Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: Its effects on morphological and physiological traits of source leaves and whole-plant growth

Abstract

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non-structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5-bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down-regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole-plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus. In this article, we conducted reciprocal grafting experiments using two varieties of Raphanus sativus with high or low hypocotyl sink activities to reveal whether hypocotyl sink activities are regulated by the above-ground parts. We also tested how physiology and morphology of source leaves and whole-plant growth are affected by the hypocotyl sink activities through the accumulation of total nonstructural carbohydrates (TNC) in the source leaves. Our results show that (1) the hypocotyl sink activity is not affected by the above-ground parts and determined autonomously in the hypocotyls; (2) whole-plant growth was highest in the grafted plants with lowest sink activity; (3) the feedback regulation of photosynthesis was not induced by the accumulation of TNC; (4) leaf morphological traits could be also affected by the sink activity through the accumulation of TNC. These findings will help us understand mechanisms regulating the hypocotyls growth and leaf development in relation to sink-source balance.