Eccentric growth and growth stresses associated with negative gravitropism in inclined stem of gymnosperm Cycas revoluta

Abstract Trees typically form reaction wood in response to stem inclination, where significant mechanical stress develops during secondary growth. This process contributes to the development of negative gravitropism. Cycads, as gymnosperms, exhibit morphologically unique features; however, their formation of reaction wood and the mechanisms underlying negative gravitropism remain unclear. This study investigated eccentric growth and growth stress distribution in inclined stems of Cycas revoluta to elucidate the mechanisms of negative gravitropism and assessed anatomical characteristics of reaction wood formation, microfibril angle ( MFA ), and crystallinity index. Eccentric growth of vascular bundles was observed on the lower side of inclined stems, although no typical compression wood was identified. The radial distribution of released strain did not differ between inclined and vertical stems. MFA in the vascular bundles showed no difference between the upper and lower sides; however, cellulose crystallinity increased in the outermost vascular cylinder, especially on the lower side. Based on these results, it was concluded that negative gravitropism in C. revoluta is primarily achieved through primary growth, which controls the direction of stem elongation, rather than through the formation of reaction wood related to secondary growth, as observed in other gymnosperms.