Variation of leaf water potential, relative water content and SPAD under gradual drought stress and stress recovery in two medicinal species of Plantago ovata and P. psyllium

To evaluate the effect of drought stress on the variation of leaf water potential, relative water content and SPAD in two plantago species (Plantago ovata and P. psyllium) as well as the improvement rate of these parameters after re-irrigation, an experiment was conducted at Agriculture Research Center, Melbourne University, Australia in 2006. Leaf water potential was constant within five days after irrigation stop and afterwards, it decreased to -1.6 and -2.0 MPa for P. psyllium and P. ovata at eighth day and -2.3 and -2.8 MPa at tenth day, respectively. Relative water content of both species was 90% up to two days after irrigation stop and had no significant difference with control treatment, but as stress intensified, relative water content of both species significantly decreased. With the decrease in relative water content and further decrease in leaf water potential, SPAD increased in both species, so that leaf chlorophyll content was 17 and 37% in P. ovata and P. Psyllium at relative water content of 60% and/or leaf water potential of -1.5 MPa, respectively.