Israeli, Y., Department of Agricultural Botany, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76100, Israel, Jordan Valley Banana Exp. Station, Zemach 15132, Israel
Schwartz, A., Department of Agricultural Botany, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76100, Israel
Plaut, Z., Department of Soil Science, ARO, Bet-Dagan 50250, Israel
Yakir, D., Department of Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel

Banana plants (Musa sp., Musaceae) were grown for 2 years in the field in 12×20 in plots under irradiance levels incident upon the canopy of 100, 81, 62 and 32% of sunlight Time-integrated parameters such as leaf 13C, yield and leaf mass to area ratio were linearly correlated with irradiance level (R2>0.9). In contrast, midday CO2 assimilation and transpiration efficiency (A/g) decreased significantly and substomatal CO2 concentrations (cst) increased significantly only at the lowest irradiance levels (below 81% irradiance). Diurnal gas exchange measurements indicated that the linear response of the long-term parameters may be associated with the significant variations in photosynthetic activity among the irradiance treatments observed only in the early morning hours. The linear fit between yield and irradiance level (per cent of control) had a slope of 0.82 (with apparently constant yield to biomass ratio). These results directly demonstrate the significance of variations in incident light, such as may be associated with increasing cloudiness, on productivity of tropical plants such as the banana. The importance of using time-integrated indicators in general, and the reliability of using 13C discrimination in particular, in evaluating the responses of plants to changes in incident irradiance is demonstrated.