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We established a greenhouse study on five different Venezuelan soils to evaluate the external phosphorus (P) requirement of cowpea (Vigna unguiculata var. Tuy). The external requirement for the various soils was almost identical (approximately 1.00 μg/ml) and independent of edaphic factors, such as texture and mineralogy. The values differ greatly from those previously reported for the same species growing in a high P-fixing Hawaiian soil. We propose that pot effect, variety differences, and the presence (or absence) of my-corrhizal associations influence the differences between these external P requirements.

Evidence for the operation of CAM in the deciduous climber, Cissus trifoliata L., was obtained in field and laboratory studies. Under natural conditions, diurnal oscillations of titratable acidity and colorimetric measurements of night CO2 fixation, determined for a period of two and a half years, suggested that acid accumulation was related to plant water status, assessed through the daily courses of stomatal resistance and xylem water potential during dry and rainy seasons. These findings were confirmed by gas exchange studies under controlled conditions which showed that the plant fixed all its CO2 during the day when it was well irrigated; as water stress increased, dark CO2 uptake gradually replaced fixation during the day until the plant only performed dark fixation. In severe water stress, even the rate of the latter process decreased until leaves fell.