POTENTIAL PHOTOSYNTHESIS OF CASSAVA AS AFFECTED BY GROWTH-CONDITIONS

Cassava (Manihot esculenta Crantz) is a hot-climate tropical crop that is also grown in cool areas of the highland tropics and subtropics, where yields are reduced. Because of its wide range of cultivation, there is some concern as to how growth conditions influence photosynthesis. Therefore, the purpose of this study was to evaluate potential photosynthesis of cassava as affected by growth environment. Three cultivars from contrasting habitats were grown in 40-L pots outdoors at a high-altitude site (cool climate) and were then allowed to acclimate and develop new leaves at a mid-altitude site (warm climate). Leaf gas exchange was monitored by infrared gas analysis under controlled laboratory conditions. In all cultivars, photosynthesis of potted cassava was substantially reduced in cool-climate as compared with warm-climate leaves. The cool-climate leaves partially recovered their photosynthetic capacity after warm-climate acclimation for 1 wk. Broad optimum temperature from 30 to 40-degrees-C was observed in the hot-climate cultivars, while the cool-climate cultivar showed an upward shift in optimum temperature from 25-degrees-C in cool-climate leaves to 35 to 40-degrees-C in both the acclimated and warm-climate leaves. Field trials using a number of cultivars were conducted under rainfed conditions at the mid-altitude site and leaf gas exchange was monitored throughout the growing season. Maximum photosynthetic rates of 40 to 50 mumol CO2 m-2 s-1 were obtained in field-grown cassava under wet conditions, and photosynthesis did not saturate up to an irradiance of 1800 mumol photosynthetically active radiation m-2 s-1. Cassava requires high air temperature and high solar radiation for optimal leaf development and for the expression of its photosynthetic potential.