Growth and Water Relations of Corn (Zea mays L) as Influenced by Relative Humidity.

The impact of relative humidity (RH) has often been neglected in estimating the effect of climatic factors on growth, development and yield of crops. The goal of this study was, therefore, to study the effect of RH on photosynthesis, transpiration and water use efficiency (WUE) and how RH interacts with water relations of sweet corn (Zea mays L. var. rugosa ‘Seneca’). The experiment was conducted in three climate controlled chambers of the “Georgia Envirotron” of The University of Georgia. Gas exchange and environmental parameters were measured continuously with PTM-48M (Photosynthesis and Transpiration Monitor). The temperature in each chamber was 30°/25°C during 14-daylight /10-dark hours. RH treatments were 30, 60 and 85 percent . Daily cumulative CO₂ was 56, 48 and 38 gr.m^-2 (LSD_0.05 =30) for the respective high, medium and low humidities. These results were below the maximum theoretical assimilation  (85 gr m^-2) but close to the  theoretical low value. Daily transpiration was 3.4, 3.9 and 6.4 mm (LSD_0.05 =0.4mm) in high medium and low humidity respectively and the differences were significant at a 0.05 level. Transpiration was positively correlated (r²=0.72) to vapor pressure deficit (VPD). Large VPD and dry air were associated with low canopy temperature. An increase of one kPa in VPD resulted in a decrease of 1^oC difference between canopy and air temperature. Transpiration during ten dark hours was 0.86, 0.51 and 0.46 mm for RH of 30, 60 and 85% respectively. That is, in all treatments more than 13% of the total water loss occurred at night. It was found that increased VPD caused dramatic increase of water consumption that resulted in a strong decline in WUE.  
Key Words: Photosynthesis, Transpiration, water use efficiency (WUE), VPD