Effect of vegetational shade and its components on stomatal responses to red, blue and green light in two deciduous tree species with different shade tolerance

• Vegetational shade enhances shade-tolerator’s stomatal red and blue light responses.
• UV attenuation has almost as strong an effect on stomata as full vegetational shade.
• Neutral shade (decreased irradiation) has no effect on stomatal light responses.
• Shade-avoider’s stomatal light responses are not modified by growth environment light.

Stomatal responses to red, blue and green light were measured in shade-midtolerant Nothofagus alpina and shade-avoidant Betula pendula seedlings grown in full sunlight (control, C-set), simulated vegetational shade (IUW-set), or with a single component of vegetational shade: decreased light intensity (I-set) or attenuated UV irradiance (U-set). In N. alpina, stomatal opening response to red light switch-on was stronger in IUW-set and U-set, than in C-set. The addition of medium blue light into the red (stomatal opening) induction light increased the stomatal response, and the stomata also responded sensitively to weak blue light in the IUW- and U-sets. The addition of green light into the red induction light did not change the stomatal response in any set. Yet in the U-set, the addition of green light into the red + (medium) blue or weak blue induction light decreased the stomatal response. Thus, vegetational shade had increased the photosynthesis-mediated stomatal light response, activated an additional stomatal response through blue light receptors, and let stomata insensitive to green light. The attenuated UV irradiance had the same effects as vegetational shade on the stomatal photosynthesis-mediated response and the additional blue light receptors-mediated response, but it also activated the green light reversal of blue light response. The stomata of leaves grown under decreased irradiance exhibited the same stomatal responses as the full-light-grown leaves. In B.pendula, stomatal responses to all the different types of light were remarkably weaker than those in N.alpina, and did not differ between themselves or between the different shade treatments. Thus, the modulation of stomatal light sensitivity participates in shade acclimation in shade-tolerating, but not in shade-avoiding deciduous tree(s).