Purpose: Evaluate the structural damage and the changes in the photosynthesis and transpiration rates of aquatic lirium leaves caused by ultrasound (US) irradiation in search of environmentally friendly methodologies for the control of this weed.
Materials and methods: Aquatic lirium plants were extracted from Xochimilco water canals in Mexico City. A part of the group of plants was selected for irradiation, and the rest formed the control group. The irradiation plants group was exposed to US irradiation of 17 kHz frequency and 30 W × 4 output power for 2 h, at noon and 25 °C room temperature. The structural analysis was done with a MOTICAM 1 digital camera, 800 × 600 pixels, incorporated into the MOTIC PSM-1000 optical microscope and edited with Motic Images Plus 2.0 ML software. The total stomata density and the damaged stomata density were determined by dividing the numbers of total and damaged stomata by the visual field area (67,917 mm2), respectively. The leaves' photosynthesis and transpiration rates were measured using an LI-6400XT Portable Photosynthesis System.
Results: Significant damage was observed in the stomata and epidermal cells, finding that the average ratio between the damaged and total stomata densities as a function of time (days) showed an exponential increase described by a Box-Lucas equation with a saturation value near unity and a maximum rate of change of the density of damaged stomata on zero-day (immediately after irradiation), decreasing as the days go by. The transpiration rate showed a sudden increase during the first hour after irradiation, reaching a maximum of 36% of its value before irradiation. It then quickly fell during the next 6 days and more slowly until the 21st day, decreasing 79.9% of its value before irradiation. The photosynthetic rate showed similar behavior with a 37.7% maximum increment and a 73.6% minimum decrease of its value before irradiation.
Conclusions: The results of structural stomata damage on the ultrasound-irradiated aquatic lirium leaves are consistent with an excessive ultrasound stimulation on stomata's mechanical operation by guard cells that produce the measured significant increase of the photosynthetic and transpiration rates during the first hour after irradiation. The initial high evaporation could alter the water potential gradient, with a possible generation of tensions in the xylem that could cause embolism in their conduits. The loss of xylem conductivity or hydraulic failure would be consistent with the observed significant fall in the photosynthesis and transpiration rates of the aquatic lirium leaves after its sudden rise in the first hour after irradiation.
Keywords: Aquatic lirium; hydraulic failure; photosynthesis; transpiration; ultrasound irradiation.