Gas exchange and leaf ultrastructure of Tinnevelly senna, Cassia angustifolia, under drought and nitrogen stress

Choice of crops is limited in dry agro-ecosystems. Senna, Cassia angustifolia Vahl, a tropical medicinal plant, has potential as a dryland crop. This study was conducted to investigate gas exchange and leaf surface ultrastructural responses of senna to drought and foliar-applied nitrogen under greenhouse conditions. Drought was imposed cyclically as four, 1-wk spells by withholding water. Additional nitrogen was provided as a foliar treatment of 1% (w/v) urea at the end of each of first three drought cycles. Drought-stressed plants had the same net photosynthesis (P(net)) as well-watered plants until the leaf water potential dropped below approximately -0.8 MPa. Leaf water potential of approximately -2 MPa completely suppressed P(net). Foliar nitrogen treatment increased P(net) > 30% in both water-stressed and well-watered plants. Intrinsic water use efficiency was unaffected by cyclic drought. Leaf area per plant increased 156% because of foliar nitrogen treatment in well-watered plants but remained unchanged in drought-stressed plants. Stomatal count per leaflet decreased 25% or more because of drought regardless of foliar nitrogen treatment on either abaxial or adaxial leaf surface. Foliar nitrogen treatment decreased adaxial but increased abaxial stomatal count regardless of drought treatment. Trichome count was more than eight times greater on the abaxial than the adaxial leaf surface under any treatment combination. Adaxial trichome number increased under drought, but decreased under nitrogen treatment. Abaxial trichome count increased because of drought only in plants deprived of nitrogen treatment. Maintenance of high carbon gain and water use efficiency, drought deciduousness, and plasticity in stomatal and trichome counts under both drought and nitrogen stress appeared to confer stress tolerance in senna.