338 included a tree from each treatment × species combination. Measurement cycles were repeated throughout the day until gs on all trees were mea- sured. Stomatal conductance data were measured on five mature, full-sun leaves from each tree. Each year (prior to budbreak and in Novem- ber), trunk diameter 15 cm above soil level was measured on each tree using a digital caliper (Mitutoyo Corp., model 500-196, Japan). For each tree, trunk cross-sectional area increase was determined as the difference between spring and autumn measurements. Also in the spring of each year, 10 arbitrarily selected shoots on each tree were selected, and shoot elongation (based on growth from the bud scale scar to the termi- nal bud) was measured on selected shoots in late autumn. At the termination of each growing sea- son, each tree was defoliated and total leaf area for each tree was measured using a portable leaf area meter (model LI-3000 with LI-3050A conveyor attachment; LI-COR, Lincoln, Nebraska, U.S.). Because statistical trends in yearly data were similar (irrigation regime effects did not differ between years), water relations and gas exchange data for each variety, irrigation treatment, and year were pooled (pre-dawn ψl and gs for each species cies × irrigation treatment interactions. Therefore, only main effects (irrigation regime or species) data are presented. However, an irrigation by species interaction occurred for shoot elongation. There- fore, shoot elongation irrigation × species means are presented. In addition, gs versus VPD data (VPD ©2015 International Society of Arboriculture × irrigation regime were taken as the mean of 66 and 165 measurements, respectively). In addition, trends in yearly growth data were similar. Therefore, growth data from each growing season were also pooled (shoot elongation, trunk cross sectional area increase, and total tree leaf area for each species × irri- gation regime were taken as the mean of 90, 9, and 9 measurements, respectively). Data were exposed to ANOVA appropriate for a randomized block design (three randomized irrigation regime blocks with three trees of each species planted randomly within each irrigation block). If differences were identified, means were separated by Fisher’s least significance difference procedure (LSD, P ≤ 0.05) (SAS Institute Inc., Version 9.2 for Windows® operating system). For trunk cross-sectional area increase, total tree leaf area, pre-dawn ψl , and gs, there were no spe- Montague and Bates: Response of Maple to Reduced Irrigation at time of day when gs data were measured) were analyzed by regression analysis. Based upon sig- nificance of the equation and coefficient of deter- mination (R2 each species × irrigation treatment (SAS Institute Inc., Version 9.2 for Windows operating system). RESULTS AND DISCUSSION Total volume of water applied to each tree in the low-irrigation regime ranged from 483 L dur- ing the first growing season, to 2,072 L during the third growing season. Irrigation to each tree receiving intermediate irrigation ranged from 967 L during the first growing season to 4,145 L during the third growing season. During the first growing season, each high-irrigation regime tree was irri- gated with 1,466 L. While during the third grow- ing season, each high-irrigation tree received 6,443 L. Throughout the experiment period, all trees of each variety survived and appeared healthy. An- nual precipitation in Lubbock, Texas, U.S., aver- aged 48.2 cm from 1997 to 2007 (Anonymous 2008). During the experiment period, total yearly precipitation was greater than average during the second year of the study (68 cm), but lower than average during the first and third years of the study (21 cm and 33 cm, respectively). Weather variables measured at the experiment site varied throughout each growing season, but trends were similar between growing seasons. In addition, cli- matic conditions during each growing season were typical for summer days in the Texas High Plains. In the first growing season, maximum daily air temperature and maximum daily VPD were 39.5°C and 6.5 kPa, respectively (Figure 1). Minimum, maximum daily air temperature and minimum, maximum daily VPD for the first growing season were 15.9°C and 0.8 kPa, respectively. During the second growing season, maximum daily air tem- perature was recorded as 38.8°C, while maximum daily VPD was 6.2 kPa. Climatic data from grow- ing season number three indicate maximum daily temperature was 40.2°C, and maximum daily VPD was 6.8 kPa. Minimum, maximum daily air tem- perature, and minimum, maximum daily VPD for growing season number three were recorded as 8.2°C, and 0.14 kPa, respectively. Mean, maxi- mum daily temperature for each growing season was 31.5°C, 30.5°C, and 31.3°C for growing sea- ) value, linear curves were selected for
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