Dry matter accumulation and water use relationships in wheat crops

Abstract

In order to better understand crop growth (dry matter accumulation, DM), crop evapotranspiration (E_t), and their interrelationships, we studied dryland crops of wheat (Triticum aestivum L. cv. Timgalen) sown at various dates and seeding densities in each of three years at Tamworth, N.S.W. Soil water stress was minimal before anthesis in each year, but in two years substantial stress arose before maturity. DM was increased consistently by increased plant density, and decreased at anthesis and maturity by later sowing. Crop growth rates determined over 2-week intervals around anthesis ranged from 3 to 20 g m^-2 d^-1, representing a range in efficiency of utilization of intercepted total solar radiation of 0.48 to 2.35%, variation which was adequately explained (R² = 0.80) by ontogeny (days from anthesis) and E_t / E_p ratio (E_p = class A pan evaporation).

E_t at anthesis, but not at maturity, was increased slightly by higher seedmg density; crop E_t was not consistently affected by sowing date. E_t / E_p over 2-week periods around anthesis was related to leaf area index, and to a lesser extent to available soil water and E_p (R² = 0.58). For the period from the first sowing date in June or July until the middle of October, the relationship of total E_t to DM production was linear and close each year, but the slope varied from 6.2 g m^-2 mm^-1 (cold dry year) to 14.0 g m^-2 mm^-1 (wet year). This variation could be attributed to annual variation in the soil evaporation component of E_t, and in the ratio of DM to crop transpiration (= transpiration efficiency, TE). For 2-week periods around anthesis, TE ranged from 2.9 to 5.4 g m^-2 mm^-1 and was inversely related to E_p (R² = 0.56). Provided soil evaporation can be allowed for, since it ranged from 18 to 41% of crop E_t from sowing to maturity, it is argued that the crop transpirationtranspiration efficiency approach is particularly useful for analysing the growth and water use of dryland wheat.