Gas Exchange and Chlorophyll Fluorescence's Characteristics of Hyoscyamus muticus L. at Different Phenological Stages under Extreme Arid Environmental Conditions, South-Western Desert, Egypt

HYOSCYAMUS muticus L. is an adaptable plant of hyper arid environment. Changes in photosynthesis rate, transpiration, stomatal conductance, instantaneous water use efficiency, chlorophyll fluorescence and photosynthetic pigments characteristics were analyzed in the course of phenological stages including seedling, vegetative, flowering and fruiting to reveal the adaptive mechanisms of this species to withstand high irradiance during its life cycle. All measurements were taken under wide range of photosynthetically active radiation (PAR). H. muticus showed significant differences of physiological processes at different phenological stages, the highest rates of photosynthesis (5 and 5.23 mu molm(-2)s(-1)) was recorded at vegetative stage and the highest transpiration rate was recorded at relatively high PAR at seedling stage. Otherwise, lowest transpiration rates (0.18mmolm(-2)s(-1), 0.19mmolm(-2)s(-1) and 0.197mmolm(-2)s(-1) were noticed at vegetative, flowering and fruiting stages, respectively) at highest PAR (2500 mu molm(-2)s(-1)). Stomatal conductance limitations in seedling, flowering and fruiting stages of H. muticus exhibited reductions of photosynthesis rate (Pn), transpiration rate (E) and instantaneous water use efficiency (WUE) in response to PAR increments. The plant fluoresces behaviour recorded significant increments and decrements in minimal fluorescence from dark-adapted leaf (F-0) and maximal fluorescence from dark-adapted leaf (F-m) respectively, and significant declines in the ratio of PSII photochemistry maximum quantum efficiency to quantum yield (F-v/F-m) (0.75200, 0.75167, 0.68833 and 0.4800) along with progressing phenological stages. In parallel with the later ratio decline, there was a significant decrement in chlorophyll a, chlorophyll b, chlorophyll (a+b) and carotene H. muticus has adapted different adaptive mechanisms to thrive the combination of water scarcity and high irradiance along its life cycle in hyper arid environment.