Photoprotection strategies of 'Cabernet Sauvignon' with different rootstocks under combined high temperature and strong light stress

Abstract

The photosynthetic pigment content, leaf photosynthesis, and chlorophyll fluorescence of ‘Cabernet Sauvignon (CS)' with different rootstocks were measured at 9:00, 15:00, and 21:00 for three consecutive days under high temperature and strong light (HTSL). HTSL led to the inactivation of the CS oxygen evolution complex (OEC), thylakoid dissociation, damage to the receptor and donor sides of photosystem II (PSII), and obstruction of electron transfer between PSII and photosystem I (PSI), which reduced photochemical activity. After grafting, rootstocks decreased the L-band, K-band, and G-band; increased the photosynthetic reaction center's performance index (PI_abs), the number of OEC centers, and electron transfer between PSII and PSI; and maintained the stability of electron transfer and the energy distribution of the CS leaf optical system under HTSL. Under the combined stress, the PI_abs and PI_total were much higher for the CS/HTH combination than for other combinations (9.55 and 8.52, respectively). As a rootstock, HTH actively down-regulated the capture of light energy by CS, and enhanced the photosynthetic performance of CS PSII and the electron transfer from primary quinone electron acceptor (Q_A) to secondary quinone electron acceptor (Q_B), which promoted a balanced energy distribution, and strengthened the connectivity between PSII and PSI. The rapid oxidation fraction (δMRfast/MRo), reduction fraction (δMRslow/MRo) and redox rate (Vox and Vred) of plastocyanin PC⁺ and P₇₀₀⁺ were also enhanced in this combination. TOPSIS entropy weight analysis proved that HTH as a rootstock enhanced the ability of CS to resist combined HTSL stress.