Abstract

Plants under hydrothermal stress show a variety of responses aimed at reducing leaf growth and total leaf area, thereby reducing physiological processes such as transpiration and photosynthesis. The effects of hydrothermal stress on cassava plant growth parameters were thus investigated. Twenty cassava varieties representing a broad range of genetic diversity were used. Plants were grown in the field, physical measurements were made and percentage changes in growth parameters and biomass accumulation recorded bimonthly. Significant variation among varieties was found for the response to stress of leaf growth rate (losses of between 60 and 100% after stress) and duration (losses within 1 to 2 months) which equally resulted into reduction in leaf area. Variations were also observed in leaf retention (0 to 40%) and expansion rate (lamina width (25 to 33%) and length (14 to 58%) among different varieties. Differences were also observed for the time and rate of leaf loss during stress period. Based on observed differences, varieties were grouped under three distinct clusters including early recovering varieties, stay green varieties and the susceptible varieties. Alterations in leaf properties were highly correlated to harvest index where low harvest index (0.2 to 0.4) was observed for stay green and susceptible varieties compared to early recovering varieties (0.4 to 0.7). From the observation, different coping mechanisms, important in selection of drought stress tolerant genotypes were identified and pointed to specific genetic mechanisms for leaf retention/loss and biomass accumulation. The results suggest that rate of growth and duration differences are due to different physiological mechanisms, and can be combined to select for hydrothermal stress tolerant varieties.

Key words: Leaf expansion rate, leaf area, leaf development, recovery mechanisms.