Determination of chlorophyll contents often requires destructives analysis and involves lengthy procedures while measurement on photosynthetic rate requires specialized equipment that often limits extensive in-field application. SPAD chlorophyll meter gives indirect measurement of leaf chlorophyll contents in a rapid, non-destructive and convenient manner. The use of SPAD meter for each individual plant species require calibration curve for each individual plant species, as the accuracy of the estimation are highly affected by the leaf anatomical characteristics. This study reports the result of correlating SPAD-502 chlorophyll index readings to the actual extracted chlorophyll content and photosynthetic rate of oil palm leaf as well as the leaf nitrogen and potassium concentration. Concentration of oil palm leaf nitrogen and chlorophyll pigment as well as photosynthetic rate were highly correlated to SPAD-502 chlorophyll index with R 2 of above 0.80. With good calibration equation, the SPAD-502 chlorophyll meter are extremely versatile for rapid determination of oil palm leaf nitrogen and chlorophyll concentration as well as photosynthetic rate.

Abstract The cyanobacteria Microcystis flos-aquae can cause harmful algal blooms in waterbodies, which threaten the normal functioning of aquatic ecosystems and human health. The plant extracts are considered as the promising algaecides. In this study, the effects of ten plant extracts (Cinnamomum camphora, Ginkgo biloba, Firmiana platanifolia, Salix babylonica, Euphorbia humifusa, Erigeron annuus, Solidago canadensis, Alternanthera philoxeroides, Thalia dealbata and Eichhornia crassipes) against M. flos-aquae were investigated. The results showed that all ten plant extracts had a significant inhibitory effect on M. flos-aquae growth after 96 h (P < 0.01). The inhibition rates of S. babylonica, E. humifusa, S. canadensis and A. philoxeroides were over 70.00%. Furthermore, the E. humifusa extract had the best inhibitory effect on the photosynthesis of M. flos-aquae, with the effective quantum yield of photosystem II (YII) and maximal relative electron transport rate (rETRmax) decreasing by 97.50% and 97.00%, respectively, after 96 h. Additionally, the E. humifusa extract was found to be nontoxic to non-target organisms such as Brachydanio rerio and Vallisneria spiralis within 96 h. This study contributes to the existing knowledge and data of freshwater cyanobacteria blooms, and provides insights for their control and the restoration of freshwater systems affected by cyanobacteria blooms.