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It is said that only about 1-5% of water used by plants is used in photosynthesis while the rest is transpired. Suppose that a plant grows in an environment where the temperature is not too high but humidity is very high, e.g. in the shadow of a forest. Would the plant grow faster if the humidity was lower? To what extent is the photosynthesis limited by reduced transpiration?

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Simple Answer: There is no simple answer, because transpiration is just one of many factors that affect plant growth, and even in controlled experiments, the conclusions deviate according to different species.

Background: First of all, we know that increase in atmospheric humidity leads to less transpiration. A graph between transpiration and humidity, as given by Wikipedia, is as:

graph

There have been some experiments regarding effect of transpiration on growth of plants. In an experiment, Mishra et al and team tried to get answers by reducing transpiration of Lycopersicon esculentum (tomato). They sprayed chemicals like 2-chloroethyl trimethylammonium chloride and 8-hydroxyquinoline 7 times a week and observed a reduction of as much as 80% in stomatal opening, early flowering, but no overall reduction in yield. Also, wilting was delayed for up to 8 days. Apart from this, TNAU concludes that very high and very low relative humidity severely affects grain yield. They reported reduction of 144 kg/ha yield by 1% increase in mean monthly RH.

Though it is quite common knowledge about what role transpiration plays in plant's metabolism, yet I want to include this part from University of Nebraska for the sake of convenience:

Plant growth and development relies on water for transpiration, photosynthesis, and respiration. The unique ability of water to regulate temperatures, dissolve molecules of life, and allow gas exchange, is essential for all life on earth. Transpiration is essential for evaporative cooling, CO2 acquisition, maintaining plant turgor, and mineral nutrient uptake.

Now, when we talk about transpiration along with photosynthesis, then we see formation of a positive feedback loop. When plant starts photosynthesis, it opens up stomata for exchange of CO2 and O2 with atmosphere. This also allows water to evaporate through stomata and causes transpiration, which slowly dries up the cells. To overcome this, the plant takes up more water from roots to maintain turgor pressure inside cells, which in turn leads to more transpiration. Thus, a continuous loop is formed by which photosynthesis enhances transpiration4. Also, when we talk about how photosynthesis is dependent on transpiration, then again we have no simple answer as it depends on many other factors too. For example, in a research, Graham et al concluded that the answer to this question also depends on whether the plant is grown hydroponically on in soil.

From the above examples, we can conclude that high humidity indeed causes reduction in growth. So how did I conclude that there is no simple answer? Well, there are counterexamples also present. In their research, Ford et al and colleagues found an overall increase in plant growth on increasing humidity. They found that when external humidity was high, then there was overall increase in growth in sugar beet, kale and wheat. However, water loss per plant depended on vapor pressure deficit of air, leaf area and species. Also, water loss per unit leaf area was less for wheat than sugar beet and kale.

Thus, in short, plant growth involves so many factors, along with transpiration, that it is almost impossible to give a simple answer to this question, and it might even take a booklet to just list all the factors involved!

EDIT: As you asked in comments, I searched for more research papers on the relation between cultivation method and transpiration rate, and came up with two conclusions. First, as I am saying already, there has not been reported a straight forward relation between the two, and second, there has not been much research too on this subject. However, I succeeded in finding a couple of papers. One paper has indicated the presence of a direct relation between transpiration rate and the amount of water present in soil, be it irrigation or any other process7. On the other hand, another paper has concluded that in comparison to plants grown hydroponically, plants grown in soil require a larger root surface area to maintain the same transpiration rate and growth rate as that of plants grown hyrdropoincally8. Again, its quite difficult to draw any conclusion from these papers. So, I'll try to add some more papers as I find them.

References:

  1. Mishra D, Pradhan GC. Effect of Transpiration-reducing Chemicals on Growth, Flowering, and Stomatal Opening of Tomato Plants . Plant Physiology. 1972;50(2):271-274

  2. Agrometeorology - Relative Humidity and Plant Growth - TNAU Agritech Portal

  3. Plant Growth Processes: Transpiration, Photosynthesis, and Respiration; David R. Holding, Anne M. Streich

  4. Transpiration - Kimball's Biology Pages

  5. The effect of increased transpiration on photosynthesis of corn part II. Comparisons between hydroponically and soil-grown plants; M.E.D Graham

  6. Effects of Atmospheric Humidity on Plant Growth; Margaret A. Ford, Gillian N. Thorne

  7. Silvestre, J. and Ferreira, M.I. 2000. EFFECTS OF IRRIGATION ON TRANSPIRATION AND WATER RELATIONS OF VINEYARDS, IN THE TEJO VALLEY (CENTRAL PORTUGAL). Acta Hort. (ISHS) 537:305-312

  8. The effect of root surface area on the water uptake and transpiration rate of maize (Zea mays) plants (1998) He, W.S. Nakayama, K. Yu, G.R.

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  • $\begingroup$ The references you provided are very good. I especially like the reference about the difference between hydroponically and soil-grown plants. I don't have access to the full paper though. Do you think reduction of photosynthesis as reaction to reduced transpiration is less for hydroponically grown plants because of the increased water availability in hydroponics? If you had any further references about interactions between cultivation methods and transpiration that would be very helpful. In other words which cultivation methods are relatively unaffected by high humidity and its consequences. $\endgroup$ – CuriousIndeed Mar 5 '17 at 9:29
  • $\begingroup$ Actually my former comment is wrong. Increased transpiration was associated by REDUCED photosynthesis in the paper by Graham. Still, does this have to do with higher water availability in hydroponics? $\endgroup$ – CuriousIndeed Mar 5 '17 at 9:40
  • $\begingroup$ @L-Theanine I've added a couple more papers. Are those satisfactory? $\endgroup$ – another 'Homo sapien' Mar 6 '17 at 8:33
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    $\begingroup$ You seriously made an effort to elaborate on all the facets of this question. Thank you! $\endgroup$ – CuriousIndeed Mar 6 '17 at 9:34
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Mineral nutrients are conveyed up the tree via the xylem by transpiration. The phytohormone cytokinin that releases buds/shoots is synthesized in the root tips and is also conveyed via the transpiration stream. Transpiration is driven by the water potential of the air --> ln(rH); transpiration stops when the relative humidity is 100%.

Even though some species are able to generate root pressure that will force water plus minerals plus cytokinin up the xylem, growth must, nevertheless, be faster at moderate relative humidity than when rH is near 100% (because of the reduced availability of minerals for tissue synthesis in the lateral and apical shoot meristems).

Fertilizing will make minimal or no difference at 100% rH as there is not transport up the plant by transpiration. However, mineral nutrition could be via 'foliar feeding' wherein a fertilizer solution is applied to the leaves could be partially effective in overcoming the mineral nutrition issue of 100% rH. Minerals are then distributed by being actively loaded into/out of the phloem tubes.

Photosynthesis would be unaffected at 100% rH since all tissues would be hydrated. The consumption of water by PhotoSystem II, however, is the only mechanism that could cause some transport of water and minerals through the xylem when rH = 100% aside from root/stem pressure generated by osmosis.

REFERENCES:

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  • $\begingroup$ Thanks for your answer. But when humidity is high the stomata are more open than when humidity is low, therefore enabling more CO2 to be acquired. Combine this with the fact that plants are mostly composed of carbon from CO2 one might wonder how large the effect of reduced mineral availability really is. Can you provide a scientific reference? $\endgroup$ – CuriousIndeed Mar 2 '17 at 21:02
  • $\begingroup$ Acquiring more CO2 is pointless if you don't have the light energy to convert it to sugars, so if high humidity is associated with shade (as you explicitly said in your example) that's probably going to be the main block to growth. Add to that that trees that grow in the shade don't want to grow fast; growing fast is useful for the competition for light, and shade-tolerant trees have usually sacrificed that in favor of resilience/durability, so their carbon allocation scheme may be different (in favor of denser wood for example). $\endgroup$ – Oosaka Mar 3 '17 at 6:40
  • $\begingroup$ IIRC, potassium is required to keep the stomata open, but if they are already in the leaf it is not an issue. The energy currency of 'all' plants is ATP. If no more phosphorous is acquired then the metabolic capacity remains fixed. More chlorophyll cannot be made without more nitrogen and a dab of magnesium. Photosystem 2 requires iron as well. New leaves require lots of nitrogen, potassium, and phosphorous. Without more, the carbon fixing capacity stays fixed. While it may be that the plant can keep growing by simply fixing CO2, the rate of growth is less at 100% rH than at lower humidity. $\endgroup$ – Jim Young Mar 3 '17 at 9:58

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