CASSAVA VARIETAL RESPONSE TO PHOSPHORUS FERTILIZATION .1. YIELD, BIOMASS AND GAS-EXCHANGE

Cassava (Manihot esculenta Crantz) yield can be limited by phosphorus (P) supply. Previous trials conducted at CIAT indicated a wide range of response to P fertilization in cassava germplasm. The purpose of this study was to elucidate possible mechanisms and morphological traits underlying cassava's differential response to P fertilization. Four contrasting varieties were grown under rainfed conditions for 10 mondis in the field for two consecutive seasons under four fertilizer treatments. There were three dosages of P fertilizer at 0, 50 and 100 kg P ha-1 with 100 kg NK ha-1, plus a control with no fertilizer. Sequential harvests that included biomass determination were conducted. Single-leaf gas exchange was monitored by portable infrared gas analysis systems. All varieties showed a significant positive response in total and aerial biomass as well as mean leaf area index (LAI) to P fertilization, at least in the second year. The variety M Col 1684 was the only one that did not significantly respond in terms of dry root yield and number of storage roots to P fertilization over the years. This variety was among the most responsive for peak LAI, number of aerial apices and reproductive organs (flowers and fruits). Yield and storage root number correlated positively with net photosynthetic rates and negatively with reproductive organs. Variety M Col 1684 could not maintain a high harvest index with increasing aerial growth. In contrast, CM 489-1 balanced aerial growth enhancement with intensive root formation, augmenting root sink strength and yield. It is concluded that yield response to P fertilization depended upon the balance between aerial growth potential of varieties and storage root sink strength.