Effect of potassium fertilizer on lemon balm (Melissa officinalis L.) grown under water stress conditions

This work was carried out to study the effect of K fertilizer rates and water stress levels on the growth and essential oil content of Melissa officinalis L. A pot experiment was carried out under the natural conditions of the greenhouse of the National Research Centre, Dokki, Giza, Egypt. Growth characters (herb fresh and dry weights g plant ^-1 leaves number and leaf area) and essential oil content of Melissa officinalis L. were significantly decreased with the rise in water stress levels, but proline synthesis was stimulated in response to water stress levels. Application of K fertilizer rates counteracted the above adverse effects of water stress. Irrigation at 80% available soil moisture and fertilization with 0.8g K pot ^-1 dose in the 1^st cut resulted in the highest mean values of herb fresh and dry weights (51.01 and 12.05g plant ^-1, respectively) and leaves number (420.80 plant ^-1), while 40% available soil moisture treatment resulted in the lowest values (13.73; 3.22 and 113.60) of these parameters, respectively. The maximum mean value of essential oil content (0.131%) was obtained from plants irrigated with 80% available soil moisture with K fertilizers rate (0.6g K pot ^-1), while the lowest mean value (0.097%) was determined in the plants irrigated with 40% available soil moisture with no potassium fertilization in the 3^rd cut. Increasing the dosage of K fertilization significantly increased the proline content while, increasing water irrigation decreased the proline content. The highest mean value of proline was determined in the plants received 0.8g K pot ^-1 and irrigated with 40% of available soil water increase an average of 68.69% than that for the plants irrigated with 80% available soil moisture in the 3^rd cut. Geranial and neral were identified as the two major compounds in the essential oil extracted from Melissa officinalis L. Increasing both of K rates and available soil moisture tended to increase the contents of geranial and neral. The results revealed that, the highest amount of geranial (49.75%) was recorded from the combination of irrigation at 80% available soil moisture and fertilization with 0.8g K pot ^-1 treatment in the 3^rd cut while, the highest amount of neral (35.71%) was recorded at 80% available soil moisture treatment in the 2^nd cut.