2008 Joint Annual Meeting (5-9 Oct. 2008): Photosynthetic Contribution of Non-Leaf Structures to C Balance of Soybean and Snap Bean.

While leaves are the primary photosynthetic organs, the photosynthetic contributions of other green structures such as pods, petioles, and stems can play a significant role in crop C balance, especially under defoliation stress.  Therefore, we conducted field experiments on soybean and snap bean canopies, and measured carbon exchange rate (CER) and electron transport rate (ETR) of leaf, pod, petiole, and stem tissues in response to photosynthetic photon flux density (PPFD).   The light response all the way to full darkness, allowed us to compute gross CER and estimate the potential contribution of photosynthesis.  While leaf CER increased up to 1500 umol PPFD m^-2 s^-1, the CER of pods and petioles increased up to 500 umol m^-2 s^-1, i.e., light-saturation point.  The gross CER of pods and petioles approached 8 to 10 umol CO₂ m^-2 s^-1 in full sun, values about one-third that of leaf tissue.  The crucial question is the actual PPFD intensity absorbed by these organs under field conditions, which can probably be predicted better by models than measured (at random points).   Reverse order defoliation versus pod removal studies were made on intact field canopies and canopy CER response to PPFD was determined.  Then, the photosynthetic contribution of pods versus leaves was determined by difference.  A synthesis of CER and ETR of leaf, pod, and petiole will be discussed relative to the canopy CER results.