Cold Soil Temperatures Research Articles

Relationship Between Laboratory Indices of Soybean Seed Vigor and Field Emergence1

There have been few comparisons of the relationship between laboratory estimates of soybean [Glycine max (L.) Merr.] seed vigor and field emergence under adverse soil conditions. These studies were conducted in 1973‐75 to compare the results of standard germination, two vigor tests (accelerated‐aging germination and 4‐day germination), and several vigor indices to field emergence in varying soil environments. Each vigor index was computed by converting the results of each laboratory test into a vigor rating ranging from 0 to 10. The three vigor indices evaluated and the tests used in each were: VR‐1 (standard germination, 4‐day germination, and accelerated‐ aging germination), VR‐2 (standard germination and accelerated‐aging germination) and VR‐3 (standard germination and 4‐day germination). The prediction accuracy (those seed lots that emerged in the same performance level in the field as predicted in the laboratory) was determined for each laboratory test and index. All laboratory tests consistently overpredicted field emergence when seed was planted in adverse soil conditions (severe crusting and cold soil temperatures). The prediction accuracy for all seed lots averaged across years with adverse field conditions was very low (8.3%) for standard germination and improved to 46.7% for accelerated‐aging germination and 56.9% for 4‐day germination. The prediction accuracy increased to 65.0% and 68.3% for the VR‐3 and VR‐1 vigor indices for the same comparisons. The VR‐1 index, using all three laboratory tests provided a better estimate of field emergence than either vigor index (VR‐2 and VR‐3) when only two tests were used. The prediction accuracy of the VR‐1 vigor index was approximately 25% higher than averaging the results of the same three tests. The vigor index concept offers the possibility of combining additional laboratory parameters that relate to seed and seedling vigor.

Physiological control of water flux in conifers : A computer simulation model.

A water flux model with daily resolution is described which permits one to assess how changes in the rooting volume, amount of sapwood, leaf area and conductance properties interact to affect water uptake, internal storage, and transpiration.A root zone water compartment is defined for a particular tree on the basis of root depth, lateral extension and moisture holding characteristics of the soil. Water is taken up from different subcompartments of the root zone as a function of vertical position, soil water content, and water deficit within the sapwood. Excess water entering the root zone is channeled into runoff or seepage.The sapwood compartment of the model is restricted to the main stem of the tree and does not include sapwood in the branches or roots. The model assumes whatever water deficit is built up in the sapwood during the day will be replenished at night if the root zone water supply/capacity ratio exceeds 20%. A complex exponential equation describes the amount of water extractable from 20% to 0 capacity when no uptake is possible. The maximum change in volume of water in the sapwood of a large Douglas-fir is estimated to represent more than a 10 day supply for transpiration.Water loss through transpiration is predicted as a function of the mean daily absolute humidity deficit, leaf area, leaf conductance and daylength. Leaf conductance is controlled by predawn plant moisture stress which in turn is a function of the rooting zone water supply.The model incorporates two special constraints upon water uptake and transpiration. The first accounts for the effect of cold soil temperatures reducing the possible uptake by Douglas-fir to half at 2°C and to 0 at-2°C. The second represents a critical absolute humidity deficit sufficient to cause stomatal closure which results in leaf conductance being reduced to a minimum.The model is employed to compare trees of different sizes and those with different stomatal behavior. From this experience, it is suggested that future studies include, at a minimum, simultaneous measurements of: absolute humidity deficit, leaf area, sapwood volume and change in water content, predawn stress and leaf conductance.