Water Efficiency of Cassava. II. Differing Sensitivity of Stomata to Air Humidity in Cassava and Other Warm‐Climate Species1

Abstract

Measurements of CO2 and H20 exchange and the calculated leaf conductance of attached leaves in well‐watered plants were conducted over a range of leaf‐air vapor pressure differences (VPD) (1.0 to 4.0 kPa) to compare the response of cassava that of other warm‐climate species. Species tested were cassava (Manihot esculenta Crantz), andropogon (Andropogon gayanus Kunth), beans (Phaseolus vulgaris L.), siratro [Macroptilium atropurpureum (DC) urb], rice (Oryza sativa L.), eucalyptus (Eucalyptus deglupta Blume), amaranth weed (Amaranthus retroflexus L.) and grain sorghum [Sorghum bicolor (L) Moench]. Plants were grown in pots outdoors at the CIAT headquarters, Palmira, Colombia, South America. All except andropogon showed a decrease in leaf conductance with increase in VPD. The degree of stomatal sensitivity decreased as follows: cassava > siratro, amaranthus, eucalyptus, bean > sorghum, rice > andropogon. The greater sensitivity in cassava was associated with reduction in transpiration and stable leaf water potential (ψl) at large VPD. In other less sensitive species, transpiration increased and bulk leaf water potential decreased at large VPD. The response of cassava to changes in VPD resulted in higher water use efficiency (WUE = μmol CO2 uptake per mmol H2O loss) compared with other C3 species. This may contribute to the comparative advantage of cassava when grown under conditions of limited availability of water. The WUE of the C4 species (sorghum, andropogon, amaranthus) were higher than those of the C3 species. This greater WUE of C4 species was attributed mainly to the higher photosynthetic rates of the C4 species rather than to a lower transpiration rate.