Photosynthesis results in 2H-depleted carbohydrates, but why?

When you grow plants in the light, the hydrogen isotopic composition (δ²H) of plant compounds such as cellulose show lower δ²H values (are ²H-depleted) relative to plants grown heterotrophically in the dark. Therefore, it is logical to assume that photosynthetic reactions introduce ²H-depleted hydrogen atoms into carbohydrates. But where in the C reductive pathway (Calvin-Benson-Bassham cycle, CBB) does this occur? Or more interestingly, can we interpret the degree of ²H-depletion of plant compounds with respect to this key reaction(s)? With the recent resurgence of studies offering hydrogen isotopes as a new proxy for plant central carbon and energy metabolism, such a fundamental question seems pertinent to answer.

We 1) examine the stereospecific mechanism of hydride transfer via NADP(H) catalyzed by oxidoreductases (ferredoxin-NADP⁺ reductase, glyceraldehyde 3-phosphate dehydrogenase) as a key reason why photoproduced NADPH is not directly the source of ²H-depletion of autotrophically produced carbohydrates, 2) reconcile the site-specific deuterium abundance pattern differences between C₃ and C₄ (NADP-ME) species of hydrogen bound to position C-4 in glucose, and 3) urge greater investment in position-specific and complimentary metabolomic analyses to progress the development of hydrogen isotopes as a metabolic proxy.