We propose a simplified alternative method for quantifying the partitioning of excitation energy between photochemistry, fluorescence and thermal dissipation. This alternative technique uses existing well-defined quantum efficiencies such as Phi(PS II), leaving no 'excess' efficiency unaccounted for, effectively separates regulated and constitutive thermal dissipation processes, does not require the use of F(o) and F'(o) measurements and gives very similar results to the method proposed by Kramer et al. [(2004) Photosynth Res 79: 209-218]. We demonstrate the use of the technique using chlorophyll fluorescence measurements in grapevine leaves and observe a high dependence on thermal dissipation processes (up to 75%) at both high light and low temperature.

Exposure to high irradiance results in dramatic changes in nuclear gene expression in plants. However, little is known about the mechanisms by which changes in irradiance are sensed and how the information is transduced to the nucleus to initiate the genetic response. To investigate whether the photoreceptors are involved in the response to high irradiance, we analyzed expression of EARLY LIGHT-INDUCIBLE PROTEIN1 (ELIP1), ELIP2, ASCORBATE PEROXIDASE2 (APX2), and LIGHT-HARVESTING CHLOROPHYLL A/B-BINDING PROTEIN2.4 (LHCB2.4) in the phytochrome A (phyA), phyB, cryptochrome1 (cry1), and cry2 photoreceptor mutants and long hypocotyl5 (hy5) and HY5 homolog (hyh) transcription factor mutants. Following exposure to high intensity white light for 3 h (1,000 mmol quanta m 22 s 21 ) expression of ELIP1/2 and APX2 was strongly induced and LHCB2.4 expression repressed in wild type. The cry1 and hy5 mutants showed specific misregulation of ELIP1/2, and we show that the induction of ELIP1/2 expression is mediated via CRY1 in a blue light intensity-dependent manner. Furthermore, using the Affymetrix Arabidopsis (Arabidopsis thaliana) 24 K Gene-Chip, we showed that 77 of the high lightresponsive genes are regulated via CRY1, and 26 of those genes were also HY5 dependent. As a consequence of the misregulation of these genes, the cry1 mutant displayed a high irradiance-sensitive phenotype with significant photoinactivation of photosystem II, indicated by reduced maximal fluorescence ratio. Thus, we describe a novel function of CRY1 in mediating plant responses to high irradiances that is essential to the induction of photoprotective mechanisms. This indicates that high irradiance can be sensed in a chloroplast-independent manner by a cytosolic/nucleic component.

Molecular analyses of plants have revealed a number of genes whose expression changes in response to high light (HL), including the H 2 O 2 scavenger, ASCORBATE PER-OXIDASE 2 ( APX2 ). We carried out a screen in Arabidopsis thaliana for lesions that alter HL-induced expression of APX2 to identify components in abiotic stress signalling pathways. High light was used as it can be instantaneously applied or removed and accurately measured. We identified a number of alx mutations causing altered APX2 expression. Here we describe the gain-of-function mutant, alx8 , which has constitutively higher APX2 expression and higher levels of foliar abscisic acid (ABA) than wild type. In fact, exogenous ABA increased APX2 expression and the APX2 promoter contains ABA response elements. Furthermore, we have shown that HL stress increases ABA in wild-type plants, implicating ABA in the regulation of HLinducible genes. The alx8 mutant is drought tolerant, exhibits improved water-use efficiency and a number of drought-tolerance genes are upregulated. Additionally, alx8 demonstrates the complexity of ABA-dependent and ABA-independent transcriptional networks as some components in both pathways are upregulated in alx8 . This study provides evidence for common steps in drought and HL stress response pathways.