Long-term deficit and excess of irrigation influences quality and browning related enzymes and phenolic metabolism of fresh-cut iceberg lettuce (Lactuca sativa L.)

The effects of five drip irrigation treatments (excess 50%, excess 25%, control, deficit 25%, and deficit 50%) on the quality and shelf life of fresh-cut iceberg lettuce (Lactuca sativa L) were examined in 6 different harvest dates during 3 consecutive years. To minimize climatic differences between harvest dates, rainfall (mm) and crop evapotranspiration (ETc) (mm) were also considered during the whole growing period. The total amount of water was calculated considering the irrigation treatment + rainfall - ETc. Four irrigation regimes were established as R1 (0-100), R2 (101-200), R3 (201-300) and R4 (301-400) mm for the better interpretation of the data. Lettuce quality characteristics at harvest showed that for the irrigation regime with most water, fresh head weight, electrical conductivity and dry matter content were lower while maturity stage was higher. Visual quality of the fresh-cut lettuce was reduced at the highest irrigation regime while off-odours increased. However, because of the storage in active modified atmosphere packaging (MAP), browning of the fresh-cut lettuce was well controlled in all irrigation regimes. However, when the midrib tissue during storage in air was specifically studied, different responses caused by the different irrigation regimes were detectable in the activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD), as well as in the content of individual and total phenolic compounds. The analysis of midrib tissue revealed a more than 17-fold increase in PAL activity of the midrib tissue from highly irrigated lettuce. Likewise, PPO activity and percentage of activation increased with a concomitant rise in browning development at the cut edge, particularly for higher irrigation regimes. Phenolic compounds, mainly caffeic acid derivatives, increased at 2 d, but the increase was more noticeable in the less irrigated samples because of the low PPO activity and therefore less consumption of PPO substrates. POD activity was less affected by different irrigation treatments. In conclusion, the quality and shelf-life of the fresh-cut lettuce was better preserved by the reduction in the irrigation quantity during the growing period. This result may provide an opportunity to lettuce growers for improving sustainability in growing areas highly dependent on water. (C) 2012 Elsevier B.V. All rights reserved.