Photosynthetic enhancement, studied in the wavelength pair 650/717 nm, is expressed as rj PSesO + 7X7 — PS717 J p P§650 + 717 — PS650 E6!o<7i7) = — and as E7i7(65o) = — . The maximum PbfiSO PS717 values of enhancement are considered to represent the ratio between the “potential cooperative activities” (PCA) of the two photosystems: PCA7*7 PCA650 rmax rV/AI or»H pmax E717(6 50) – PCA7,7 and t650(7X7) – pca„56 Under conditions of P-deficiency E”|o(7i7) tends to increase. However, when deficient plants have reactivated their photosynthesis (PS) by P-uptake, it decreases. This result demonstrates that far red light to a certain degree overcomes the inhibiting effect of P-deflciency. A tentative explanation is based on the hypothesis that in P-deficient plants PS operates with the less efficient pseudocyclic photophosphorylation at 650 nm as a 12-quanta process, whereas with background light of 717 nm a switch takes place to a more efficient PS, operating with cyclic photophosphorylation. PS, making use of the cyclic pathway, is thought to operate in plants with normal P-content, thus showing the normal high efficiency.