Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age.

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO2 assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO2 (gc) and H2O (gs), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO2 assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 μmol s(-1) m(-2). Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher gc and gs than sun-grown leaves. Shade leaves of taro had greater gc and g3 rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.