MICROCYSTIN-LR INHIBITS PHOTOSYNTHESIS OF PHASEOLUS-VULGARIS PRIMARY LEAVES - IMPLICATIONS FOR CURRENT SPRAY IRRIGATION PRACTICE

Photosynthesis of Phaseolus vulgaris L. leaves was measured after trea tment with microcystin-LR (MC-LR), a potent cyanobacterial toxin and i nhibitor of protein phosphatases 1 and 2A. The net photosynthetic rate fell by over 50% within 8 h of dipping leaves in MC-LR solution. This inhibitory effect was observed when leaves were treated once with con centrations above 10(-2) mol m(-3) At 10(-2) mol m(-3), the inhibition caused by MC-LR was transient, and net photosynthesis rates essential ly recovered after 5 d, whereas at 10(-1) mol m(-3), the net photosynt hesis rate in treated leaves was still 42% of controls after 8 d. Necr osis was observed at the higher concentration, but not usually below 1 0(-2) mol m. Analysis of net photosynthetic rate as a function of inte rnal CO2 concentration and photosynthetically active photon flux densi ty indicated that both the CO2-saturated rate a of photosynthesis and the carboxylation efficiency were lowered when MC-LR-treated leaves we re exposed to photosynthetically-saturating light. When the leaves wer e exposed to 2.0x10(-5) mol m(-3) MC-LR solution repeatedly, the photo synthetic rate was significantly reduced after 7 d, suggesting that in termittent exposure of P. vulgaris leaves to low concentrations of MC- LR brings about an effect on photosynthesis more inhibitory than that of a single exposure to high concentrations of MC-LR. These results in dicate that relatively low concentrations of MC-LR cause damage to the photosynthetic apparatus of P. vulgaris in situ. We discuss the signi ficance of these findings in relation to current spray irrigation prac tice for crop plants involving the use bf water containing cyanobacter ial blooms and microcystins.