No abstract

Sypnosis Increases in soil temperature accelerated rate of emergence, rate of growth, and promoted earliness in corn. Corn growth rates approximately followed the Van't Hoff law, the Q10° c. of that law being from 2.0 to 2.8 for the average temperature range tested, 60° to 80° F. Corn yields appeared to increase with increases in soil temperature up to a point and then decreased with further increases in soil temperature. The most favorable soil temperature at the 4‐inch depth for corn growth in central Iowa appears to be about 75° F.

Plants growing under field conditions are usually subjected to varying degrees of water stress, but the effect of temperature on the development of plant water stress and subsequent recovery is not well‐known. Therefore, a study was conducted in growth chambers to determine the combined effects of temperature and soil water supply on the development of plant water stress and subsequent recovery in spring wheat (Triticum aestivum L. ‘Waldron’). Measurements were made of leaf water potential, photosynthesis, and stomatal diffusion resistance on the fifth leaf at tillering and the flag leaf at heading, flowering, and grain‐filling growth stages for plants grown at 10, 18, and 27C. Stomatal closure of stressed plants was affected by both leaf position and age. Closure occurred at −13, −13, and −15 bars leaf water potential at tillering and at −18, −17, and −26 bars at heading for 10, 18, and 27C, respectively. As the flag leaf matured, stomata closed at progressively lower leaf water potential. In nonstressed check plants, temperature greatly influenced leaf water potentials. At heading, plants at 10C had a significantly lower leaf water potential than plants at 18 and 27C, probably resulting from chill‐induced water stress caused by increased internal resistance to water movement in the plant. At 18C, leaf water potential at tillering and at heading recovered to prestress levels 6 hours after stress ceased, but not until after 48 hours at the grain‐filling stage. Stomatal diffusion resistance recovered to prestress levels within 2 hours, except at grain‐filling, when recovery was incomplete after 48 hours. Recovery of photosynthesis was related to recovery of stomatal diffusion resistance at tillering and heading, except that photosynthesis never fully recovered to prestress levels, and photosynthesis of plants stressed at grainfilling showed no sign of recovery due to stress‐induced senescence.

Mechanistic crop simulation models can aid in directing research and improving farm management. Recent research on winter wheat (Triticum aestivum L.) canopy development was consolidated into a model of aboveground vegetative development and growth called SHOOTGRO 1.0. The model assumes that water, nutrients, and light do not limit development and growth. Initial conditions of cultivar type (dwarf, semidwarf, mid-tall, and tall), seeding rate, planting depth and date, and latitude are inputs. Daily temperature, expressed as growing degree-days, drives SHOOTGRO 1.0. Processes are simulated with a daily time step. Computer code is standard Fortran 77.The morphological nomenclature used allows each leaf and node plus associated internode to be identified and their appearance, development, growth, and senescence to be followed from planting through completion of main stem flag leaf growth (shortly before booting). Three cohorts of plants are simulated based on time of emergence. Simulated growth of leaves and internodes is a function of leaf number and accumulation of thermal time, as is leaf senescence.

Evaporation from two-layered soil systems with varying depths to water table is described for steady-state conditions. The difference in evaporation rate from a system of fine-textured soil overlying a coarse soil compared to a homogeneous profile of fine-textured soil may be relatively small. A reversed condition of layers may have a large effect on evaporation rates.It appears that for many practical purposes the presence of inhomogeneities may be of little consequence when the water table is relatively deep. It also appears that a weighted-average capillary conductivity curve might give satisfactory results in calculating estimated evaporation rates.