Preharvest heat stress affects rocket salad leaf transcription and metabolism at harvest and after chilled postharvest storage

Background and Aims
Climate change is resulting in increasingly variable weather patterns with spikes of high temperatures adversely affecting crop production. Here the effect of elevated temperature just before harvest was investigated in wild rocket (Diplotaxis tenuifolia), a popular brassicaceous salad. The key aim was to investigate how pre-harvest stress affects postharvest responses.

Methods
Mature rocket plants were subjected to 3 d of elevated daytime temperature (35 °C) before harvest. Leaves were then stored at 6 °C to mimic postharvest supply chain conditions. Physiological data were collected at harvest and after 7, 14 and 21 d of storage. Volatile organic compounds (VOCs) were analysed by gas chromatography–mass spectrometry, changes in metabolite profiles were analysed through flow injection electrospray high-resolution mass spectroscopy, and gene expression was assessed by RNAseq and real-time PCR.

Key Results
Transcriptomic analysis showed a mild heat stress signature affecting both metabolic and regulatory genes, including those related to hormone signalling. Models for effects on circadian clock genes and regulation of cold/dark stress responses are derived based on Arabidopsis thaliana pathways. After 7 d of storage, there were also significant effects of the pre-harvest heat stress on leaf VOC profiles, with distinct patterns compared with those at harvest. The metabolome was also affected after 7 d of storage, with specific effects on several lipid classes, amino acids and sugars. However, the direction of gene expression changes did not always match effects on VOCs. After 21 d of storage, pre-harvest heat stress adversely affected chlorophyll content and photosynthetic capacity, promoted ion leakage, and resulted in increased stomatal closure. Conclusions Cold storage affects the physiology, gene expression and metabolome of rocket leaves and these effects are perturbed by exposure to heat stress before harvest.