Late season xylem production in an herbaceous legume is associated with enhanced shoot growth and fruit yield
In temperate environments, herbaceous annuals often complete the reproductive phase of their life cycle during the hottest and driest part of the summer. Not only do herbaceous annuals compete with other plants to acquire limited water supplies from the soil, but once water reaches the xylem, they must maintain a continuous water column in their vascular bundles. If embolisms form during critical stages of fruit development, total yields can be significantly reduced. We examined stem tissues during the reproductive phase of soybean (Glycine max L.) to determine if late season xylem formation influenced final fruit yields. Soybeans, as legumes, can produce interfascicular secondary xylem tissues in addition to primary xylem that forms in vascular bundles. Our objective was to determine if variation in secondary xylem formation was present among soybean cultivars and whether such variation could impact fruit production. We measured proportionate allocation among stem tissues during the reproductive growth phase of soybean. Four Virginia soybean cultivars were sampled at the beginning of seed fill and again at completion of reproduction. Stem tissues were measured using histological microscopy and quantitative analysis performed using ImageJ-Fiji (NIH).
Soybean stem tissues showed little variation among cultivars, but we detected significant within-plant proportionate allocation. The middle and upper regions of the main stem and branches were composed of 49% secondary xylem. However, the lowest region of the main stem, above the root-shoot junction, contained nearly 70% secondary xylem, indicating significant investment in secondary growth in the stems of this herbaceous plant. Our measurements of stem tissues at the beginning and end of pod fill (seed weight gain) lead us to important ecological discovery. The total area of secondary xylem in soybean stems increased linearly with stem area (R2=0.99). Additionally, 95% of the variation in fruit dry weight accumulation during the final stages of fruit maturation is related to stem area, and thus secondary xylem growth. Thus, cultivars engaging in late-season xylem production may gain a fitness advantage. New xylem tissues are less likely to show cavitation, thus maintaining constant water supply to growing fruits. These plants would also have greater strength in support of increased fruit weight. We propose that late season secondary xylem production is an adaptation in herbaceous legumes, and its role in other herbaceous plants should be explored.