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Water Use Efficiency of Cassava. III. Influence of Air Humidity and Water Stress on Gas Exchange of Field Grown Cassava¹

Field measurements of gas exchange were made to study the response of field grown cassava (Manihot esculenta Crantz) to changes in air humidity under well-watered and unirrigated conditions. ‘M Col 1684’ was used as testing material at Palmira and Santander de Quilichao, CIAT, Colombia, South America. The infrared gas exchange technique was modified to allow for field sampling of leaf CO₂ exchange rates using the syringe injection method at locations far from the laboratory. There was a remarkable decrease in the leaf photosynthetic rate with decreasing air humidity in both well-watered and unirrigated plants. Leaf photosynthetic rate was significantly correlated with VPD and leaf conductance but was not correlated with leaf water potential at midday and early afternoon. The midday leaf water potential of the plants without irrigation was slightly higher than that of the well-watered plants. Photosynthetic rate of the plants without irrigation was always lower than the well-watered ones. Transpiration initially increased with increased leaf to air vapor pressure difference (VPD) and then sharply declined with further increases in VPD. Leaf water use efficiency (WUE) decreased in curvilinear pattern with VPD. The results agree with previous reports on laboratory controlled experiments. Dry root yield and total biomass production of cassava was increased by increasing the relative humidity in the crop canopy by artificial misting. There was 91,15, and 27% increase in dry root, top and total dry matter, respectively, after 40 days of misting as compared with the control. These differences in biomass production were neither associated with changes in soil water nor with changes in leaf area indices but rather with enhanced photosynthetic rate per unit leaf area of the misted plants. The leaf photosynthetic rate of cassava decreased with leaf age with a maximum rate of 26 /ümol CO₂ m^-2 s^-1 for young leaves as compared with 13 ümol CO₂ m^-2s^-1 for old leaves lower in the canopy. Photosynthetic saturation irradiance was around 1500 and 800 ümol m^-2s^-1 for young and old leaves, respectively. The modified syringe injection method proved reliable and suitable for handling a large number of field samples in a short time. Photosynthetic rates of several C₃ and C₄ crop species were similar to reported values obtained with the conventional infrared gas exchange technique.

Key Words: Photosynthesis • Transpiration • Stomata • Syringe Injection technique • C₃ and C₄ species • Corn • Sugarcane • Amaranth • Field bean • Dry matter production • Misting

² Physiologist, former graduate student, (now: programa de mandioca, CNPMF/EMBRAPA, Cruz das Almas, Bahia, Brazil) and visiting scientist, respectively.

Received for publication April 6, 1984.

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