Growth, Gas Exchange, And Harvest Index of Field-Grown Cassava in a Subtropical Short-Season environment

Cassava ( Manihot esculenta

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Crantz L.) is an important food and industrial crop in the tropics and holds potential in the subtropics. Cassava productivity is similar to C4 crop plants but its single-leaf net photosynthetic rate (Pn) is lower than C4 plants. This field investigation was conducted from 1982 to 1984 with seven cassava genotypes to study growth and gas exchange in a short-season environment. The Pn was determined on fully mature and attached leaves in an open system and canopy photosynthesis (CPn) was determined in a closed system between 1100 and 1500 hr using a portable plexiglass chamber. Leaf area index (LAI) of all cassava genotypes was similar except for Aug 1983 sampling and ranged from 2.40 to 6.16. The stomatal density was about ten times higher on the abaxial than the adaxial surface of cassava leaves. The abaxial stomatal conductance of water ranged from 0.23 cm s^-1 to 1.94 cm s^-1. Canopy photosynthesis differed significantly only during 1983 and varied from 1.31 to 1.97 mg CO₂m^-2s^-1. The Pn ranged from 0.48 to 1.21 mg CO₂ m^-2s^-1. ‘Senorita’ and ‘M Ven 218’, the two semi-forking genotypes, had generally higher HI than the forking genotypes. The HI was significantly correlated with CPn (r= 0.76*) and storage root yield (r=0.92*) during 1984. Cassava HI in a short season environment in the subtopics is similar to that in tropics. The relatively high productivity of cassava may be attributed to its high HI.