Effects of cadmium and salicylic acid on growth, spectral reflectance and photosynthesis of castor bean seedlings

Salicylic acid (SA) is a potent signaling molecule in plants and is involved in eliciting specific responses to biotic and abiotic stresses. The aim of this study is to investigate whether the exogenous application of SA can improve cadmium (Cd) induced inhibition of photosynthesis in castor bean (Ricinus communis L.) plants. The effects of SA and Cd on plant growth, spectral reflectance, pigment contents, chlorophyll fluorescence and gas exchange were examined in a hydroponic cultivation system. Results indicate that Cd exposure significantly decreased the dry biomass, photosynthetic rate (P(n)), stomatal conductance (G(s)), intercellular CO(2) concentration (C(i)), pigment contents, quantum yield of PS II photochemistry (F(v)/F(m)), and effective quantum yield of PS II (I broken vertical bar PS II) in the plants. Pretreatment with SA alone reduced the biomass and P(n) in castor bean plants, whereas pigment contents, F(v)/F(m) and I broken vertical bar PS II remained unaffected. Reduced G(s), C(i) and E, as well as increased stomatal limitation (L(s)) and water use efficiency (WUE), were observed in plants pretreated with 500 mu M SA alone, whereas plants treated with 250 mu M SA were unaffected. Under Cd stress, SA pretreatment led to a significant decrease in P(n), G(s), E, C(i), and chlorophyll contents (Chl a, Chl b, Chl a+b, Car, Chl a/b), and an increase in L(s) and WUE. Cd exposure enhanced spectral reflectance in the range 550-680 nm and 750-1,050 nm. It also decreased the normalized difference vegetation index ((chl)NDI), the modified red edge simple ratio index (mSR(705)), the red edge position (REP), water band index, and red/green ratio, whereas the structure independent pigment index (SIPI) was increased. Significant correlations (P < 0.01) between spectral indices (mSR(705), (chl)NDI, REP, red/green ratio) and pigment contents. SA significantly worsened plant growth and photosynthesis in Cd-stressed castor bean plants, in which a stomatal limitation was involved. The leaf spectral reflectance is a sensitive indicator in determining Cd toxicity in plants.