HORTSCIENCE 48(7):848–854. 2013. was 25% greater using cyclic irrigation com- pared with the traditional method of applying a single cycle in the morning (Beeson and Carbon Isotope Discrimination, Haydu, 1995; Fain et al., 1999; Witmer, 2000). These studies, however, were conducted in the Gas Exchange, and Growth southeastern United States or in greenhouses and may not reflect conditions common in of Container-grown Conifers Under nurseries in cooler climates such as the upper midwestern United States. Improving our understanding of the phys- Cyclic Irrigation iological mechanisms behind the growth re- Amanda J. Taylor1 and R. Thomas Fernandez2 sponses to irrigation is important in developing Department of Horticulture, Michigan State University, East Lansing, effective water management strategies. Up to 97% of water used by plants is transpired MI 48824 through stomata, and an initial response to Pascal Nzokou2 limited water availability is stomatal closure (Halevy, 1972; Hand et al., 1982; Medrano Department of Forestry, Michigan State University, East Lansing, MI 48824 et al., 2002; Taiz and Zeiger, 2006), which Bert Cregg2,3 ultimately inhibits CO2 uptake and reduces photosynthetic assimilation (A). It has been Department of Horticulture and Department of Forestry, Michigan State postulated that cyclic irrigation increases University, 214 PSSB, East Lansing, MI 48824 growth by removing or reducing midday water limitations, thereby increasing cumulative pho- Additional index words. water use efficiency, nursery production, Picea glauca, Picea tosynthesis (Beeson, 1992; Witmer, 2000). For pungens var. densata, Abies fraseri, Pinus strobus, needle conductance example, applying daily irrigation in two split Abstract. The objective of this study was to quantify the effects of cyclic irrigation on applications consistently increased midday growth and physiology of container-grown conifer species in pot-in-pot (PIP) production stomatal conductance (gS) to water vapor for in the upper Midwest. Trees of four conifer species (Picea glauca var. densata, Picea plants from three Rhododendron cultivars pungens, Abies fraseri, and Pinus strobus) were grown in 25-L containers and assigned to (Scagel et al., 2011). Cyclic irrigation, by main- one of four combinations of irrigation rate (low or high) and daily irrigation cycle taining higher substrate moisture content in frequency (one or four). Irrigation rates were based on common nursery practice in the the midday hours than one single application, region (2 cm rainfall equivalent/day) and one-half the standard rate (1 cm rainfall delays or prevents stomatal closure, resulting equivalent/day). Cyclic irrigation increased relative height growth and relative caliper in higher cumulative A (Halevy, 1972; Warren growth of Pinus strobus by over 80% and 35%, respectively, compared with once-daily and Bilderback, 2002). irrigation. The high-rate irrigation increased relative caliper growth of Picea pungens by Water use efficiency (WUE) is the rela- 40% compared with the low rate. The effects of irrigation regime on needle- or shoot- tionship between assimilation and water loss level gas exchange varied by species and date of measurement. Carbon isotope and can be used to measure how efficiently discrimination (D13C) of needle and wood tissue was positively correlated (r ‡ 0.64, P < a plant is using water to produce biomass (Anyia and Herzog, 2004). Several studies 0.001) with needle conductance to water vapor (gwv) and negatively correlated (r £ L0.60, P< 0.001) with intrinsic water use efficiency (WUEi). Carbon isotope discrimination have demonstrated increases in WUE when of wood and needle tissue decreased with the low irrigation rate, indicating increased trees are exposed to water-limiting irrigation 13 regimes (Anyia and Herzog, 2004; Ningbo WUEi associated with reduced gwv. Cyclic irrigation had relatively little effect on D C except for Pinus strobus. Our findings suggest that carbon isotope composition of wood et al., 2009; Warren and Bilderback, 2002). and needle tissue provides a sensitive and accurate representation of plant response Two methods are commonly used to estimate to varying moisture availability. From a water management perspective, identifying WUE for trees and shrubs: instantaneous optimal irrigation rates appears to be more important than number of daily cycles for WUE from measurements of leaf gas ex- these crops grown in the midwestern United States. change and integrated WUE based on stable carbon isotope discrimination. Instantaneous WUE measures WUE at the Nursery producers are in need of water- overwater by applying a fixed rate of irriga- leaf level and is the ratio of the rate of A to conserving irrigation methods that will allow tion that may exceed plant water use (Tyler the rate of transpiration or the rate of leaf them to adapt to increased water costs and et al., 1996); this can result in excessive water conductance to water vapor conductance. The stricter water withdrawal and discharge reg- withdrawals and leaching of nutrients and ratio, A/gwv, is often referred to as WUEi,be- cause it removes the effect of vapor pressure ulations (Beeson et al., 2004). Growers often chemicals into surface and groundwater. The deficit inherent in WUE estimated from tran- development of irrigation programs that con- spiration rate (Roussel et al., 2009). Intrinsic serve water without reducing tree growth and water use efficiency typically increases as Received for publication 20 Dec. 2012. Accepted quality can enable the nursery industry to plants undergo water stress, because g often for publication 1 May 2013. adapt to increased input costs and stricter wv Funding for this study provided by Michigan State decreases more rapidly than A as water be- water use regulations. comes limiting. University Project GREEEN. In-kind contributions One potential method of conserving water provided by Nursery Supplies, Inc., Scotts Co., Estimating WUE from gas exchange can Renewed Earth, Inc., and Peterson’s Review is cyclic irrigation, in which a fraction of present logistical challenges in measurement Nursery. a plant’s daily water allowance is applied and interpretation because gas exchange rates We thank Dana Ellison, Brad Rowe, Erik Runkle, several times a day. Plant responses to cyclic vary diurnally and seasonally. Because of and three anonymous reviewers for their helpful irrigation include higher growth index, root this, stable carbon isotope (13C) analyses reviews of a previous version of this manuscript. growth, trunk diameter, shoot dry weight, 1 have become an important method of assess- Former Graduate Research Assistant. Current height, and crop yield than plants receiving address: North Carolina Cooperative Extension, ing plant responses to environmental stress, 120 Hospital Drive, Suite 1, Lenoir, NC 28645. one irrigation event per day (Beeson and especially water deficits (e.g., Farquhar et al., 13 2Associate Professor. Haydu, 1995; Fain et al., 1999; Ismail et al., 1989). Plants discriminate against C during 3To whom reprint requests should be addressed; 2007; Keever and Cobb, 1985; Ruter, 1998; photosynthesis, resulting in a lower 13C iso- e-mail [email protected]. Witmer, 2000). In some cases, caliper growth topic composition in plant tissue than in the 848 HORTSCIENCE VOL. 48(7) JULY 2013 CROP PRODUCTION atmosphere. Empirical and theoretical stud- var. densata], and eastern white pine (Pinus units (trees) for each species · irrigation ies have shown that the degree of discrimi- strobus L.) (Table 1). All plants were origi- rate · cycle frequency combination. The plot nation against 13C is determined largely by nally obtained from a local nursery (Petersons of study trees was surrounded by a guard row intercellular CO2 concentration, which is Riverview Farm, LLC, Allegan, MI) as seed- of similarly sized trees. controlled by the ratio of A to gwv (Farquhar ling transplants in 2006 and then grown for 2 Growth. Tree growth (caliper and height) et al., 1989). Stable carbon isotope discrim- years in 10-L containers in an 80% pine was measured at the beginning and end of ination may be used to estimate WUE during bark:20% peatmoss (v:v) substrate. each growing season. Initial tree height was the time in which plant tissues, usually leaves Irrigation treatments. Daily irrigation measured with a meter stick to the highest or wood, are formed. Ecological studies have volumes were based on daily water use live point of the tree. Trunk caliper was documented decreases in D13C of coniferous estimates of similar species from Warsaw assessed using a digital caliper. Two caliper species in response to limited water avail- et al. (2009). Four trees of each species were measurements were taken perpendicular to ability (Aranda et al., 2010; Brandes et al., randomly assigned to one of four irrigation each other, north–south-oriented and east– 2007; Olivas-Garcia et al., 2000; Zhang and regimes. Treatments consisted of a combina- west-oriented, at a height level even with the 13 Cregg, 2005). Because both D C and WUEi tion of daily irrigation rate (low or high) and rim of the container. Relative growth rates are functions of A and gwv, they are often daily irrigation cycle frequency (one or four). were calculated as height (or caliper) growth strongly correlated. However, D13C may pro- There were eight rows of trees in the study. during the study divided by the initial height vide some practical advantages in evaluating Each row was assigned at random to receive (or caliper). plant response to water availability because it once-daily irrigation (1·) or cyclic irrigation Gas exchange measurements—Picea spp. integrates physiological responses over time applied four times daily (4·). Once-daily and Abies fraseri. Photosynthetic gas ex- rather than presenting a series of ‘‘snapshot’’ irrigation events occurred at 0600 HR. Cyclic change was measured on three dates in the measurements. irrigation treatments were applied at 0600, 2009 growing season using a portable gas Based on previous studies, we hypothe- 1000, 1400, and 1800 HR daily to apply 25% exchange system (LI-6400; LI-COR Inc., sized that cyclic irrigation would reduce tree of the daily total at each watering.