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Shoot-bending has become a standard cultural practice in cut-fl ower rose (Rosa hybrida L.) production. Physiological effects of shoot-bending on leaf net photosynthesis (A), stomatal conductance (gs), transpiration rate (E), and stem water potential (ψ) were investigated for rose plants. With saturating light conditions, shoot-bending decreased rates of A, gs, and E in comparison with the rates prior to shoot-bending. A, gs, and E of bent shoots were signifi cantly lower than those of the control shoots that were not bent. The differences in A between bent and control shoots decreased over time, disappearing within 3 weeks after bending. Bent shoots exhibited reduced ψ. Leaves projecting upward on a bent stem were found to have higher A, gs, and E than those projecting downward. This was probably due to the destruction of xylem vessels serving the leaves attached to the lower side (compression side) of the bent stem. Our results support the hypothesis that hydraulic conductivity is reduced in bent shoots probably due to disturbed xylem tissues, and that reduced photosynthetic rates of bent shoots are a function of water status. Received for publication 10 Nov. 2003. Accepted for publication 15 Mar. 2004. We thank Alexandra Rosati and Jonathan Guan from the NSF Young Scholars Program for carrying out stem water potential measurements and preliminary photosynthesis measurements, respectively. We thank Drs. Loren Oki and Mark Roh for helpful comments on the manuscript. We also thank Dr. Mike Parrella for letting us use the photosynthesis system. This research was supported in part by grants from the California Cut Flower Commission and the Kee Kitayama Research Foundation, and the Joseph H. Hill Memorial Foundation. 1Corresponding author and present address: Soo-Hyung Kim, USDA–ARS, Alternate Crops and Systems Laboratory, Bldg. 001 Rm. 342, BARC-W, Beltsville, MD 20705. Tel: (301) 504-5343; Fax: (301) 504-5823; e-mail: sookim@asrr.arsusda.gov In commercial rose production for cut-fl owers, shoot-bending has become a standard growing technique in conjunction with container-based soil-less media culture. Rose growers have been replacing or augmenting pruning with bending where nonproductive shoots are bent down into the canopy or toward the aisle. It is thought that shoot-bending results in increased foliage area to intercept light for photosynthesis by forming an extended horizontal canopy of bent shoots. Therefore, in terms of utilization of resources, plants would benefi t from the increased capture of light falling between rows. Kool and Lenssen (1997) reported that increased light interception was achieved by shoot-bending during the period of basal-shoot formation. As a consequence, plant growth rate was increased. Kim and Lieth (2004) reported that applying shoot-bending to nonproductive shoots resulted in increased stem length of harvestable fl owering shoots at the expense of the number of fl owering shoots per unit ground area. It has also been speculated that shoot-bending may break apical dominance by changing the hormonal balance within the plant and promoting the formation of axillary shoots (Cline, 1991; Hosokawa et al., 1990). The fl ower bud is normally removed when bending a shoot in the commercial greenhouses. Studies suggest that removal of any plant parts or changing the plant structure in any manner may change the balance of source and sink relations as well as the water relations in the plant (Heichel and Turner, 1983; Larson, 1998; Pinkard and Beadle, 1998). This in turn could give rise to changes in the rates of photosynthesis, transpiration, and stomatal conductance. Rose plants in the commercial greenhouses are disturbed throughout the year by practices such as harvesting, pruning, pinching, and shoot-bending. The effects of some of these practices on growth and production of cut-fl ower roses have been reported (Faber and White, 1977; Zieslin and Mor, 1981). The objectives of the study were to assess how shoot-bending infl uences gas-exchange characteristics of rose plants and to determine the associated physiological mechanisms. We measured the rates of net photosynthesis (A), transpiration (E), and stomatal conductance to water vapor (gs) of rose leaves before and after shoot-bending. We also measured the stem water potential (ψ) of bent and control (erect) shoots. As the fl ower bud is usually removed when a shoot is bent, we investigated the effect of fl ower removal on gas-exchange rates in combination with shoot-bending. We tested the hypothesis that shoot-bending decreases the rates of photosynthesis, transpiration and stomatal conductance as a consequence of impaired water transport due to bending. Materials and Methods PLANT CULTURE FOR GAS-EXCHANGE EXPERIMENTS. Three-yearold ʻKardinal ̓rose plants grafted onto ʻNatal Briar ̓rootstock in May 1997 were grown in the greenhouse in the Dept. of Environmental Horticulture at the Univ. of California, Davis. The plants were grown in UC Mix [1 sand : 1 redwood sawdust : 1 peatmoss (v/v)] in 8-L containers. The bending treatment was applied to individual shoots, which were the experimental units. Individual plants were used as blocks. An automated, tensiometer-based irrigation system with set points of 1.0 and 3.0 kPa irrigated the plants with modifi ed half-strength Hoaglandʼs Solution No. 2 plus NovJournal.indb 896 10/26/04 11:34:46 AM 897 J. AMER. SOC. HORT. SCI. 129(6):896–901. 2004. micronutrients (Oki et al., 2001). Plants were grown under natural light. The air temperature set points inside the greenhouse were 24 °C in the day to 20 °C at night. Average daily air temperature was maintained between 18 and 24 °C. GAS-EXCHANGE EXPT. 1. The effect of shoot-bending on leaf gas-exchange was determined using seven ̒ Kardinal ̓rose plants from Nov. 2000 to Jan. 2001. Two young fl owering shoots at a similar stage of development were selected on each plant. One shoot was carefully bent down at the internode above the second leaf with fi ve leafl ets from the base (Fig. 1A). The second shoot (control) remained unbent and erect. A young fully expanded fi ve-leafl et leaf (≈20 d after unfolding) in the middle (position 2; Fig. 1A) of each shoot was selected for the gas-exchange measurements. Leaf age was determined by recording the unfolding date of the selected leaves to ensure that leaves of similar ages were used for the measurements. Gas-exchange measurements were carried out on bent and erect shoots before applying shootbending (day 0), then 1, 3, 5, 7, 14, 21, and 35 d after bending repeatedly using the same leaves. Photosynthetic light responses were generated using a photosynthesis system (LI6400; LI-COR, Lincoln, Neb.) with a red/blue LED light source (LI6400-02B) at photosynthetically active radiation (PAR) levels of 2000, 1500, 1000, 500, 200, 100, 50, and 0 μmol·m–2·s–1, and ambient CO2 concentration (360 μmol·mol–1). Leaf temperature and relative humidity inside the leaf chamber were kept at 25 °C and near 50%, respectively. The rates of A, gs, and E at saturating light conditions (PAR = 1500 μmol·m–2·s–1) were used for analyzing the effect of bending. Paired t tests were used for comparisons of the gas-exchange rates between erect and bent shoots for each measured date, and between before (day 0) and 7 d after bending (day 7) of the same shoots. To assess how shoot-bending affects photosynthetic light response properties, the following model (Pasian and Lieth, 1989) for leaf net photosynthesis (A) as a function of PAR was used: The parameters are quantum effi ciency (α), photosynthetic capacity (Amax), and curvature factor (θ). PAR is represented by I and Rd is leaf dark respiration rate. Data from individual leaves measured 0 and 7 d after bending was fi tted to the model to test if shoot-bending alters the photosynthetic parameters of the model. Differences between parameter estimates of individual leaves on erect and bent shoots were tested using analysis of variance (ANOVA). GAS-EXCHANGE EXPT. 2. Eight ̒ Kardinal ̓rose plants with several developing shoots at similar stages were randomly selected. A 2 × 2 factorial experiment in a randomized complete-block design with each plant serving as a block (shoot-bending vs. control; fl ower bud removal vs. intact) was conducted. Four young fl owering shoots at a similar stage of development were selected on each plant. Two shoots were bent down at the internode above the second leaf (fi ve-leafl et leaf) from the base (Fig. 1A). The other two shoots remained unbent and erect. Flower buds were removed using shears from one bent shoot and one control shoot. Five leafl et leaves at three different positions were selected from each stem (Fig. 1A): close to the fl ower bud (position 1), middle (position 2), and proximal to the base (position 3). The orientation of the leaves immediately after shoot-bending was recorded (i.e., the direction that the terminal leafl et points: upward, horizontal, or downward). Gas-exchange rates (A, gs, and E) of the leaves were measured using the LI6400 with the terminal leafl et before bending (day 0), then 2 and 7 d after bending. The gas-exchange measurements were made under light saturated conditions (PAR = 1500 μmol·m–2·s–1) using the LED light source at a leaf temperature of 25 °C and ambient CO2 concentration (360 μmol·mol–1). Data collected 2 d after bending were used to examine the effects of shoot-bending and fl ower removal on the gas-exchange properties using analysis of covariance (ANCOVA). Values collected before the treatment (day 0) were used as the covariable for adjustment. Three-way ANCOVA accounting for the effects of shoot-bending, fl ower removal, and leaf position revealed that all interactions involving leaf position were not signifi cant at P = 0.05. Based on this result, pooled values over the three leaf positions for each stem were used for testing the effects of shoot-bending and fl ower removal in order to get the values more

Bending Alters Water Balance and Reduces Photosynthesis of Rose Shoots