Monitoring stomatal conductance of Jatropha curcas seedlings under different levels of water shortage with infrared thermography

Model simulations and experimental measurements were used to investigate the applicability of infrared thermography for the estimation of stomatal conductance and drought stress under sub-optimal meteorological conditions. The study focused on the stomatal conductance index Ig, calculated from the leaf temperature and the temperature of a dry and wet reference leaf. The simulations revealed that Ig is influenced by leaf dimension, wind speed and air temperature and not or hardly by leaf angle, albedo, relative humidity or insolation. In addition, Ig was found to be very sensitive to differences in wind speed, air temperature, insolation, leaf dimension and leaf angle between the measured and the reference leaves. In the experimental part, we evaluated if infrared thermography can be used to improve the knowledge on the water use of Jatropha curcas L., a tropical biofuel crop. Thermal images from Jatropha seedlings grown under three different drought treatments were made on a day with very variable insolation and a day with very low insolation. Smaller newly formed leaves and the active control of the leaf angle proved efficient ways of Jatropha to protect leaves under drought stress from overheating. Ig, assessed in four different ways, and four simplified drought stress indices were derived and related to the measured stomatal conductance (gs) of the seedlings. The strongest correlation with gs and the highest discriminative power between the different water treatments were achieved when Ig was calculated by taking the average leaf temperature per plant and the temperature of the dry and wet reference leaves of this plant, rather than the average temperature of several reference leaves. Using the difference between the dry reference and the measured leaf (Tdry − Tl) as a simplified index gave similar results, although correlations were weaker. Other simplified thermal indices were not well correlated with leaf stomatal conductance or with water treatment. Recommendations were formulated for the measurement of Ig and (Tdry − Tl).

Research highlights
► Under drought, Jatropha reduces its leaf temperatures efficiently by adjusting the leaf angles.
► Ig can be used to estimate gs or detect drought stress when insolation is very low or very variable.
► Ig is sensitive to different microclimate or leaf properties between measured and reference leaves.
► (Tdry − Tl) is a promising drought stress index with potential for application at field scale.