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[1] The purpose here is to reexamine the ecological importance of dew in arid and semiarid regions with a focus on the eastern Mediterranean area. This reevaluation is of particular importance under the controversial perspective that dew is insufficient as a source of water for plants but is sufficient to promote the spread of plant diseases. Adana, Turkey, was selected as an appropriate semiarid test ground with well-documented meteorological data and a newly developed photosynthesis and transpiration rate monitor (PTM), which was used to detect the response of transpiration and photosynthesis to the presence of dew on the leaves. A convolution theoretical model was used to simulate no-dew days; simultaneously, PTM measurements were used to obtain actual situations with dew. Contrary to expectations, we detected separate, early peaks of photosynthesis and late peaks of transpiration, leading to an average ratio of about 2:1 units of water use efficiency (WUE) for dew-affected versus no-dew conditions. The impressive performance of the dew-affected WUE was explained by a synergy between (1) low transpiration during dew-affected morning hours and (2) high CO 2 gradient toward the canopy. The first resulted from dew formation that created a humid environment in the near vicinity of the leaf followed by a low leaf to air vapor pressure deficit, which minimized transpiration. The second resulted from night respiration that induced a high CO 2 gradient from the air toward the canopy. This synergy resulted in intensive carbon intake at a low water cost and explained the ecological importance of dew.Citation: Ben-Asher, J., P. Alpert, and A. Ben-Zvi (2010), Dew is a major factor affecting vegetation water use efficiency rather than a source of water in the eastern Mediterranean area, Water Resour. Res., 46, W10532,

Four temperature treatments were studied in the climate controlled growth chambers of the Georgia Envirotron: 25/20, 30/25, 35/30, and 40/35 °C during 14/10 h light/dark cycle. For the first growth stage (V3-5), the highest net photosynthetic rate (P N ) of sweet corn was found for the lowest temperature of 28−34 μmol m −2 s −1 while the P N for the highest temperature treatment was 50−60 % lower. We detected a gradual decline of about 1 P N unit per 1 o C increase in temperature. Maximum transpiration rate (E) fluctuated between 0.36 and 0.54 mm h −1 (≈5.0−6.5 mm d −1 ) for the high temperature treatment and the minimum E fluctuated between 0.25 and 0.36 mm h −1 (≈3.5−5.0 mm d −1 ) for the low temperature treatment. Cumulative CO 2 fixation of the 40/35 o C treatment was 33.7 g m −2 d −1 and it increased by about 50 % as temperature declined. The corresponding water use efficiency (WUE) decreased from 14 to 5 g(CO 2 ) kg −1 (H 2 O) for the lowest and highest temperature treatments, respectively. Three main factors affected WUE, P N , and E of Zea: the high temperature which reduced P N , vapor pressure deficit (VPD) that was directly related to E but did not affect P N , and quasi stem conductance (QC) that was directly related to P N but did not affect E. As a result, WUE of the 25/20 o C temperature treatment was almost three times larger than that of 40/35 o C temperature treatment.Additional key words: maize; quasi stem conductance; transpiration rate; vapor pressure deficit; water use efficiency.

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