ALBERT

Notes: Summary Cowpeas (Vigna unguiculata [L.] Walp.) were subjected to six irrigation treatments and two levels of soil nitrogen during two seasons under field conditions. Withholding irrigation during the vegetative stage in a rain-free environment following pre-irrigation resulted in lower water use (−11 to −20%) but negligible effects on seed yields (−2 to +3%) providing the irrigation interval during flowering and pod filling was not too great. Lengthening the irrigation interval resulted in less vine elongation and less shoot biomass production. Significant reductions in seed yield were observed only with the longest irrigation interval. The higher level of soil nitrogen resulted in small or negligible increases in shoot biomass production and seed yield. It is proposed that water use of cowpeas can be reduced while maintaining seed yields by planned-water-deficit irrigation. Major increases in water use efficiency may be achieved by withholding irrigation from plant emergence to the first appearance of macroscopic floral buds, providing a moderate supply of water is present in the soil profile and no precipitation occurs. Seed yields will be maintained if cowpeas are irrigated during flowering and podfilling so that there is only a mild to moderate check on plant growth. Maximum shoot biomass and hay production would require frequent irrigation after the appearance of macroscopic floral buds. Methods for guiding planned-water-deficit irrigation are discussed in the following paper.

Notes: Summary Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO2 for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO2 concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C3 metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO2 was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C3 species tested and in one CAM species, Aechmea magdalenae at high CO2. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C4 grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C3 species suggest that elevated CO2 may increase photosynthetic rate relative to ambient CO2 over a wide range of light conditions. The response of CO2 assimilation to internal Ci suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO2. This experiment suggests that: (1) a global rise in CO2 may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO2.

Notes: Summary In a previous study conducted at the University of California at Riverside, it was shown that water use of cowpea could be reduced while maintaining seed yields by withholding irrigation during the vegetative stage in a rain-free environment, and then irrigating when estimates based on potential evapotranspiration, indicated 40% depletion of available moisture in 90-cm depth of soil. The general applicability of this efficient irrigation management method was tested by experiments conducted at the West Side Field Station in the San Joaquin Valley of California with six irrigation treatments, three different row spacings (single rows on 76- and 102-cm beds, and double rows on a 102-cm bed), a semi-erect cultivar of cowpea (Vigna unguiculata [L.] Walp.), and a prostrate cultivar of lima bean (Phaseolus lunatus L.). Withholding irrigation during the vegetative stage following pre-irrigation substantially reduced dry matter at anthesis (−17% to −38%) and water use (−101 mm) of cowpea, but did not influence seed yield or shoot dry matter at harvest for either cowpea or lima bean. Increasing the irrigation interval until 75% nominal depletion of available water in 90-cm depth of soil reduced water use (−139 cm), but did not affect seed yield of cowpea. Lima bean, however, showed a significant decrease in shoot dry matter production (−17%) and in seed production (−18%) at the longest irrigation interval involving 75% nominal depletion. The different row spacings used in this experiment did not affect shoot dry matter or seed production of the semi-erect cowpea. However, shoot dry matter and seed yield were significantly greater for the prostrate lima bean grown with double rows on a 102-cm bed. Seed yield was 46% and 18% greater than with single rows on 76-cm and 102-cm beds, respectively. Generally, variations in seed yields of lima bean were positively correlated with variations in shoot dry matter production. Nominal depletion of available soil water provided a practical method for scheduling irrigations, but the results with cowpea indicated that the critical level, which resulted in the greatest reductions in water use while maintaining maximum seed yield varied from 40% (at Riverside) to 75% (at West Side Field Station). Additional methods are needed to fine-tune irrigation which is based mainly on nominal depletion of available water. Generally, pressure chamber estimates of leaf water potential exhibited too little variation among plants subjected to different irrigation treatments for it to be useful for fine-tuning irrigation schedules for either cowpea or lima bean. However, differences in temperature between canopy and air, when expressed as a function of either vapor pressure deficit or canopy temperature, and related to percent reduction in yield, appeared to have sufficient resolution to provide a practical method for fine-tuning irrigation schedules for cowpea during flowering and pod-filling, but not lima bean. Normalizing temperature differences with vapor pressure deficit was more effective, but normalizing with canopy temperatures is more convenient because it does not require a measurement of air humidity.

Notes: Summary In a concurrent study we reported that cowpea (Vigna unguiculata [L.] Walp.) had the ability to maintain seed yields when subjected to drought during the vegetative stage provided the subsequent irrigation interval was not too great. A 4 day irrigation interval produced maximum vegetative and seed yields, while an 8 day interval following a vegetative stage drought produced maximum seed yields and water-use-efficiency. Soil and plant measurements were made during flowering and podfilling to develop general methods for scheduling irrigation. Tensiometers provided adequate resolution for maintaining either shoot biomass production or seed yield. Predawn estimates of leaf water potential provided adequate resolution for maintaining seed yield but not biomass production. Measurements of percent actual depletion of available soil water or afternoon values for temperature differences between canopy and air divided by the vapor pressure difference gave adequate resolution for maintaining biomass production. However, nominal depletion of available soil water based upon predicted evapotranspiration appears to be the most practical procedure for scheduling irrigation of cowpeas.

Notes: Summary Sugarbeets (Beta vulgaris L.) on a Panoche clay loam soil were subjected to 3 different irrigation frequencies and 3 irrigation cutoff dates prior to harvest to determine the effects on evapotranspiration, growth, and sucrose yield. Lengthening the irrigation interval from 1 to 3 weeks reduced evapotranspiration without a significant decline in sucrose production. Increased irrigation cutoff from 3 to 7 weeks prior to harvest significantly increased sucrose percentage within the root and resulted in similar total sucrose yields. Lengthening the irrigation interval only slightly reduced both fresh vegetative biomass and leaf area index (significant differences occurred only at one plant sampling date). The combination of less frequent irrigation and an early cutoff date increased the amount of soil water extracted by sugarbeets. The water use of sugarbeets can be reduced without a significant decline in sucrose production through optimizing irrigation frequency to about 14 to 20 days on this soil and cutting off irrigations about 40 to 45 days before harvest, provided irrigations replenish soil water depletions.