The Shoot Plays An Important Role In Potassium Deficiency-Induced Senescence In Cotton (Gossypium hirsutum L.) Seedlings

Premature senescence induced by potassium (K) deficiency has been a major restriction in cotton (Gossypium hirsutum L.) production in China. This study was carried out by grafting to elucidate whether shoot plays a role in senescence in cotton. Expereiment was conducted in gowth chamber, seedlings senescence were induced by K deficiency (0.03 mM). Early senescence cultivar CCRI41 and late senescence cultivar SCRC22 were reciprocally or self-grafted, including one scion onto one rootstock (I-type), one scion onto two rootstocks (A-type), and two scions onto one rootstock (Y-type). We found that CCRI41 maintained its early senescence character (despite affected by SCRC22 rootstock in a less extent), which was suggested by reduced level of chlorophyll, soluble protein and photosynthetic rate in the youngest fully-expanded leaf, regardless of genotypes of rootstocks (I-type), even in A- and Y-type grafting. On the other side, SCRC22 maintained its late senescence character (despite affected by CCRI41 rootstock in a less extent) in all I-, A- and Y-type grafts. Furthermore, we determined cytokinin (zeatin+zeatin riboside, dihydrozeatin+dihydrozeatin riboside,and isopentenyl+isopentenyl adenine) and abscisic acid (ABA) in the youngest fully-expanded leaves and xylem exudate collected either under or over graft union of I-, A- and Y-type grafts. In accordance with the results of chlorophyll level etc. indicating senescence degree, cytokinin concentration in CCRI41 leaves were always lower than those in SCRC22 leaves. In contrast, ABA concentration in CCRI41 leaves were higher than those in SCRC22 leaves. Therefore, we concluded that shoot plays an important role in K deficiency-induced senescence in cotton seedlings probably by sending long-distance feedback signals to regulate cytokinin and ABA in root. Additionally, when cytokinin and ABA level were compared between leaves and xylem exudate (both under and over graft union), it appears that the site for feedback regulation of shoot located above the graft union.