Physiological response of Olea europea L. to variable irrigation regimes
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Olives are a historic horticultural crop in the Mediterranean where a long, hot growing season and a relatively cool winter with minimum temperatures make it the perfect climate for growing this crop. An increase in the global demand for olive products, especially olive oil, has led to many countries outside the Mediterranean to develop extensive olive production areas. Olive oil is a staple in most Mediterranean diets and has been linked to improving health quality with its free-radical scavenging properties, and has gained popularity around the world. New olive orchards are typically high-density orchards that range from 200 – 300 trees per hectare, to a super-high density system of 1,500 – 2,500 trees per hectare. A benefit to high-density orchards is the ability to mechanically harvest the crop, which cuts down on labor costs for the orchards. These high-density orchards rely on supplemental irrigation systems to meet the needs of the plants. Additional irrigation can cut the amount of time it takes to bring an orchard into full production and possibly eliminate the alternate bearing characteristic of olive trees. However, with water supplies rapidly depleting, it is important for olive growers to be able to limit irrigation without harming the overall health and production of the trees. Olives are a drought tolerant crop and there has been extensive research completed in the Mediterranean area to understand how drought affects the trees in that area. However, there is limited research available for newer production areas, and none of that is specific to the south Texas production area. Therefore, the purpose of this research was to understand the physiological effects of drought on olives in south Texas. The field portion of this research took place at three different olive orchards in Texas: Texas Olive Ranch in Carrizo Springs, Conly Olive Orchard in Asherton, and Central Texas Olive Ranch in Walburg. At each orchard three different levels of irrigation, low, medium and high, were applied to the established ‘Arbequina’ trees. During the growing season, May – August, monthly mid-day stomatal conductance and SPAD readings were taken in 2010 and 2011. Shoot growth was measured at the end of each growing season. At Texas Olive Ranch in September 2010, individual olives were harvested from the trees under the different levels of irrigation. Overall means at the three orchards indicated little difference in stomatal conductance between the different irrigation treatments, indicating that less water can be applied without physiologically harming the trees. The greenhouse portion of this research was conducted to evaluate the effects of drought on four different olive varieties when subjected to varying levels of fertility. Four fertility treatments (control, low, moderate, and high) were applied to ‘Arbequina’, ‘Arbosona’, ‘Koroneki’ and ‘Mission’ olives. These trees were then subjected to two different drought experiments, acute and sequential. Mid-day stomatal conductance, leaf temperature and pre-dawn leaf water potential measurements were taken and indicated that ‘Arbequina’ can withstand the effects of drought better than ‘Mission’. Also increasing fertility rates enhance the effects of drought regardless of variety. Both the greenhouse and field studies play a role in better understanding the effects of drought on olive trees.