Control of fusiform initial orientation in the vascular cambium of Abies balsamea stems by indol-3-ylacetic acid

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Abstract
Fusiform initial reorientation, deduced from changes in tracheid arrangement, was investigated in helical bridges left after Abies balsamea stems were girdled. The first responses included an increase in the number of S- and Z-oriented anticlinal divisions without any change in their ratio, a decrease in fusiform initial length, and the formation of traumatic resin canals. Subsequently, normal tracheid differentiation resumed, the frequency of anticlinal divisions further increased, and there was an increase in the number of oriented intrusive growth events. The orientations of the anticlinal wall and of the intrusive growth were predominantly in the direction that eventually would result in the fusiform initials being aligned parallel to the bridge angle. The application of exogenous indol-3-ylacetic acid to the upper edge of a helical bridge inhibited fusiform initial reorientation, mainly by decreasing the frequency of oriented intrusive growth events. However, immediately below the application point, exogenous indol-3-ylacetic acid also induced the fusiform initials to begin temporarily to realign so as to be perpendicular to the bridge angle. The transport of a pulse of [1-14C]indol-3-ylacetic acid applied to the shoot apex was inhibited 1 day after girdling in both helical and longitudinal bridges, particularly the former, whereas it was inhibited only in helical bridges at the end of a subsequent 11-week growing period. The degree of inhibition in the helical bridges was greater the day after girdling than at the end of the growing period, during which interval fusiform initial reorientation occurred at the bottom of the bridge width. The results support the conclusion that the orientation of fusiform initials is parallel to the direction of indol-3-ylacetic acid transport.