With the prediction of rising global air temperatures, drought may become more frequent and severe in areas where problems with water resources already exist (Wujeska et al., 2013). New Mexico has an arid to semiarid climate. Mean annual precipitation is 200 mm and average annual temperature is 20 °C. Pecan is one of the most important agricultural crops in New Mexico. In 2012, New Mexico produced 31.3 million kilograms of pecan nuts (in-shell basis), which accounted for ≈23% of the U.S. production [U.S. Department of Agriculture (USDA), 2012]. However, the extreme demand for water coupled with periods of drought and increasing pecan acreage has reduced the available water for irrigating pecan (Johnson, 2004; Samani et al., 2011). Because of ongoing drought in the Rio Grande Basin, irrigation districts have allocated much less water to farmers than in the past. Farmers in the region recently have been forced to rely heavily on water pumped from wells to irrigate orchards. Orchards without available well water have not been irrigated adequately and typically have died.
Pecan requires adequate water for development of the nut. Pecan nut development can be divided into two stages: nut sizing and kernel filling (Wells et al., 2007). In nut sizing, the nut enlarges and the volume of endosperm increases (Herrera, 1990). Water deficit during nut sizing diminishes the ability of pecan roots to extract soil water and this reduces growth and nut size and increases nut split (Wells et al., 2007). Low soil moisture during this stage also reduces embryo development, resulting in poor kernel fill (Herrera, 1990; Wells et al., 2007).
Drought reduced pecan yield 5% to 24% when the applied water was reduced from 5% to 52% relative to control (Garrot et al., 1993). Root growth and number of growing roots of black walnut (Juglans nigra) approached zero as soil water potential ranged from –0.5 to –1.0 MPa (Kuhns et al., 1985). Yield of almond (Prunus dulcis) trees exposed to water deficits decreased 7.7 kg/tree for each 1-MPa decline in midday stem water potential below –1.2 MPa during the previous growing season (Esparza et al., 2001). Midday stem water potential of –0.7 to –1.4 MPa reduced fruit relative yield percentage of apple (Malus ×domestica) trees (bearing medium cropload) by less than 15%, whereas Ψsmd of –1.5 to –3.0 MPa reduced the fruit relative yield by 20% to 100% (Naor et al., 2008).
Stomatal closure is among the first processes to occur in the leaf in response to water deficit (Cifre et al., 2005). As plants transition from no or low water deficit to moderate plant water deficit, gS generally declines into the 0.05 to 0.15 mol·m−2·s−1 H2O range, photosynthesis and intercellular CO2 decrease slightly, and stomatal limitations to gas exchange dominate (Cifre et al., 2005). At severe water deficits (gS less than 0.05 mol·m−2·s−1 H2O), Pn further decreases and ci increases indicating that non-stomatal limitations become significant (Cifre et al., 2005). However, the water stress study of Cifre et al. (2005) was conducted on grapevine (Vitis vinifera). Information on Pn and gas exchange in response to water stress of mature pecan trees is practically non-existent. When red oak (Quercus rubra) leaf water potential was between –1.5 and –2.0 MPa, CO2 assimilation reduction relative to control trees was 60% (Weber and Gates, 1990). Subjecting rubber trees (Hevea brasiliensis) seedlings to water deficit decreased gas exchange and hydraulic conductivity parameters. However, these variables had recovered to control levels after 5 d of rewatering (Chen et al., 2010).
Irrigation scheduling is the process used to determine when to irrigate the orchard, how much water to apply, and improve orchard irrigation efficiency in agricultural areas (Food and Agriculture Organization of the United Nations, 1996). Timing of irrigation usually requires continuous monitoring of the variables that best represent moisture status of trees and applying irrigation only when such a variable drops below a certain threshold value (Cifre et al., 2005). However, most New Mexico pecan growers use the calendar and soil moisture “by feel” to schedule irrigation (Kallestad et al., 2008). Less than 3% of New Mexico growers refer to plant and soil moisture-sensing devices to schedule irrigation (Kallestad et al., 2008; USDA, 2008). This could be attributed to the inconsistent results of these instruments during the growing season. Midday stem water potential has been proposed as a precise plant water status indicator for scheduling irrigation of pecan orchards (Othman et al., 2014). Klein et al. (2001) showed that the threshold Ψsmd value for leaf persistence in mature almond trees was –1.8 MPa. At this level of Ψsmd, the decline in Pn and gS was 30% and 20%, respectively. However, no report we are aware of has established water deficit thresholds for irrigating field-grown pecan while linking the irrigation threshold to precise pecan growth and development functions.
Under field conditions, water deficit develops gradually over an irrigation cycle rather than suddenly like in most studies designed to assess the impact of drought on trees (Damour et al., 2009). In fact, Poorter et al. (2012) cautioned that water deficit studies that are conducted on plants grown in small containers or pots may lead to sudden drought and plant nutrient deficiency, which are unlikely to occur under field conditions. In a previous study (Othman et al., 2014), leaf-level physiological measurements were made during the cyclic irrigation to determine which of these leaf-level parameters best represented changes in moisture status. We concluded that of the leaf-level physiological response variables tested, Ψsmd was the best for detecting moisture status in pecan. Therefore, Ψsmd was used as a water deficit reference against which to test the responses of Pn and gas exchange variables. The objectives of this field study were to 1) assess the impact of water status developed during the flood irrigation dry-down cycles on Pn and gas exchange; and to 2) establish values of Ψsmd that are needed to maintain Pn and gas exchange of pecan. These data could determine water deficit thresholds of Ψsmd needed for irrigation scheduling in pecan orchards.
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