*3.3. Correlations between Growth and Fluorescence: A Case for Rapid Phenotyping*

Our data showed many interrelations between chlorophyll *a* fluorescence and growth-related traits (PLA, no. of visible leaves, average leaf size), suggesting that light use efficiency of photosystem II electron transport generally correlated with growth (Tables 2 and 3; Figures 5–7). Also, a strong negative and highly significant correlation (*p* < 0.001) between ΦPSII and NPQ across treatments and accessions supports a link between photosynthetic efficiency and photoprotection (Table 2). This is remarkable, as this link is not often apparent at the relatively low light intensities that these measurements were conducted at (90 μmol m−<sup>2</sup> s<sup>−</sup>1).

Our data again emphasize chlorophyll *a* fluorescence imaging as a powerful tool for rapid plant phenotyping, enabling the analysis of photosynthesis-related traits in many accessions under multiple (and sometimes rapidly changing) environmental conditions [6,28,29]. Rapid plant phenotyping aims to simultaneously and repeatedly determine a large number of traits on a large number of plants. To that end, several weighing and imaging tools exist to determine, e.g., whole shoot and root growth [30], plant architecture, relative or absolute transpiration rates, as well as leaf temperature, photosynthetic capacity, spectral absorptivity [31], thickness, pigmentation and sugar concentration [32]. Since chlorophyll *a* fluorescence is often closely related to actual photosynthesis rates, and since photosynthesis reacts in a highly sensitive manner to intrinsic and extrinsic (e.g., environmental) factors, chlorophyll *a* fluorescence is a great tool to determine differences in photosynthesis within plants, across plants, and over time.
