Spatio-temporal redox changes in rice leaves exposed to high light

Abstract

Despite plant growth and development are systemic and spatio-temporal highly regulated processes, the biochemical mechanisms involving redox metabolism are lacking. In this work we evaluated the role of leaf age and position in redox response to high light, aiming to establish a relationship between spatio-temporal changes and redox metabolism. Rice (Oryza sativa) hydroponically cultivated in greenhouse for 45 days. Afterwards, plants were transferred to a growth chamber, where they were divided in two groups: low light - LL (300 μmol photons m-2 s-1 PPFD) and high light – HL (1,800 μmol photons m-2 s-1 PPFD) for 9 hours. After this, the canopy leaves were separated in 3 parts: bottom or senescing leaves (L6 and L7), middle or fully expanded leaves (L8 and L9) and top or last fully expanded and expanding leaf (L10 and L11). Periodic measurements of chlorophyll content were taken in all leaves. Lipid peroxidation and electrolyte leakage were similarly and strongly increased in bottom leaves, independently of light, in comparison to top and middle leaves. In contrast, these stress indicators and H2O2 content were intensely increased in HL only in top leaves. Ascorbate redox state was strongly decreased from bottom to top parts and slightly dependent of light intensity. Interestingly, CAT and APX activities were strongly stimulated by HL in bottom and middle leaves but not in top, whereas SOD activity didn’t present any variation in response to light or plant part. Top portion displayed high but not-HL stimulated CAT activity whereas APX activity in these leaves were lower than bottom leaves. Soluble protein were much lower in bottom and, thus enzyme activities will depend on the chosen basis: mg of protein or g of fresh weight. Here, we choose FM because it is a more stable basis. According to our data, redox responses displayed by leaves along rice canopy exposed to contrasting light are dependent on the leaf physiological stage or of a spatio-temporal contextualization.