Fitotossicità dell'acido acetico sulla germinazione di differenti varietà di riso

Green mulch is an agronomic management largely adopted in organic rice cultivation, to fertilize soil and control weeds. However, when it is associated with submersion technique, fermentation of fresh organic matter takes place by anaerobic microorganism activity. Consequently, organic acids are released and phytotoxic effects both for rice and weeds are recorded. The aims of this study are 1) to define an analytic method to be used to determine the phytotoxic effects of acetic acid on rice, 2) to analyse phytotoxicity effects of acetic acid on different rice varieties, 3) to assess the varietal sensitivity and to identify varieties more tolerant to phytotoxic effects. The experiment was conducted in a growth chamber in which the dose-response effects of varieties Salvo, Selenio and Sant'Andrea were evaluated. Seeds of the three varieties have been placed in different acetic acid concentrations and left to germinate for eight days, simulating a conventional agronomic field management. The trial was organized as a randomized complete block design at a controlled-temperature (20 °C) and continuous light. The experiment consisted in 18 cans of 5 cm in diameter, containing each 5 seeds per variety. Concentration of acetic acid were 0 ppm (test), 9 ppm, 18 ppm, 36 ppm, 54 ppm, 72 ppm. The following measurements were assessed: 1) the number of germinated seeds during the first three days in order to determine the germination curve; 2) qualitative effects, on the eighth day, to identify chromatic alterations; 3) biometric evaluations of shoots and roots, studying images obtained by scanning, processed with Corel PhotoPaint Software and then analysed with WinRHIZO Software. Biometric results were length, diameter and feret ratio of each vegetable portion (roots, shoot, first leaf). Germinated seeds results showed a greater germination rate induced by acetic acid, especially at high concentrations, for all varieties. A significant reduction of the radical length was observed starting from 36 ppm, independently from the varieties. Sant'Andrea showed a greater tolerance to acetic acid. Indeed, a more gradual root shortening was measured, compared to the others varieties, with increasing magnitude when concentration increased. Acetic acid also had an effect on the root diameter, which increased at high concentrations. Shoots of all the varieties showed a better tolerance to acetic acid if compared to the roots. Length of shoots decreased only at 72 ppm for Sant'Andrea and Selenium and at 54 ppm for Salvo. No effect was detected for diameter measures. Length and diameter of first leaf were significantly reduced only at 72 ppm in all varieties. Chromatic and morphological alterations, such as yellowing, redness, whitening and curling of shoots, started from 18 ppm. Yellowing has been observed in Sant'Andrea, while redness was only present in Salvo and Selenium. Instead, shoots of all varieties showed bleaching and curling. In conclusion, an analytic method to study varietal response at increasing concentration of acetic acid has been defined and the methodology will be applied to other varieties. Sant'Andrea was the most tolerant variety to acetic acid. Therefore, it is probably the most suitable variety, among the three tested, for green mulching technique when sown in water.