Food waste is a significant problem globally and one that we have written about previously in the context of wasted resources and climate change. Taking data from the Food and Agriculture Organisation of the United Nations, the authors of the paper we are looking at today report that the amount of food loss, being food successfully produced but which must be discarded before being sold to consumers, amounts to something in the vicinity of 45% of food produced globally.
A large portion of fresh food requires refrigeration in order to extend shelf life. However, refrigeration throughout the transportation and storage periods after harvest can be problematic to maintain. Further, cold storage post-harvest can effect the nutrient values of the produce and can effect the look, feel and taste at the time of purchase and consumption.
The researchers behind a paper published recently in the Plant Biotechnology Journal studied the physiological and hormone profiles of detached cabbage leaves stored at room temperature and in cold conditions in an attempt to characterise differences between the two types of storage conditions and identify those differences that could be the cause of the increased senescence of those stored at room temperature.
Given the significant amount of research that has characterised abscisic acid (ABA) as a plant hormone which affects responses to stress (we’ve previously reported on a mini-review of the role of ABA in plant stress response here), the researchers further studied the response of the detached cabbage leaves when ABA or pyrabactin was applied to the leaves and then stored at room temperature. Pyrabactin is a synthetic sulfonamide which can mimick the effects of ABA but is less photosensitive, possibly more selective in its effects on plants and probably easier to apply by farmers.
The effects of cold storage compared to room temperature storage
The paper details that cabbage leaves were detached and placed either in storage at 4°C (cold storage) or at 25°C (room temperature) and monitored over 10 days.
After 10 days in storage, leaves stored at room temperature lost half their initial weight compared to only 20% when kept in cold storage.
All other methods of analysis were performed on the separated apical, medial and basal strips of leaves stored at either temperature.
Water loss from apical strips of cold stored leaves was less than those stored at room temperature, while there was little difference in water loss between the basal strips.
Photosynthesis efficiency was significantly reduced across all strips stored at room temperature after 4 days of storage compared to no change under cold storage. The medial and basal strips had decreased chlorophyll and carotenoid levels under room temperatures, characterised by yellowing of the leaves and water loss.
Figure 1 from article. Differences in relative water content (RWC) and chlorophyll levels in apical, medial and basal strips of cabbage leaves stored at room temperature and 4°C.
Hormonal profiling showed significant changes between the two treatments, with decreases in levels of a number of cytokinins in leaves stored at room temperature and ABA contents increasing to a greater extent in leaves stored at room temperature compared to those stored in the cold.
The effects of applying ABA and/or Pyrabactin on senescence
Given the difference in ABA levels in leaves stored at room temperature, ABA and/or Pyrabactin (Pyr) was applied to leaves to test whether it had any effect on the deterioration of leaves stored at room temperature. Either ABA alone, Pyr alone or a combination of ABA and Pyr were applied to leaves every second day for eight days and then analysed in comparison to control leaves.
Membrane stability, analysed via a number of different methods, demonstrated that each of the three treatments increased membrane stability compared to control. Despite increased stability, photosynthesis efficiency was not significantly different between experiment and control.
Figure 3 from article. Water retention, chlorophyll, malondialdehyde and Membrane Stability Index measurements compared between leaves at the day of harvest and between each treatment after 8 days.
Hormone profiling did however show some significant changes caused by the ABA/Pyr application. Endogenous ABA levels were doubled in treated leaves with the application of ABA (but not Pyr alone) and several cytokinins showed increased levels with treatment.
Transcriptomic analysis was then performed on the apical strips of control leaves and each of the three treatments. Despite the doubling of endogenous ABA after treatment, ABA transcription showed no marked changes but ABA catabolism and ABA release from reversible conjugation were down-regulated. ABA transporters in the cytosolic membrane were expressed while those in the vacuole were not expressed, likely due to the increase in ABA from the application and therefore reducing sensitivity to the hormone.
Ethylene metabolism and regulation showed some significant changes with the increase in ABA content. At the end of the treatment period the presence of the precursor to ethylene, AAC, was halved in treatments containing ABA with genes encoding the syntheses of ACC and ethylene both down-regulated. Negative regulators of the ethylene response were also down-regulated. Pyr treatments showed little or no effect.
The manipulation of hormones by the ABA/Pyr treatments suggested to the researchers that the treatment may delay senescence in the cabbage leaves. Many photosynthesis genes were up-regulated as a result of treatment, as were genes coding calvin cycle enzymes. Genes involved in the reduction of reactive oxygen species were also up-regulated, while genes involved in maintenance of cell walls and membranes were also found to be up-regulated by the ABA/Pyr treatment. These findings gave plausibility to the possibility that senescence was being delayed in the treatment group.
In a possible trade off for the delay in senescence, the ABA/Pyr treatments appeared to down-regulate the defence mechanisms of the leaves to biotic stress. Treatment with ABA resulted in reductions in salicylic acid-related defence responses.
The researchers state in the conclusion that ABA’s role in regulating senescence isn’t overly surprising given its role in assisting plant responses to abiotic stress such as drought, a stress imposed on leaves when they are detached from the plant. Previous research has indicated that the role ABA plays in the stress response of leaves depends on the age of the leaf; ABA stimulates senescence in older leaves while resisting senescence in younger leaves. By applying ABA to leaves immediately post-harvest, the treatment promoted homeostasis and assisted the leaves to resist the senescence observed in the untreated leaves.
The findings of the paper suggest a plausible method of reducing food loss post-harvest. However, the practicability of applying ABA to leaves immediately post-harvest remains to be seen and the transferability of the findings to other fresh produce will be interesting to see. Also of interest would be the difference in senescence delay between ABA application and cold storage to determine whether there is any ability to reduce reliance on cold storage.
The research should lead to further investigations into the role of the individual components of the senescence response of harvested plants and how significant a role manipulating ABA responses can play in securing food availability. Further, the applicability of transgenic methods in generating the same responses without the need for exogenous application of ABA could be a research target which could generate interesting results.