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 Plais ant. Tadmor leaves had reduced stomatal conductance (g(s)) compared to Plai sant leaves both under well-watered conditions and during water stress. Bot h Tadmor and Plaisant barley seedlings were progressively acclimated to hig h temperature (39 degrees C) and high photon flux density (1600 mu mol phot ons m(-2) s(-1)). During acclimation, the chlorophyll content of Tadmor lea ves further decreased whereas the carotenoid concentration remained virtual ly unchanged, leading to a marked increase in the carotenoid: chlorophyll r atio. The chlorophyll content of acclimated Tadmor leaves was reduced to ap proximately 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 flu x density). In the improved variety, both chlorophylls and carotenoids accu mulated during acclimation to heat and strong light, leading to an almost c onstant carotenoid: chlorophyll ratio. The loss of chlorophyll in the Syria n landrace was associated with limited changes in the photosynthetic charac teristics of the leaves (oxygen evolution, electron transport quantum yield , chlorophyll antenna size of photosystem II). Plaisant leaves, but not Tad mor 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 decreas es leaf absorbance which, in turn, reduces the potentially damaging heating effect of high solar radiation in droughted plants whose stomata are close d.