As a leafy vegetable, lettuce is particularly vulnerable to stresses that affect turgor, growth, and nutrient uptake.

*   **Water Stress (Drought):** Negatively affects growth and productivity, with low irrigation levels significantly reducing yield.
*   **Drought Effects on Biomass:** Drought leads to lower dry aerial weight and dry root weight.
*   **Yield Reduction:** Drought stress can decrease lettuce yield by up to 50%.
*   **Nutrient Stress:** Nutrient unbalance or deficiency, particularly of nitrogen and phosphorus, can negatively affect crop productivity.
*   **Combined Water and Nutrient Stress:** When combined, these stresses have a more significant negative impact than when applied individually.
*   **Cold Stress:** Cold soil temperatures and frosts can cause cellular damage and increase disease pressure.
*   **Waterlogging:** Can lead to anoxic conditions in the root zone, stressing the plant.
*   **Light Stress:** Lettuce is sensitive to continuous light, especially at high intensities, which can induce oxidative stress.
*   **Biotic Stress (Fungal):** Fungal diseases like Fusarium wilt can have a greater negative effect on lettuce than water or nutritional stress, reducing fresh weight by up to 69%.
*   **Phenolic Compounds:** Stressed lettuce plants show a higher phenolic index, indicating an induced defense response.
*   **Anthocyanins:** Biotic stress can lead to a significant increase in anthocyanin content.
*   **Nitrate Levels:** Drought stress has been reported to decrease fresh biomass-related nitrate levels.
*   **Calcium Deficiency:** Both overly humid and overly dry conditions can interrupt transpiration, leading to calcium deficiency, which manifests as tip burn or internal browning.
*   **Silicon Application:** Foliar applications of silicon can strengthen plant cells, protecting the crop from cold stress, wind damage, and high humidity.
*   **Antioxidant Response:** Lettuce plants have an antioxidant defense system to overcome oxidative stress from factors like continuous light.
*   **Root-to-Shoot Ratio:** Under continuous light, lettuce plants may show a greater root-to-shoot ratio as an adaptive response.
*   **Chlorophyll Fluorescence:** This parameter can be used to monitor the plant's response to light stress.
*   **Spectral Indices:** Indices like the Pigment Specific Simple Ratio (PSSRa) can effectively detect biotic stress before visual symptoms appear.
*   **Combined Abiotic and Biotic Stress:** The combination of stresses can lead to unique responses; for example, lettuce subjected to both water and biotic stress showed the highest phenolic index.