by Roberto G. Lopez - [email protected]
Figure 1. Hydroponic nutrient film technique (NFT) greenhouse basil production.
Visual symptoms of chilling injury in basil can vary by cultivar, with some growers indicating that sweet basil is the least sensitive.
Figure 2. Typical packaging of potted greenhouse or vertical farm produced herbs. Potted herbs are then marketed in grocery store produce departments.
Interveinal brown spot or discoloration of leaves and sometimes browning of the stem is often followed by wilting, epinasty, loss of glossy appearance (Figures 3, 4, 5, and 6) and aroma. Leaf abscission of older leaves typically occurs a few days after exposure to warmer temperatures and can be attributed to chilling-induced ethylene production (Figure 5).
Figure 3. Characteristic chilling injury symptom of brown discolorations (top left) on cut basil in a plastic clam shell. The clam shells were not in the display cooler with the other cut herbs.
To complicate matters even more, it has been reported that greenhouse produced basil is prone to Botrytis cinerea when there is excess humidity in the packaging and storage temperature are above 54 ºF.
Figure 4. Chilling injury symptoms of potted basil in a garden center. Photo: Christopher Currey, Iowa State University.
A review of the literature indicates that researchers around the world have explored a wide variety of treatments and technique to prevent or reduce chilling injury of basil with success. Recent research has shown that the time of day that ornamental cuttings such as euphorbia and lantana are harvested influences their post-harvest life. Aharoni et al (2010) reported that harvesting sweet basil at 8 AM (2 hours after sunrise) resulted in severe leaf browning, decay, and abscission after 5 days at 54 ºF plus 2 days at 63 ºF (17 ºC). However, harvesting basil at 4 PM resulted in reduced browning, decay, and abscission after exposure to the above chilling temperatures. Previous work conducted at Michigan State University by Lange and Cameron (1994 and 1997) indicates that shelf life of sweet basil stored at temperatures at or above 50 ºF (10 ºC) improved when harvested during the afternoon. They conclude that a possible solution to large losses caused by chilling injury might be to harvest in the late afternoon or early evening and then store chilling-sensitive leafy crops for 1 day at 50 ºF before refrigerated shipment at 41 ºF. It has also been reported that increased resistance to chilling injury of afternoon-harvested basil can be correlated to a 10-fold accumulation of carbohydrates such as starch.
Figure 5. Young potted basil after exposure to 3 days of 43 ºF in a cooler, followed by 3 days at 72 ºF. Notice yellowing and brown discoloration of bottom leaves. Photo: Jennifer Boldt, USDA-ARS.
Another study by Aharoni et al ( 2010) showed that chilling injury severity of cut basil harvested in the morning was reduced by up to 42% by exposing the packages to humidified air (96 to 98%) at an air temperature of 100 to 104 ºF (38 to 40 ºC) for 4 hours prior to exposure to storage temperatures of 48 ºF (9 ºC).
Figure 6. Chilling injury of potted basil in a grocery store produce department with an air temperature set point of 50 ºF.
To summarize, chilling injury may be avoided by storing and shipping fresh cut and potted basil at or above 54 ºF (12 ºC). Basil harvested in the afternoon will have a better post-harvest life than plants harvested in the morning. Finally, if you are not able to ship basil at temperatures above 50 ºF, condition plants for one day at 50 ºF before shipping. As with all research, growers are advised to conduct their own trials before implementing large scale changes as your growing conditions can impact results.
References
- Aharoni, N., Kenigsbuch, D., Chalupowicz, D., Faura-Mlinski, M., Aharon, Z., Maurer, D., Ovadia, A., and Lers, A. 2010. Reducing chilling injury and decay in stored sweet basil. Israel Journal of Plant Science 58:167- 181.
- Lange, D.L . and A.C Cameron. 1994. Postharvest shelf life of sweet basil (Ocimum basilicum). HortScience 29:102–103.
- Lange, D.L. and A.C Cameron. 1997. Pre- and postharvest temperature conditioning of greenhouse-grown sweet basil. HortScience 32:114–116.