A relationship between transpiration and soil suction and soil‐water content has been derived. Good agreement between the theory and results of laboratory experiments was obtained. The results suggest that once plants wilt, the transpiration rate should be roughly proportional to the available water content of the soil. The lower limit of water available for transpiration occurs at a suction well above 15 bars.

No abstract

The thermocouple psvchromiieter (leveloped by Richardls and Ogata (1)

Synopsis An equation describing the uptake of water by plants is described. Greenhouse experiments support the equation and show that the lower limit of water availability is determined by the rate of water transmission through the soil to the plant roots. An appreciable amount of water is shown to move vertically in the root zone.

Climatological factors and the boll load from the first fruiting cycle were evaluated as primary causes for low boll retention by cotton (Gossypium hirsutum L.) during midseason. Boll retention was permitted from incipient flowering, or after June 26, July 15, July 30, or August 14, by the daily removal of flowers. Boll retention was greater than 75% initially, but decreased to less than 50% after bolls equivalent to 500 to 1,200 kg lint/ha (1 to 2 bales/acre) were retained and less than 20% after bolls equivalent to 700 to 1,300 kg lint/ha (1.25 to 2.25 bales/ acre) were retained. The fruit load was the primary cause for low boll retention and cessation of flowering during midseason. No direct relationship between low boll retention and high maximum or minimum temperatures or high relative humidity was observed.

Synopsis Below a characteristic diffusion pressure deficit value for each plant, the transpiration rate was proportional to the potential transpiration. Above this value, the transpiration rate tended to decrease, rapidly at first and then more slowly, with increasing DPD. The relative rate of water loss from initially turgid detached leaves decreased very markedly with decreasing water content at a water content of about 90% of that at full turgor and corresponding to a DPD of about 10 to 15 bars.

This study was conducted to determine whether low soil oxygen levels or high soil ethylene reduce tomato (Lycopersicon esculentum) yield in clay soil, when high temperatures are combined with trickle or furrow irrigation. Treatments were daily or weekly trickle irrigations (100 or 120% of pan evaporation) and furrow irrigations (5‐ or 10‐d intervals). Soil oxygen and ethylene levels and soil matric potentials were measured at the 20‐ and 40‐cm depths immediately before and during part of the fruiting cycle. Daily trickle irrigations (100 or 120% of pan evaporation) resulted in oxygen levels of 3 to 6% and matric potentials of 0 to −7 kPa. Furrow irrigation or weekly trickle irrigations resulted in oxygen levels that were about double the values measured for the daily trickle irrigation, and in soil matric potentials of −1 to −60 kPa. Irrigation frequency or rate did not have a significant effect on yield of tomatoes. Soil ethylene levels were low for all treatments (< 1 ppm). Because of higher oxygen level, weekly trickle or furrow irrigation would be preferable to daily trickle irrigation when oxygen may be limiting or when the crop grown is oxygen sensitive.

The total impedance to water movement from the soil into the plant was compared with that predicted for the soil alone. When soil suction was below 0.6 bar, the impedance was largely in the plant. When suction was greater than 1 or 2 bars, the soil became the limiting factor. Water movement to the plant roots takes place primarily in the liquid phase.