Summary• Stomatal formation is affected by a plant's external environment, with longdistance signaling from mature to young leaves seemingly involved. However, it is still unclear what is responsible for this signal. To address this question, the relationship between carbon isotope discrimination (Δ) and stomatal density was examined in cowpea (Vigna sinensis).• Plants were grown under various environments that combined different amounts of soil phosphorus (P), soil water, and atmospheric CO 2 . At harvest, stomatal density was measured in the youngest fully expanded leaf. The 13 C :12 C ratio was measured in a young leaf to determine the Δ in mature leaves.• Results indicated that stomatal density is affected by P as well as by amounts of water and CO 2 . However, stomatal responses to water and CO 2 were complex because of strong interactions with P. This suggests that the responses are relative, depending on some internal factor being affected by each external variable. Despite such complicated responses, a linear correlation was found between stomatal density and Δ across all environments examined.• It is proposed that the Δ value is a good surrogate for the long-term mean of the intercellular (C i ) to the atmospheric (C a ) CO 2 concentration ratio (C i : C a ) and may be useful in understanding stomatal formation beyond complicated interactions.

The results suggest that the efficiency of root morphological plasticity is largely determined by the size of the P-enriched patch. Furthermore, the results imply a novel aspect of P-uptake physiology that roots in heterogeneous P cannot demonstrate their potential capacity, as would be observed in roots encountering P continuously; this effect is probably mediated by an internal root factor.

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The cultivars of NERICA (New Rice for Africa), which are characterized by early maturity and high yield potential under rainfed conditions, have the potential to increase rice production in Tanzania, where rice cultivation is greatly affected by a short rainy season. Trials were conducted in Zanzibar to examine the yield performances of 14 NERICA cultivars at five locations during the long-rains season (Masika) and at another five locations during the short-rains season (Vuli). The NERICA cultivars produced significantly higher yields than local cultivars at five locations. Yields of 12 NERICA cultivars were associated with rainfall (R 2 = 0.367 to 0.732) such that they yielded well during Masika (109 to 343 g m-2) and poorly during Vuli (11 to 68 g m-2). Spikelet number per panicle and percentage of filled spikelets (% filled spikelets) accounted for 70 to 90% of the yield variation in all cultivars, suggesting that yield was determined mainly during the later part of the growth period. In some cultivars, yield was associated with rainfall during the later part of the growth period but the yield of the remainder was associated with rainfall during the early part. A selected group of farmers, extension workers and researchers evaluated grain quality. Some cultivars scored well, especially NERICA 1. We conclude that NERICAs are generally suitable for production during Masika and that NERICA 1 especially should be promoted due to its high grain quality. However, for double cropping of NERICAs, measures must be implemented for increasing or maintaining the water status of the soil during Vuli.