Response of Bean (Vicia faba L.) Plants to Low Sink Demand by Measuring the Gas Exchange Rates and Chlorophyll a Fluorescence Kinetics

BACKGROUND: The decline of photosynthesis in plants under low sink demand is well known. Previous studies focused on the relationship between stomatal conductance (g (s)) and net photosynthetic rate (P (n)). These studies investigated the effect of changes in Photosystem II (PSII) function on the P (n) decline under low sink demand. However, little is known about its effects on different limiting steps of electron transport chain in PSII under this condition. METHODOLOGY/PRINCIPAL FINDING: Two-month-old bean plants were processed by removing pods and flowers (low sink demand). On the 1(st) day after low sink demand treatment, a decline of P (n) was accompanied by a decrease in g (s) and internal-to-ambient CO(2) concentration ratio (C (i)/C (a)). From the 3(rd) to 9(th) day, P (n) and g (s) declined continuously while C (i)/C (a) ratio remained stable in the treatment. Moreover, these values were lower than that of control. W(k) (a parameter reflecting the damage to oxygen evolving complex of the donor side of PSII) values in the treatment were significantly higher than their corresponding control values. However, RC(QA) (a parameter reflecting the number of active RCs per excited cross-section of PSII) values in the treatment were significantly lower than control from the 5(th) day. From the 11(th) to 21(st) day, P (n) and g (s) of the treatment continued to decline and were lower than control. This was accompanied by a decrease of RC(QA), and an increase of W(k). Furthermore, the quantum yield parameters φ (Po), φ (Eo) and ψ (Eo) in the treatment were lower than in control; however, C (i)/C (a) values in the treatment gradually increased and were significantly higher than control on the 21(st) day. CONCLUSIONS: Stomatal limitation during the early stage, whereas a combination of stomatal and non-stomatal limitation during the middle stage might be responsible for the reduction of P (n) under low sink demand. Non-stomatal limitation during the late stages after the removal of the sink of roots and pods may also cause P (n) reduction. The non-stomatal limitation was associated with the inhibition of PSII electron transport chain. Our data suggests that the donor side of PSII was the most sensitive to low sink demand followed by the reaction center of PSII. The acceptor side of PSII may be the least sensitive.