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?
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:
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.
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.