Anatomical effects of 2,4-DB on tomato internodes

Abstract

Anatomical changes induced in the first internode by an aqueous spray of 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) (0.56 kg acid equivalent per hectare) confirmed the intolerance of tomato to this herbicide. Hypertrophy, hyperplasia, and plastid destruction occurred rapidly in most tissues. Many of the hyperplastic phloem cells differentiated into unusual supernumerary sieve tube members which were shorter and narrower than normal and which approximated the size of the co-differentiating companion cells. The supernumerary sieve tube members usually possessed several sieve plates and formed sieve tubes which did not follow a vertical course. Although obliteration of the supernumerary sieve tube members was stimulated, it was not associated with the formation of necrotic masses. Secondary wall formation was prevented in the protophloem fibers which became multiseptate following the stimulation of mitoses. The cambial initials were converted into a tissue of squat cells with little organization. Xylem which differentiated after treatment lost its normal heterogeneity and became a tissue of squat tracheary elements and parenchyma with scanty secondary thickening resembling wound xylem. Included phloem differentiated from parenchymatous masses within the xylem, and tylosis formation was stimulated. Pith volume increased by hypertrophy unaccompanied by hyperplasia. Although protophloem fibers did not mature in the internal phloem and limited hyperplasia and hypertrophy did occur, the internal phloem was much less affected than the external. Similarities between the anatomical effects of 2,4-DB and those reported for certain growth regulators and pathogens were noted.