There's yellow, and then there's yellow-Which one is YCS?

YELLOW CANOPY SYNDROME (YCS) symptoms include leaf yellowing of the mid canopy, a reduction in photosynthesis and yield decreases of up to 30%. The cause of YCS remains unknown. It has been speculated leaf yellowing from YCS is produced by triggering the early onset of senescence, or through implications of water-stress. Inability to differentiate accurately between YCS and other conditions leading to leaf yellowing is a serious limitation for research and the development of strategies to manage YCS. The aim of this work is to determine if there are distinct metabolic or gene expression changes in YCS which would allow discrimination between YCS, leaf senescence and water-stress induced yellowing. For this purpose, the metabolome and transcriptome of YCS affected leaves, water-stressed leaves and leaves undergoing senescence were compared at the early stages of still-green lamina and after visible yellowing of the lamina. Differential gene expression data uncovered a uniqueness among YCS affected yellow leaves. The separation of YCS yellowing involved genes related to plant diurnal cycle and carbon metabolism. We suggest that YCS yellowing symptoms are produced by an alternate trigger to that determining the developmental (age-related) senescence process, and may be related to excess sugar accumulation in the leaf. Based on the new information, gene expression patterns unique to YCS could be used as a potential diagnostic, and open up an area for further investigation into understanding YCS.