Fifty shades of fade: linking transmittance loss to cellular death during leaf dehydration.

Drought-induced tree mortality is associated with hydraulic failure and subsequent cell death. Leaf vitality is a critical indicator of tree water stress; however, current methods for assessing cell death are destructive and unsuitable for large-scale measurements. The optical method, which detects changes in leaf light transmittance, has been traditionally used to monitor vein cavitation. However, emerging studies have reported transmittance changes extending beyond veins, suggesting a potential link with mesophyll cell death. In this study, we investigated whether the loss of light transmittance in leaves reflects underlying cellular mortality. We subjected seedlings from six tree species with contrasting drought resistance to progressive water stress. Changes in leaf light transmittance were continuously monitored using the optical method while cell mortality was quantified using classical methods. Cryo-SEM imaging provided anatomical insights into cell structural degradation and its influence on transmittance. Our results revealed a strong synchronicity between transmittance loss and cell mortality increases across species, supporting the use of light transmittance as a nondestructive proxy for cellular damage. Overall, our results highlight the potential of the optical method for early detection of drought-induced stress and mortality in trees and offer a promising, scalable, and noninvasive tool for monitoring tree health under drought.