Effect of Water Deficit Stress on Photosynthetic Characteristics of <i>Jatropha curcas</i>

Abstract

The need to mitigate climate change cannot be more emphasized, which arises, as a result of increases in CO 2 emissions due to anthropogenic activities. Given the current world energy problems of high fossil fuel consumption which plays a pivotal role in the greenhouse effect, Jatropha curcas biodiesel has been considered a potential alternative source of clean energy (biodiesel is carbon neutral). However, the ability of Jatropha curcas , as a candidate source of alternative of clean energy, to grow in marginal and dry soils, has been poorly elucidated. This study, therefore aimed at investigating whether Jatropha cur cas leaves could switch from carrying out C 3 photosynthetic pathway to Cras sulacean Acid Metabolism (CAM) as a strategy to improve its water deficit to lerance. Thirty-five-day-old Jatropha curcas accessions, from three different climatic zones of Botswana, viz ., Mmadinare (Central zone), Thamaga (Southern zone) and Maun (Northern zone), were subjected to water stress, by with-holding irrigation with half-strength Hoagland culture solution. Net photo synthetic rate, transpiration and stomatal conductance were measured at weekly intervals. The leaf pH was measured to determine whether there was a de crease in pH (leaf acidification) of the leaves during the night, when the plants experienced water deficit stress. All the accessions exhibited marked reduction in all the measured photosynthetic characteristics when experience water deficit stress. However, a measurable CO 2 uptake was carried out by leaves of all the accessions, in the wake of marked decreases in stomatal conductance. There is evidence to suggest that when exposed to water stress J. curcas ac cessions switch from C 3 mode of photosynthesis to CAM photosynthetic pathway. This is attested to by the slightly low leaf pH at night. Thamaga accession exhibited an earlier stomatal closure than the other two accessions. This resulted in Thamaga accession displaying a slightly lower dry weight than both Mmadinare and Maun accessions. It could be concluded that Jatropha curcas appeared to tolerate water deficit stress due to its ability of switching from C 3 photosynthetic pathway to the CAM photosynthetic path way , but with a cost to biomass accumulation, as demonstrated by slightly more reduced CO 2 assimilation by Thamaga accession, than the other two accessions.