Arturo Alvino, Mauro Centritto, Sebastiano Delfine, Francesco Loreto, D. Tricoli, Loreto, F, Tricoli, D, Centritto, M, Alvino, A, and Delfine, S
Short-term fumigation with 1% methanol in air was carried out to investigate effects on the photosynthetic apparatus of horticultural species characterized by leaves with different stomatal distribution. Methanol decreased the photosynthetic capacity of all species. The hypostomatous cherry (Prunus avium L.) was the most sensitive species. Between the two amphistomatous species, the effect was smaller in pepper (Capsicum annuum L. var. annuum) than in melon (Cucumis melo L.). A 4-minute fumigation caused a stronger inhibition of photosynthesis than a 90-second fumigation. The time course of the inhibition of the photosynthetic electron transport following a methanol fumigation of cherry leaves suggests that methanol starts inhibiting photosynthesis and photorespiration after ≈60 seconds and that the effect is complete after 180 seconds. This inhibition is not permanent, however, since gas-exchange properties recovered within 24 hours. Methanol vapor effects were greatest when leaves were fumigated on the surfaces with stomata. However, fumigation with methanol does not affect stomatal conductance. Therefore, inhibition of photosynthesis following methanol fumigation can be attributed to a temporary inhibition of biochemical reactions. reports show that methanol does not affect the CO2 compensation point (Poskuta et al., 1995, van Iersel et al., 1995), an indication that photorespiration is not preferentially suppressed by methanol. Simultaneous measurements of gas exchange and chlorophyll fluorescence, allow one to partition the electron transport rate between photosynthesis and photorespiration (Di Marco et al., 1993). We used this approach to accomplish our second goal, that is, to understand if methanol vapor rapidly affected biochemistry of photosynthesis and selectively inhibited photorespiration. It is possible that the different results obtained with methanol sprays reflect differences in the exposure and acquisition of methanol by the leaf. Under field conditions these cannot be controlled accurately. For instance, acquisition may be inefficient because of temperature-dependent volatilization of the methanol solution. The third goal of this study was to eliminate these uncertainties while investigating methanol effects on stomata and biochemistry. We used gaseous methanol in a totally controlled environment. This eliminated the variability of environmental factors and allowed us to study a direct effect of methanol vapor rather than an effect that may depend on the environment-controlled evolution of methanol vapor from sprays. Materials and Methods Seedlings of pepper (Capsicum annuum ‘Quadrato d’Asti’) and melon (Cucumis melo ‘Supermarket’) were grown for ≈2 months in 3-dm pots. Two-year-old cherry (Prunus avium ‘Gisella’) trees were grown in 15-dm pots. Soil was a mixture of 1 sand : 1 peat : 1 lime. Pots were regularly watered to soil saturation and weekly fertilized according to the Ingestad and Agren (1992) method. Plants were grown under conditions typical of summer in the experimental field of IBEV (Rome, Italy 41° 50' latitude). Daily light intensity and air temperature varied between 700 to 2000 μmol·m·s and 25 to 37 °C, respectively, depending of sky cloudiness. The experiments were conducted on the last fully expanded leaf. The leaf was enclosed in a gas exchange cuvette with 4.9-cm Foliar sprays of methanol solutions were reported to increase the yield of crops (Nonomura and Benson, 1992). Other experiments confirmed (Valenzuela et al., 1994) or disproved (Albrecht et al., 1995; Faver and Gerik, 1996; McGiffen et al., 1995; van Iersel et al., 1995) that methanol has a beneficial effect on crop yield. Yield improvement was especially noticed in C3 plants growing in arid, high sunlight environments but was not observed in C4 plants (Nonomura and Benson, 1992). Apparently, the leaf turgor of methanol-sprayed leaves was higher than in nonstressed controls. This led to speculation that methanol spraying increased stomatal conductance, in turn resulting in improved carbon acquisition (Nonomura and Benson, 1992). However, Nonomura and Benson (1992) did not report any direct measurements of CO2 and H2O exchange between leaves and air. More recently, methanol was found to improve photosynthesis and stomatal conductance but not growth of cotton (Faver and Gerik, 1996) and was ineffective on sour orange (Idso et al., 1995) and on fescue (Poskuta et al., 1995). A first goal of our work was to understand how stomata are affected by methanol. We investigated the effect of methanol on horticultural plants with different stomatal distribution as estimated by exchange of water vapor in a gas exchange cuvette. We used one hypostomatous species (cherry) and two species amphistomatous but with stomata prevalently distributed on the abaxial (pepper) and on the adaxial (melon) leaf surface, respectively. This approach also allowed us to investigate if methanol penetration occurred through stomata and/or cuticle and if the methanol effect on photosynthesis was mediated by changes of stomatal opening. Nonomura and Benson (1992) also speculated that an inhibition of photorespiration could improve carbon fixation of C3 plants exposed to methanol sprays. This would have explained why the effect was not observed in C4 plants. Gas exchange-based