Nutrient Homeostasis, C:N Ratio, and Oil Content in Cuphea Seed.

Interrelationships between densities of 14 nutrients, C:N and N:S ratios, and oil and protein contents were assessed in seeds of indeterminate Cuphea plants subjected to no (GDD0) or to source-sink manipulation by removing the top 25% of plant foliage at 100 and 200 growing degree days (GDD1 and GDD2, respectively) after flower initiation. Most traits were impacted by treatments and their interaction with years; and more so by number of seeds per capsule than by single seed weight or the packaging cost of seed (i.e., ratio of capsule tissue weight to seed weight). Carbon and N allocations responded in different quantitative manners to treatments over years; nevertheless, large variances in C and N contents and C:N ratio (R2=0.70, 0.73, and 0.82, respectively), but not in N:S ratio (R2=0.21) were explained by all factors.  Carbon:nitrogen ratio and oil content, but not protein content, can be predicted as functions of nutrient densities with acceptable, but wide range of reliabilities (R2); the GDD1 treatment consistently resulted in slightly increased oil content and in marked improvements in the reliability of its estimates. Oil and protein contents in Cuphea seed, being positively and negatively correlated with C:N ratio, respectively, may not be easily and concomitantly improved. An improved N:S ratio (<12:1), which ranged from 10.4:1 to 19.8:1 in this study, can be achieved through proper S nutrition and could lead to larger oil content.