Leaf Gas Exchange Characteristics of Jatropha as Affected by Nitrogen Supply,Leaf Age and Atmospheric Vapour Pressure Deficit

Facing a steadily increasing world energy demand, jatropha, among other energy crops, has been reported to potentially contribute to biofuel production. A basic characterisation of plant responses to abiotic environmental factors is important for assessing the model‐assisted potential of this plant in view of the many agro‐ecological zones in which jatropha is presently cultivated. Two pot experiments and two field studies were used to record gas exchange parameters in response to light, nitrogen supply, atmospheric vapour pressure deficit (VPD), leaf age and time of measurements. Variation of N supply from 0 to 16 mm resulted in lower rates of photosynthesis (A) and stomatal conductance (g_s) of treatment 0 mm N compared with other N levels, whereas the light compensation point (I_C), quantum yield (QY) and dark respiration rates (R_d) were similar in all treatments. In the field, diurnal effects were evident with higher light‐saturated photosynthetic rate (A_max) and QY and lower I_C and R_d in the morning than in the afternoon. Considering leaf age effects, fully expanded leaves had a lower A_max compared with expanding leaves and this variation in leaf gas exchange was not related to changes in the chlorophyll index value (SPAD) which steadily increased with leaf age. QY of field and greenhouse plants varied from 0.023 to 0.037 and was substantially lower than in C3 plants. A was positively correlated with g_s in a hyperbolic function. A varied from 0.64 to 21.13 μmol m^?2 s^?1 and g_s varied from 12 to 469 mmol m^?2 s^?1. With increasing VPD, g_s decreased, but this response differed between the field experiments and the two pot experiments which contrasted each other distinctively. Applying the inverse logistic function of Webb (Ecological Modeling, 56 (1991), 81), the maximal stomatal conductance of jatropha was in the range of 382 mmol m^?2 s^?1 and g_s is predicted to be close to zero at 5 kPa. These data altogether indicate that light absorption characteristics of single leaves and carbohydrate status parameters should be investigated further to explain the low QY and the pronounced diurnal variation.