The production of high-quality container-grown nursery plants requires adequate nutrients and water during production. Negative growth responses to excess N can occur from increased salinity, disruption of the balance between N and other nutrients, or increased water stress (Cabrera, 2004). Fertilizer application rates commonly used for container-grown crops are based on optimal water availability, a condition that rarely exists in commercial production systems. Fertilizer management effects on plant water use and drought stress during production for many crops has not been evaluated fully (Linder et al., 1987; Tan and Hogan, 1997). With some woody perennial plants it is possible that growth can be enhanced more by minimizing water stress than by increasing fertility (Rose et al., 1999).
The soilless substrate used during container production of nursery plants has a low nutrient and water-holding capacity and large irrigation quantities can result in nutrient losses from the container. Maintaining plant-available water near 100% field capacity in the growing substrate is believed to maximize plant growth (Beeson, 1992); however, this objective is impracticable in commercial practice. Reduced irrigation volume in combination with increased irrigation frequency can be used to decrease runoff of nutrients and water in nurseries (Fare et al., 1994). Increasing irrigation frequency can increase growth of container-grown plants or have no influence on growth (Beeson, 1992; Fare et al., 1994; Keever and Cobb, 1985). The growth responses of container-grown plants using deficit irrigation regimes have been investigated and acceptable growth was measured when the growing substrate was allowed to dry to 40% to 75% of container capacity before irrigation (Beeson, 2006; Welsh and Zajicek, 1993). Cumulative actual evapotranspiration was similar between control plants grown at 100% container capacity and plants grown at 40% container capacity (Beeson, 2006), suggesting this range of water stress has little influence on photosynthesis and carbon gain.
Nutrient uptake is a function of nutrient availability in the rhizosphere solution. Transpiration can cause large differences between the water content in the rhizosphere and that in the bulk growing substrate. Changes of a few percent in water content of a growing substrate can induce 4- to 5-fold decreases in the hydraulic conductivity of the substrate and can potentially limit nutrient acquisition by plants (Raviv et al., 1999). Increasing irrigation frequency may increase availability of nutrients and nutrient uptake efficiency and decrease the amount of fertilizer required for optimal growth. Several researchers suggest increasing irrigation frequency could compensate for certain nutrient deficiencies, and lower yields of plants irrigated less frequently may be a result of nutrient shortage rather than water shortage (Buljovcic and Engels, 2001; Silber et al., 2003; Xu et al., 2004).
Recently, we described the influence of N availability on growth and uptake and storage of N and other nutrients by container-grown evergreen and deciduous cultivars of Rhododendron spp. (Bi et al., 2007a; Scagel et al., 2007, 2008) and observed an apparent increase in water use with increasing rate of N fertilizer application. Improved knowledge of the combined influence of irrigation and nutrient management during nursery production of container-grown plants is needed to develop integrated nursery production practices that optimize nutrient and water use. The objective of this study was to investigate the influence of water stress (as induced by irrigation frequency) on growth and N uptake of one deciduous and two evergreen cultivars of Rhododendron grown with different rates of N fertilizer in containers.
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