Accumulation of Nitrate and Nitrite in Chilled Leaves of Rice Seedlings is Induced by High Root Temperature

We previously found a novel type of chilling injury in the leaves of rice seedlings (Oryza sativa L. cv. Akitakomachi). The damage was only observed when the roots were not chilled (10 °C/25 °C, shoots/roots), but not when the whole seedling was chilled (10 °C/10 °C). In this report, we show that the chilling injury induced by high root temperature required nitrate and potassium together with a trace amount of iron, manganese or both in the nutrient solution during the treatment, and that the injury was increased by nitrogen starvation before chilling. Both nitrate and nitrite accumulated in the 10 °C/25 °C leaves when the nutrient solution contained nitrate. The nitrate accumulation in the 10 °C/25 °C leaves was highest at the end of the first light period, and was followed by a decrease with a concomitant increase in nitrite during the first dark period. The photosynthetic electron transport was completely lost in both PSII and PSI in the 10 °C/25 °C leaves when the nutrient solution contained nitrate. However, the activities in the leaves of the 10 °C/25 °C plants treated with the nutrient solution lacking nitrate remained at approximately half those in the 10 °C/10°C leaves. The photochemical quenching of Chl fluorescence and the P700 oxidation state were also intermediate between those in the 10 °C/25 °C and 10 °C/10°C leaves of plants supplied with the complete nutrients. Thus, the chilling injury was closely linked to the accumulation of nitrate and nitrite, as well as to a malfunction of photosynthesis in the 10 °C/25 °C leaves.