Alternating light-induced stomatal characteristics and their effects on photosynthetic carbon assimilation and water use efficiency among 15 Cycad species.

To analyze the stomatal response of Cycad species during dynamic light induction and their effects on photosynthetic carbon assimilation and water use efficiency, we examined the stomatal dynamics and structural cha-racteristics of their pinnae in 15 Cycad species from Qingxiu Mountain Homogeneous Garden during continuous weak-strong-weak alternating light induction. The results showed that, from low to high light intensity, the duration for the photosynthetic rate (<i>A</i>) to reach 95% of its maximum value (<i>A</i>_95t) ranged from 14 minutes (<i>Cycas pachypoda</i>) to 38 minutes (<i>Macrozamia moorei</i>). The ascending rate of stomatal conductance (<i>g</i>_s) was slightly slower than that of <i>A</i>. After continuous strong light exposure, the <i>A</i> value of <i>Cycas panzhihuaensis</i> was the highest and that of <i>Cycas bifida</i> was the lowest. During the light transition from strong to weak, <i>A</i> dropped sharply, and then decreased slowly to the lowest value. Across the 15 examined cycad species, the decline rate of <i>g</i>_s during the decreasing light intensity stage was less steep than the rise in <i>g</i>_s during the intensifying light stage. For stomatal traits, the maximum <i>g</i>_s under normal condition was positively correlated with <i>g</i>_s decline rate (Sl_{max, d}) during the decreasing light intensity stage, and the constant characterizing <i>g</i>_s increasing rate during the weak to strong light treatment was positively correlated with <i>A</i>_95t. The stomatal opening duration (λ_i) was negatively correlated with the stomatal closing duration. The time constant of <i>g</i>_s descending to steady state was negatively correlated with Sl_max,d. Under the stomatal conductance, there was a significant positive correlation between stomatal length and <i>λ</i>_i. Results of principal component analysis showed that maximum photosynthetic rate, <i>A</i>_95t, stomatal limit value and stomatal length contributed the most to the variations in stomatal and photosynthetic traits of Cycads. Our results suggested that the photosynthetic potential of Cycad species was lower, but was sensitive to dynamic light resources, through efficient stomatal regulation. Cycad species could optimize CO₂ acquisition and maintain high water use efficiency, which is commonly observed in understory plants.