334 Martinson et al.: Water Stress and Geographic Origin Influence Borer Infestations in Ash watering five days per week. Trees assigned to the half watering treatment were hand watered three days per week and fitted with plastic rain excluders. Trees assigned to the low watering treatment were fitted with plastic rain excluders and watered only to maintain survival based on observations of tree condition (i.e., wilting, marginal browning, leaf dis- coloration, leaf drop). Rain excluders were made of 3.5 mm thick, clear polyethylene sheeting (Husky brand, Poly-America, Grand Prairie, Texas, U.S.), were fastened with duct tape to the bole of the tree just above the soil surface, and were draped over the sides of the pot (Figure 1). Trees were watered to the point of saturation, as evidenced by runoff from drainage holes, and then watered to runoff again. Tree water stress was verified with a Handy PEA® chlorophyll fluorometer (Hansatech Instruments, Norfolk, UK). Chlorophyll fluorescence, measured in the field at regular intervals with a fluorometer, is an indicator of photoinhibition due to water stress (Maxwell and Johnson 2000). It is calculated as Fv Fv mal fluorescence (Maxwell and Johnson 2000). This ratio of variable to maximal fluorescence indicates the maximal or potential photochemical efficiency of a plant and is a widely used indicator of plant health (Maxwell and Johnson 2000; Percival 2004; Percival et al. 2006). To examine drought toler- ance in ash trees, Percival et al. (2006) determined that Fv , the variable fluorescence, is calculated as Fm where Fm is maximal fluorescence and Fo stressed trees falls within the range of 0.76–0.85, whereas Fv /Fm ash trees was <0.76. These values were used to assess the extent of drought stress achieved in the experiment. Chlorophyll fluores- /Fm Figure 1. Experimental green and Manchurian ash trees at the Maryland Department of Agri- cultural (MDA) common garden site. To count borers, prevent their escape, and prevent colo- nization from additional borers during the second year of study, the boles, trunks, and major scaffold branches were enclosed in screening and pipe insulation. Drought stress treatments were imposed with plastic rain excluders. ©2014 International Society of Arboriculture 74.8, P < 0.0001) as well as from the entire two-year experiment (r = 0.99, t93 = 64.0, P < 0.0001). Tree cence was measured at the beginning of the experi- ment prior to the imposi- tion of water treatments and periodically through- out the experiment. Because of tree death (see Results), the number of times fluorescence was measured differed among trees. Chlorophyll fluores- cence assessed early in the experiment (July 15, 2009– August 26, 2009) corre- sponded with the flight period of EAB at this site. These fluorescence values were highly correlated with those taken through the whole first year (Pearson’s correlation, r = 0.99, t93 = in stressed in healthy, non- is mini- /Fm –Fo . ,
November 2014
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