Postharvest chilling injury affects a wide range of species and results in extensive PLW. Chilling may also trigger higher starch content, which is degraded to replenish sugars for continued osmoprotection to abate PCI. An interesting observation is that starch degradation may slow down or even halt as cold storage progresses in tomato, banana, and apple fruit, and even if fruits are rewarmed, black plastic planting pots starch degradation does not resume. In bananas, TFs that regulate this mechanism have been identified. The biological rationale for this mechanism is unclear and should be investigated in more diverse species.
Postharvest physiological deterioration Cassava root deteriorates 72 h after harvesting due to postharvest physiological deterioration , a disorder that leads to losses of 20–30%. Silencing AGPase in cassava causes sugars to accumulate in the root. A positive correlation between low PPD and high sugar content was found in the transgenic cassava that extendedroot longevity, and was likely due to sugars serving as ROS scavengers. Salinity stress Saline soils trigger sugar accumulation in the leaves, which initially serve as osmoprotectants, but over time, inhibit leaf photosynthesis. The rapid conversion of sugars to starch in tomato fruit promotes sugar export from the leaf to the fruit by mass flow, thus relieving photosynthesis. The ‘extra’ starch stored in ‘salt-stressed’ green fruit is hydrolyzed during ripening, boosting sugar content and intensifying fruit sweetness post harvest.Pathogen infestation decreases post harvest quality, but recent findings showing that pathogen colonization is associated with changes in host carbohydrate levels, offer new avenues for disease mitigation. Starch may accumulate in the host as an early response after perceiving bacterial effectors or volatile organic compounds in some species. Such accumulation could physically contain the microbes in situ, thus reducing systemic spread. If the infestation becomes advanced, accelerated breakdown of the accumulated starch to sugars may provide the host with energy and carbon for the biosynthesis of protective antimicrobial compounds.
Starch accumulation may also be induced by the pathogen after infection, and may involve reprogramming carbon allocation between source and sink. This is seen with black Sigatoka disease in bananas, and in citrus greening and grapevine red blotch where phloem starch accumulation is part of the disease response. BSD in bananas not only alters starch metabolism in vegetative tissues, but also changes the physical and chemical characteristics of starch in the harvested fruit. These examples across host species and pathogen types illustrate that spatial–temporal changes in starch metabolism may be important to host disease response. The RF1 banana mutant with BSD resistance, was associated with high levels of sugars and starch accumulation. Thus, starch accumulation could be a critical factor that could be modulated pre- and post infection to reduce the damage caused by pathogens.The postharvest quality of sink tissues, for example, fruits, roots, and tubers, depends on carbon allocation from the source , and agronomic practices, environmental factors, and disease pressure can all influence this process. Reconfiguration of carbon allocation in response to development or stress , is mediated in part by the trehalose-6- phosphate-sucrose nonfermenting-1-related kinase 1 signaling pathway, a central energy hub that senses sugar status. The T6P-SnRK1 pathway regulates carbohydrate content in potato tuber , sweet potato, and various fruits. T6P-SnRK also regulates starch physicochemical properties, anthocyanin accumulation, and CIS in various species, which all have an impact on PLW.As shown in multiple examples, manipulating the carbohydrate profile of horticultural crops by biotechnology should impact postharvest quality and directly reduce PLW.
Increasing starch accumulation, modulating starch degradation, altering starch composition, or changing sugar content are key targets. The composite Figure 4 and accompanying Table 1 illustrate many genes and regulatory factors that influence these processes.Throughout this entry, we have pointed to many unanswered questions related to carbohydrate metabolism in horticutural crops and experimental approaches to address them. Here, we highlight additional steps that could yield new knowledge to improve produce for reduced PLW. 1) Determine the extent to which starch metabolism influences fruit quality. An unresolved question is if the amount, and the rate of sugars released from starch, influence the substrate pool, and the synthesis of downstream flavor compounds . 2) Bioengineer genes encoding carbohydrate enzymes and regulatory proteins for improved postharvest sensory and nutritional quality. Many of these enzymes are regulated at the post-transcriptional level. A deeper understanding of these regulatory mechanisms could be leveraged to introduce subtle adjustments to the carbohydrate composition of tissues. Fine-tuning the amount and spectrum of carbohydrates produced could be used to optimize shelf life or the flux toward specialized metabolic pathways. 3) Establish the spatial–temporal profile of starch in fruit tissues. A starch-to-sugar atlas in fruit, through development and in response to pre- and postharvest stress, would provide fundamental and high-resolution data on core energy and carbon metabolic processes. Such an atlas would serve as a baseline to identify targets for gene editing. 4) Determine the role of fruit photosynthesis, in determining fruit quality. Fruit chloroplasts have photosynthetic and CO2 fixation capacity, an internal source of CO2, and fruit chlorophyll correlates with fruit quality. Fruit photosynthesis was activated when source photosynthesis was impaired under drought stress, and was accompanied by starch accumulation. Yet, the role of fruit photosynthesis in fruit growth, carbohydrate production, and stress response is unclear. Data that clarify these potential relationships are needed as the first step to exploiting these processes for enhanced fruit quality. 5) Leverage data from Arabidopsis to study starch metabolism in leafy greens. There is a growing mechanistic understanding of starch metabolism in A. thaliana. These data could be applied to starch-rich leafy greens such as spinach, or those of the related Brassicaceae, that is, cabbage, kale, collards, and so on. This would largely fill the knowledge gap between the current models of starch metabolism and their real-world application toward crop improvement. 6) Identify factors controlling carbon allocation from source to sink for postharvest quality and shelf life. Although it is tempting to focus on the harvested product, source-sink relationships help determine produce size, abiotic and biotic stress responses, and quality attributes . Varying the expression of key genes, for example, T6P-SnRKs, SWEETs, INVs, and so on, combined with precision agronomical practices, could improve many of these critically important attributes that influence marketability .Carbohydrate biosynthesis and degradation denote changes in energy conversion and storage, with consequences for postharvest quality. Despite the multifaceted role of starch and sugars in plant tissues, rarely are these compounds seen beyond serving as bulk reserves for direct consumption. Here, we argue that their metabolic dynamism is pivotal to the physiology and quality of the harvested organ, which, through their effect on organ size, aroma, taste, flavor, texture, drainage pot and visual appearance, will reduce postharvest waste at the consumer end. Further, because carbohydrates are vital substrates for respiration, and act as ‘stress-protectants,’ they influence storage and shelf life, and as a result, postharvest loss.
We also show that carbohydrate movement from source tissues to the harvested organ should not be ignored when investigating produce quality. Finally, the identified genes, enzymes, and regulators involved in carbohydrate metabolism we present, offer opportunities for precision modification of postharvest attributes to reduce waste and loss.Plant phenology is a key determinant of plant success and ecosystem productivity. Furthermore, as phenological events are often triggered by environmental cues, especially temperature, the study of phenology is essential for predicting how species will respond to climate change. Over the past decade, there has been a concerted effort to incorporate phenological traits, including the onset and duration of individual phenological phases, into evolutionary ecology and climate change biology. Despite the importance of phenology to plant success, however, little is known about the phenological behavior of most species. In particular, the way in which different environmental factors serve as phenological cues across the majority of species remains a mystery. This is mainly due to the difficulty of acquiring the data necessary to identify specific environmental factors that drive phenological transitions for a given species. The collection of these data has traditionally required long-term field observations or manipulative experiments that are difficult to scale up such that they capture entire regions, communities, or plant clades. Efforts to collect species-level phenological data, therefore, have been pursued in only a relatively small number of species from a limited geographic distribution and often over short timescales, resulting in a substantial gap in our understanding of phenology . To address this gap, researchers have recently turned to the vast collections of plant specimens in the world’s herbaria for phenological information. Herbarium specimens can be viewed as records of the phenological status of an individual, population, or species at a given time and place . While the phenological information provided by an individual specimen is limited, many specimens can be used collectively to assemble a long-term picture of the phenological behavior of a region and the species that inhabit it. Expanded phenological information derived from large numbers of specimens can offer insight into two key ecological phenomena: long-term shifts in phenology at a given location over decades or even centuries; and how seasonal or interannual environmental variation cues phenological transitions. It is now being recognized that herbarium specimens provide a reliable method for estimating phenological sensitivity in plants . Furthermore, specimens offer unique attributes that have the potential to greatly expand our understanding of phenology. First, specimens offer a detailed history of phenological change, with many collections dating back centuries , before the modern influence of climate change. Second, given their diversity in both phylogenetic and geographic sampling, specimens offer the opportunity to study the evolution of phenological traits in a wide range of lineages and biomes as well as how phenological traits may shape patterns of diversity under future climate change. The pace of herbarium-based phenological research has accelerated rapidly over the past decade facilitated by the increasing availability of online digitized herbarium specimens. As more of these collections are digitized and climate change research continues to advance, it is now an appropriate time to evaluate the current state of herbarium based phenological research and discuss potential limitations, areas for improvement, and opportunities for future research.For hundreds of years, botanists and naturalists have collected and preserved plants as herbarium specimens for taxonomic research, to record the flora of a region, to document their economic uses, and as a social hobby. Traditionally specimens were not collected with the specific intent to study phenology per se. As plant collection became more widespread among professional botanists in the 18th and 19th centuries, however, the ancillary information recorded and retained with each specimen became more detailed and standardized, and thus more amenable to phenological research. Most specimen labels created during the past 150 years provide information on locality, date of collection, and habitat. In addition to label data, physical specimens are rich with information regarding plant health, morphology, and phenological status. From these data researchers can derive descriptive estimates of a species’ reproductive season for inclusion in published floras, species identification, and application in horticulture. The use of such data for more detailed studies of ecological and evolutionary processes, such as phenological sensitivity to temperature, has been limited . Phenology as a field of study dates to the 18th century in Europe and even earlier in Japan and China, where observers recorded the flowering dates of culturally significant plants such as cherry trees. Careful observations of plant phenology and its relationship to meteorological records became common in many European countries, the USA, Japan, South Korea, and China during the 19th century; these observations have a rich tradition in horticulture and agriculture and natural history and in the past couple of decades have been used for climate change and ecological research. It is only relatively recently that researchers have begun to use herbarium specimens for plant phenological research.