Plasmodesmatal distribution, structure and frequency in relation to assimilation in C3 and C4 grasses in southern Africa

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Abstract
This paper addresses plasmodesmatal distribution and frequency in the leaf-blade bundles of four southern African grasses — one C3, and one each of the NAPD-malic-enzyme (ME), NAD-ME and phosphoenolpyruvate carboxykinase (PCK) C4 photosynthetic subtypes and, additionally, relates the calculated plasmodesmatal frequencies to the photosynthetic carbon-fixation rate. Plasmodesmata are, in all instances, constricted where they traverse suberin lamellae at the Kranz mesophyll-bundle sheath (KMS-BS), the bundle-sheath-vascular parenchyma (BS-VP), or the bundle-sheath-mestome-sheath interfaces (BS-MS). Frequency studies clearly show that plasmodesmata are most numerous at the KMS-BS, BS-MS and BS-VP interfaces, (31.9–76.8% of the total) and that their numbers decrease rapidly with increasing proximity to both thin- and thickwalled sieve tubes. In Themeda triandra var. imberbis (Retz.) A. Camus and Bromus unioloides H.B.K. thickwalled sieve tubes have few connections with vascular parenchyma cells and are, to all intents and purposes, almost totally isolated from the rest of the vascular tissue, indicating that the loading pathways for these sieve tubes are predominantly apoplastic. Although decreasing plasmodesmatal frequencies indicate that loading of assimilate may become progressively more apoplastic with increasing proximity to the sieve tubes, a symplastic route to the thin-walled sieve tubes cannot be ruled out. Studies of net assimilation rate indicate a good correlation of photosynthetic rate with the photosynthetic type (C3, C4 NADP-ME, C4 PCK, and C4 NAD-ME); furthermore, the lowest plasmodesmatal frequencies were associated with C3, and the highest with C4 NAD-ME types.