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Title: Do stomata respond to relative humidity?

Introduction:

In recent reports it has been concluded that stomata respond to relative humidity. However, Aphalo andJarvis (1990) suggest that particular notion was made on the wrong basis. For instance, previous authors

were equating correlation with causation. On the other hand, they also suggest two questions that would

give a better understanding of the process. (1.) Do stomata respond to both humidity and temperature?(2.) Do the responses of stomata to humidity and temperature interact in such a way that h s (relative

humidity at the leaf surface) is a more appropriate variable than X  s (mole fraction of water vapor at the

leaf surface) and yields a simpler description of the compound response.

The objective of the study was to answer the following questions: (1.) Do stomata respond to humidityand temperature. (2.) Do these responses interact in such a way that relative humidity at the leaf surface

is a more appropriate variable than water vapor saturation deficit at the leaf surface and yield simpler 

description of the compound response? Experiments were carried out to test (1.) whether  g  s (stomatal

conductance)responds linearly to D s and h s at a fixed temperature, and (b) whether  g  s changes with T 1, andwhether h s a more appropriate measure of humidity than D s.

Material and methods:

Treatment used Two different sets of plants were used in two replicates of the entire experiment. The plants were grown

12-cm-diameter plastic pots filled with a peat-perlite-vermeculite mix. The plants were transferred to a

growth chamber, and then fertilized with slow release granules (N = 14%, P = 6.1%, K = 11.6%, weight

for weight and mirconutrients at 2.5g per pot). Afterwards, they were fertilized weekly with liquidfertilizer (N = 14%, P = 1.7%, K = 11.6%, weight for weight, and micronutrients at 0.5 cm3 per pot).

Procedures

 Plant material 

1. Coutinho plants (Hedera helix subsp. Canarienis (willd.) were grown in a heated greenhousefrom cutting.

2. Set A of the plants were moved from a greenhouse to a growth cabinet at 20 °C 10 days

 before the experiment commenced, with no humidity control, and a photoperiod of 12h at

200µmol m-2 s-1 at leaf level from fluorescent tubes.3. Set B were kept for 2.5 months in a growth chamber at 20 °C , humidity at 30-60%, and a

 photoperiod of 12h at 400 µmol m-2 s-1 at leaf level from metal halide lamps.

Gas exchange measurements1. A computer-controlled, open-path gas exchange system were used which made it possible to

control and measure the water vapor and CO2 mole fraction at the leaf surface, and within the

leaf.

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2. Measurements of gs were taken throughout the day to under constant conditions to know

when it was safe to assume that time of day was not a variable to which stomata were

responding.3. Measurements of C were taken steadily after a change in humidity that after a change in

temperature.

 Experiments

1. The response of gs were measured to either h s or  D s at constant T1 of 20 °C, either a constant

h s of 60% or a constant Ds of 10 mmol  mol-1.2. All the experiments were carried out at a Cs (CO2 molar fraction at the leaf surface) of 350

µmol m-2 s-1.

3. In addition, the whole set of experiments were replicated at two quantum flux densities-200

and 340 µmol m-2 s-1 – on the plants grown under the different conditions.

Results/finding and implications:

 Response of g  s to humidityIn the Hedera helix subsp. responses of gs to humidity was observed that under constant temperature T1

and I was linear function of Ds and h s . Also, the proportional of the variation in g s that was explained bya linear regression model was 90% or more for individual plants.

 Response of g  s to temperatureReponses to temperature under constant D s was different to that under constant h s. In plants from set A,

Aphalo and Jarvis (1990) conclude that there was no response to T I in the range 15-28 °C under constant

 D s, however, when h s was kept constant, gs decreased with increasing TI and  D s. On the other hand, in

 plants from set B, there was a different and significant effect of T1 under both humidity treatments, and theeffect of T1was such that g  s was higher at lower temperatures.

Conclusions:Aphalo and Jarvis (1990) concluded that stomata so respond to humidity, and usually respond to

temperature as well. Additionally, they established that these responses do not interact in a way that

makes hs a more appropriate way of expressing humidity than x s.