Photosynthate allocation in loblolly pine (Pinustaedd) seedlings as affected by moisture stress
The effects of recent water stress on distribution of 14C-labelled photosynthate in loblolly pine (Pinustaeda L.) seedlings were examined. Leaves on a lateral branch were allowed to photosynthetically fix 14C, and 14C levels in seven biochemical fractions in the source leaves and in each of five other plant parts were determined 72 h later. Regardless of stress level, about 95% of the 14C originally presented was taken up and 60% was respired during the 72 h translocation period. Most 14C was found in exposed or source leaves and in the roots, with very little occurring above the level of the exposed branch. The bud and nonexposed leaves contained 1.2% or less of the seedling's 14C. As preexposure stress level increased, translocation to roots and other sinks decreased. Sugars contained most of the 14C in parts other than the bud and nonexposed leaves. The proportion of 14C in sugars in these parts increased as stress increased. Structural compounds contained less 14C as stress increased, indicating a lasting effect of stress on growth. In all cases, organic acids and amino acids contained very little 14C. Mild stress resulted in increased starch production in the source leaves relative to no stress, while severe stress reduced starch production.