Biomass and Pigment Production in Three Isolates of Azolla II. Response to Light and Temperature Stress

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Abstract
Two isolates of Azolla caroliniana Willd. (RAR, M-3) and one of Azolla filiculoides LAM. (LA) were compared and characterized in defined growth environments. A moderate environment (200–230 μ mol m−2 S−1, 18/6 h photoperiod and 26/22 °C thermoperiods) was selected to record growth in near-optimum conditions. All isolates doubled their fresh and dry weights in approximately 2–3 d during the first week of trials. Acetylene reduction rates were low for all isolates, but RAR (from Amazonian Colombia) generally grew the fastest. RAR also accumulated more chlorophyll, its chlorophyll doubling times (2.14 ± 0.1 d) were 1–2 d faster than those of M-3 or LA, and it retained the most stable chlorophyll a/b ratios and total chlorophyll densities. Growth data for all isolates converged during the second week. Dry-weight doubling times were longer than 3 d. None of the isolates excreted measureable amounts of nitrogen in month-long trials. Stress trials were conducted using high light (450–510 μ mol m−2 s−1) and/or high temperature (35/31° C conditions Biomass yields were usually least inhibited, if at all, with elevation of only photon flux densities. High temperature alone stimulated the dry-weight growth only of M-3 compared with its growth under moderate conditions. Biomass, chlorophyll, and acetylene reduction data suggested that M-3 may have a higher optimum growth temperature than RAR or LA. The combined effect of light and temperature stress depressed acetylene reduction rates in RAR and M-3 to a greater extent than did either parameter alone. High light levels had more effect than high temperatures in inhibiting chlorophyll doubling times and inducing anthocyanin production in RAR and M-3. These two isolates did not form any anthocyanin in elevated temperatures. Pre-incubation in darkness before exposure to elevated light/temperature conditions facilitated anthocyanin production in all isolates. LA, however, proved to be the isolate most sensitive to stress by heat, alone or augmented by light stress, according to all criteria used.