Photosynthesis and photoinhibition in two xerophytic shrubs during drought

Seasonal changes in water relations, net photosynthetic rate (PN), and fluorescence of chlorophyll (Chl) a of two perennial C3 deciduous shrubs, Ipomoea carnea and Jatropha gossypifolia, growing in a thorn scrub in Venezuela were studied in order to establish the possible occurrence of photoinhibition during dry season and determine whether changes in photochemical activity of photosystem 2 (PS2) may explain variations of PN in these species. Leaf water potential (ψ) decreased from -0.2 to -2.1 MPa during drought in both species. The PN decreased with ψ in I. carnea and J. gossypifolia by 64 and 74 %, respectively. Carboxylation efficiency (CE) decreased by more than 50 and 70 % in I. carnea and J. gossypifolia, respectively. In I. carnea, relative stomatal limitation (Ls) increased by 17 % and mesophyll limitation (Lm) by 65 % during drought, while in J. gossypifolia Ls decreased by 27 % and Lm increased by 51 %. Drought caused a reduction in quantum yield of PS2 (ϕPS2) in both species. Drought affected the capacity of energy dissipation of leaves, judging from the changes in the photochemical (qP) and non-photochemical quenching (NPQ) coefficients. Photoinhibition during drought in I. carnea and J. gossypifolia was evidenced in the field by a drop in the maximum quantum yield of PS2 (Fv/Fm) below 0.8 and also by non-coordinated changes in ϕPS2 and quantum yield of non-photochemical excitation quenching (Yn). Total soluble protein content on an area basis increased with ψ but the ribulose-1,5-bisphosphate carboxylase/oxygenase content remained unchanged. A reduction of total Chl content with drought was observed. Hence in the species studied photoinhibition occurred, which imposed an important limitation on carbon assimilation during drought.