Loss of chlorophyll with limited reduction of photosynthesis as an adaptive response of Syrian barley landraces to high-light and heat stress

The Syrian barley landrace Tadmor is adapted to semi-arid environments and characterized by a reduced chlorophyll content (ca −25% on a leaf area basis) compared to improved barley genotypes, such as the European variety Plaisant. Tadmor leaves had reduced stomatal conductance (gS ) compared to Plaisant leaves both under well-watered conditions and during water stress. Both Tadmor and Plaisant barley seedlings were progressively acclimated to high temperature (39°C) and high photon flux density (1600 µmol photons m −2 s −1 ). During acclimation, the chlorophyll content of Tadmor leaves further decreased whereas the carotenoid concentration remained virtually unchanged, leading to a marked increase in the carotenoid:chlorophyll ratio. The chlorophyll content of acclimated Tadmor leaves was reduced to approximately half of the chlorophyll content of Plaisant leaves grown under the same conditions. Loss of chlorophyll in Tadmor leaves was not observed when only one environmental factor was increased (temperature or photon flux density). In the improved variety, both chlorophylls and carotenoids accumulated during acclimation to heat and strong light, leading to an almost constant carotenoid:chlorophyll ratio. The loss of chlorophyll in the Syrian landrace was associated with limited changes in the photosynthetic characteristics of the leaves (oxygen evolution, electron transport quantum yield, chlorophyll antenna size of photosystem II). Plaisant leaves, but not Tadmor leaves, exhibited symptoms of oxidative damage during growth in strong light at high temperature. When the stomata were closed, sudden exposure to bright light caused a smaller increase in leaf temperature in Tadmor than in Plaisant. Taken together, our results suggest that the ‘low chlorophyll’ feature of Syrian barley landraces is related to their drought adaptation which is manifested by a low g S : the very low chlorophyll content decreases leaf absorbance which, in turn, reduces the potentially damaging heating effect of high solar radiation in droughted plants whose stomata are closed.