Table II.

Labeling of DMSP synthesized by S. alterniflora leaf sections given [¹³CH₃,C²H₃]SMM

Precursor	Relative Intensity of DMSP Ions
	13CH3,13CH3 (m/z 137)	13CH3,C2H3 (m/z 139)	C2H3,C2H3 (m/z 141)
	%
[13CH3,C2H3]SMM	−0.27	7.63	0.10
Unlabeled SMM	−0.01	0.09	0.02
lsd0.05	0.45	1.80	0.64

Leaf sections (0.2 g fresh weight) were incubated with 5.0 μmol of [¹³CH₃,C²H₃]SMM (experimental samples) or unlabeled SMM (controls) for 24 or 48 h. The intensities of the labeled DMSP ions are expressed relative to that of endogenous unlabeled DMSP (m/z 135, 100%); the endogenous DMSP level was 23 ± 1 μmol g⁻¹ fresh weight (mean ± se, n = 4). All signals were corrected for background noise from the matrix, and those at m/z 141 were further corrected for the contribution from the natural-abundance isotope peaks associated with [¹³CH₃,C²H₃]DMSP (m/z 139). Because labeled DMSP yields at 24 and 48 h did not differ significantly, results for both times were pooled. Data are means from eight experimental samples and four controls, and were subjected to analysis of variance. That all of the values for control sections given unlabeled SMM are close to 0 confirms the validity of the corrections applied.