Effect of transpiration on plant growth
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:


Wikipedia - Mineral
Adsorption
Water ascent in plants: do ongoing controversies have a sound basis?
Chunfang Wei, Ernst Steudle and Melvin T. Tyree, trends in plant
science, September 1999, Vol. 4, No.
9
biology-pages: Transport of Water and Minerals in
Plants