Culinary herb growth and appearance in response to high salinity and high pH treatments in substrate-based and substrate-free conditions

Low irradiance during winter glasshouse production in northern Europe induces elongation of stems and petioles in culinary herbs, leading to taller, weaker plants with a lower leaf density and a reduced shelf-life. Growers are interested in using low-cost alternatives to supplementary lighting to maintain plant quality. Plant stress responses have been shown to reduce growth in many species and appear to use an overlapping set of signals, including increased internal pH. This raises the possibility of using high pH or high salinity treatments to modify plant growth. High pH or high salinity foliar spray or substrate drench treatments were applied to two species of culinary herbs, namely coriander (Coriandrum sativum) and flat-leaf parsley (Petroselinum crispum var. neapolitanum) in three substratebased experiments with both species and one hydroponic experiment with parsley. A transient high salinity (240 mM NaCl) substrate drench treatment was effective in reducing petiole lengths by 10 - 20% in both species. The leaf area and colour of the plants remained unaffected. A high pH foliar spray treatment (20 mM KHCO3, pH 12) had a limited effect on petiole elongation. In parsley grown in a hydroponic system, high salinity (240 mM NaCl) reduced petiole elongation, but did not affect leaf area, colour, electrolyte leakage, or petiole firmness. The deterioration of plant appearance during 14 d storage at 4°C was also reduced by the 240 mM NaCl high salinity treatment. The results indicate that a sub-phytotoxic salinity treatment could provide a low-cost alternative to artificial lighting to control herb growth under commercial conditions