Characteristics of Photosynthesis and Conductance in Early and Late Leaves of Honey Mesquite

Photosynthetic rates of early and late leaves of honey mesquite (Prosopis glandulosa) were measured on native rangeland with an air-flow open system. Daily maximum photosynthetic rates of early leaves were significantly greater than those of late leaves. Greater photosynthetic rates in early leaves were associated with higher nitrogen content per unit leaf area and a thicker leaf blade. Lack of well-developed photosynthetic apparatus (functional complex leading to CO2 fixation) in the late leaves is suggested as a cause for lower photosynthesis. Photosynthetic rates of early leaves were generally higher in the morning than late leaves, and they became lower in the afternoon than late leaves. Stomatal conductance was negatively correlated with leaf-air vapor pressure difference (VPD) (R > .8) in early leaves, but a low correlation was found between conductance and VPD (R < .1 in some samples). Higher photosynthetic rates of late leaves in the afternoon were accompanied by greater leaf conductance and leaf wilting, which suggested that turgor potential is not a sole transducer of water stress.