Chlorophyll content and photosynthetic performance after heat stress. (a) Schematic representation of the heat stress (HS) regime. A. thaliana wild type, ppi1, and toc64-1 were grown under short-day conditions and were then either kept under control (c) conditions (21 °C) or submitted to heat stress conditions (42 °C) either only once on day 7 for 2 h (HS-I) or every alternate day (HS-II) until day 21. (b) Chlorophyll content from 21-day-old leaves of wild-type, ppi1, and toc64-1 plants subjected to control (c), HS-I (I), or HS-II (II) treatment as shown in (a). The values shown are the means of at least three measurements (± SD). Statistical analysis (see methods section) was performed (p < 0.05). (c,d) The kinetic of the quantum yield adaptation of photosystem II (c) or the non-photochemical quenching (NPQ; d) from 21-day-old leaves of wild-type (left), ppi1 (middle) or toc64-1 plants (right) after they have been subjected to control (black), HS-I (orange), or HS-II treatment (green) at low light conditions is shown. The maximal Φ(II) and NPQ value for HS-treated wild-type plants is indicated as a horizontal dashed line for comparison.