Fertigation Frequency Affects Growth and Water and Nitrogen Use Efficiencies of Swingle Citrumelo Citrus Rootstock Seedlings

We determined if frequency of application of irrigation water plus fertilizer in solution (fertigation) could modify root and shoot growth along with growth per unit nitrogen (N) and water uptake of seedlings of the citrus rootstock Swingle citrumelo growing in a greenhouse. In the first experiment, all plants received the same amount of water with sufficient fertilizer N but in three irrigation frequencies applied in 10 1.5-mL pulses per day, one 15-mL application per day, or 45 mL applied every 3 days. Plants irrigated at the highest frequency grew the least total dry weight and had the highest specific root length. Plants with lowest irrigation frequency grew the most and used the least water so had the highest water use efficiency. There were no irrigation frequency effects on relative growth allocation between shoot and roots, net gas exchange of leaves, or on leaf N. A second experiment used identical biweekly irrigation volumes and fertilizer rates, but water and fertilizer were applied using four frequency combinations: 1) daily fertigation; 2) daily irrigation with fertilizer solution applied every 15 days; 3) fertigation every 3 days; or 4) irrigation every 3 days and fertilizer solution applied every 14 days. Total plant growth was unaffected by treatments, but the highest frequency using the lowest fertilizer concentration grew the greatest root dry weight in the uppermost soil depths. Roots grew less and leaf N was highest when N was applied every 15 days, implying that root N uptake efficiency was increased when fertigated with the highest fertilizer concentration. All plants had similar water use efficiencies. A third experiment was conducted with irrigation every 3 days and with four different N application frequencies: every 3, 6, 12, or 24 days using four fertilizer concentrations but resulting in similar total N amounts every 24 days. There were no differences in growth, gas exchange, or water use efficiency. Given the fact that all treatments received adequate and equal amounts of water and fertilizer, fertigation frequency had only small effects on plant growth, although very high frequency fertigation decreased N uptake efficiency. High citrus production and good fruit quality usually depend on irrigation and fertilization, especially in semiarid areas, but also in subtropical humid areas like Florida, where rainfall is seasonal and sandy soils have low waterand nutrient-holding capacities (Koo, 1980). Water-saving irrigation techniques and fertilizer strategies can be used to improve efficiency of water uptake in grapevines (Vitis vinifera L.) and peach trees [Prunus persica (L.) Batsch] (Chalmers et al., 1981; Dry et al., 1996) and also fertilizer use efficiency (Quiñones et al., 2003; Russelle et al., 1981). Because tree growth and yield can be limited by low nitrogen (N) supply (Syvertsen and Sax, 1999), historical emphasis has been on applying sufficient N to maximize yield (Dasberg, 1987; Tucker et al., 1995). Nitrogen fertilization exceeding the recommendation rates {maximum rates of 225 kg N/ha for young orange trees [Citrus sinensis (L.) Osbeck] and 280 kg N/ha for mature trees; Obreza et al., 2008} can result in low N use efficiency, reduced fruit quality, and groundwater contamination (Legaz and Primo-Millo, 1988). Nitrate concentrations above the drinking water quality standards (maximum contamination limit of 10 ppm NO3-N; U.S. Department of Health, Education, and Welfare, 1962) have been found in groundwater of citrus-producing regions like Florida and Spain (Lamb et al., 1999; Riotte, 1994; WHO, 2004) as a result of the excessive or inefficient use of inorganic fertilizers in citrus production (Bacchus and Barile, 2005). In Florida, coarse-textured soils with low organic matter, shallow root systems of citrus, and intense rainfall increase the probability that fertilizer N will leach beyond the root zone. However, citrus can be grown in the sandy soils with only minimum contamination of groundwater if best management practices are followed using reduced N application rates (Paramasivam et al., 2000; Schumann et al., 2003). Schumann et al. (2003) estimated an optimal N rate of 145 kg N/ha for young orange trees [Citrus sinensis (L.) Osbeck] and 260 kg N/ha for mature trees, although optimal rates depend on factors such as soil type, land history, and yield potential. Low N fertilizer rates and increased frequency of applications of fertilizer in solution (fertigation) may enhance nutrient use efficiency and tree productivity while minimizing NO3 leaching losses (Alva and Paramasivam, 1998; Syvertsen and Smith, 1996). Good N management has been reported to decrease leaf Cl concentration in lemon trees [Citrus · limon (L.) Burm. f.] under salt strees (Gimeno et al., 2009). Fertigation management might also be beneficial to limit excessive root and vegetative flush growth (Yuan et al., 2005) because fertilizer rates can change root density of grapefruit trees (Citrus paradisi Macfad.) (Zhang et al., 1998). Intensively managed irrigation systems may reduce summer growth flush in citrus (Schumann et al., 2009), which could reduce the growth rate of pests like Asian citrus psyllid that feed on new leaves. Psyllids are the vector of Huanglongbing, one of the most devastating diseases in citrus all over the world (Bové, 1986; da Grac xa, 1991). Frequent applications of low N concentrations can minimize residual soil N that is susceptible to leaching (Willis and Davies, 1990). Intensively managed fertigation systems, in which trees are fertigated as frequently as daily or hourly, have been proposed as a tool to increase water and N uptake efficiency (Schumann et al., 2009). Our objectives were to precisely measure daily evapotranspiration (ET) and to make frequent applications of water and fertilizer to determine if different fertigation frequencies with identical rates of N and water application could alter both daily ET and root and shoot growth of potted Swingle citrumelo [Citrus paradisi Macfad. · Poncirus trifoliata (L.) Raf.] rootstock seedlings under controlled conditions in a greenhouse. We tested the hypothesis that fertigation frequency determines growth per unit water and N uptake and, thus, water and N use efficiency. Materials and Methods Plant material and growth conditions. Three separate experiments were conducted in a greenhouse at the University of Florida/ IFAS, Citrus Research and Education Center, Lake Alfred, FL (long. 28.09 N, lat. 81.73 W; elevation 51 m). For all three experiments, uniform 5to 6-month-old Swingle citrumelo seedlings were purchased from a commercial nursery and transplanted into 30-cm tall, 2.4-L pots filled with previously autoclaved Candler fine sand soil, a hyperthermic, coated Typic Quartzipsamment containing 98% sand and less than 1% organic matter (Li et al., 2006). The bottom 15 cm of each pot was filled with air-dried sand, whereas the top 15 cm, which included the entire initial root zone of 10 cm, was filled with thoroughly wet sand near field capacity. Thus, roots were not subjected to dry soil Received for publication 18 Mar. 2010. Accepted for publication 17 June 2010. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 45(8) AUGUST 201