Signals from mature to new leaves

Abstract

Stomata are microscopic pores on the surfaces of leaves, the number and density of which vary in response to changes in environmental conditions, such as carbon dioxide concentration and light. We show here that mature leaves of Arabidopsis thaliana detect and transmit this external information to new leaves of the same plant, producing an appropriate adjustment of stomatal development. As CO₂ concentration controls both stomatal opening¹ and number^2,3, and stomatal numbers also increase with higher light intensity⁴, the large gradients of CO₂ and light found within plant communities⁵ have the potential to influence stomatal development.

Main

Every year, 40% of the CO₂ in the atmosphere passes through stomata⁶. Stomatal numbers modify both photosynthesis and efficient use of water¹, and so any change in stomatal numbers⁷ in response to CO₂ and light can influence photosynthesis and atmospheric CO₂ concentration. The response mechanism described here may exert global effects that are not currently included in canopy and vegetation models of increasing atmospheric CO₂ levels.

A genetic component in the response of stomatal development to increasing atmospheric CO₂ concentration has been identified⁸ and the control of stomatal development by CO₂ concentration is known to occur during early leaf development^9,10, when ambient CO₂ concentrations may not be accurately detectable by a new leaf sheathed by antecedent leaf primordia¹¹. We therefore tested whether CO₂ concentration can be detected by mature leaves in open ambient conditions, which might then transmit a signal to induce an appropriate developmental response by the stomata of new leaves. Our experimental design using the model plant Arabidopsis thaliana is shown in Fig. 1a.

Figure 1: Mature leaves detect changes in CO₂ concentration and elicit a stomatal response in developing leaves.

a, Leaf-cuvette experiment. Plants of Arabidopsis (Columbia, Col-0) were grown for 4 weeks under ambient CO₂ (360 p.p.m.) until leaf insertions 5 to 13 had developed. These mature leaves were enclosed in transparent airtight cuvettes under CO₂ concentrations of either 720 or 360 p.p.m. Subsequent leaf insertions developed outside the cuvette under ambient CO₂. Plants were maintained in cuvettes for 7 to 9 days until the next five leaf insertions had matured, the last three of which were investigated for stomatal density (no. of stomata per mm²) and index ((no. of stomata/no. of stomata + no.of epidermal cells) × 100). b, (left) Stomatal index and density for new leaves (insertions 16 to 19) under ambient CO₂ when mature leaves (insertions 5 to 13) inside cuvettes are supplied with increased CO₂ (720 p.p.m.). Both stomatal density and index are reduced in new leaves if the supply of CO₂ is increased to the mature leaves. Right, reverse experiment: mature leaves inside cuvettes are under CO₂ at 360 p.p.m.; external CO₂ is 720 p.p.m. Stomatal density and index increase in response to the decreased CO₂ around the mature leaves. c, Effect on stomatal index of new leaves of reducing light incident on mature leaves by using neutral density filters (shade) or transparent filters (full light). Stomatal index of new leaves is reduced when mature leaves are shaded. *** P < 0.0005; * P < 0.05; bars, s.e.m.; n = 150.

Expanding leaves outside the cuvette (ambient CO₂, 360 p.p.m.), with mature leaves exposed to a high concentration of CO₂ inside the cuvette (720 p.p.m.; Fig. 1a), developed with a reduced stomatal index and density (Fig. 1b, left) compared with control plants grown entirely at ambient CO₂. Reversing the cuvette arrangement so that the mature leaves were exposed to 360 p.p.m. and the expanding leaves to 720 p.p.m. CO₂ resulted in a complete reversal (increase) of the stomatal index and density of the new leaves (Fig. 1b, right).

In these experiments, both abaxial (upper) and adaxial (lower) leaf surfaces responded in a similar manner, indicating that CO₂ concentration is detected by mature leaves which signal to expanding leaves to induce an appropriate developmental response. To our knowledge, this is the first demonstration that mature leaves both detect CO₂ concentration and transmit a long-distance signal that controls stomatal development in young leaves. Expanding leaves appeared to have no capacity to detect ambient CO₂ concentration or to respond to it directly by altering stomatal initiation; no cuvette effect was found to contribute to the responses.

This mechanism of CO₂ detection and signalling could enhance or optimize performance in plant communities. For example, the reduction in stomatal index and density with CO₂ enrichment enhances water-use efficiency^3,12, but such a response will be much less effective¹³ when leaves develop in the shade of other plants. We therefore tested whether the mechanisms controlling the response of stomatal initiation to CO₂ might also be accompanied by another that responds to light, reducing the initiation of stomata with increasing shade.

We have previously shown that there is a positive relation in Arabidopsis between irradiance and both stomatal index and density. Fully grown leaves were now placed in shaded light, with the expanding leaves under full light. The stomatal index (Fig. 1c) and density (results not shown) of new leaves were reduced, as if they had developed under shade conditions. We conclude that long-distance signalling must also be involved in controlling the response of stomatal development to light, which supports the idea that there is an ecologically important link between the responses evoked by light and by carbon dioxide.