Growth kinetics of Marquis wheat. III. Nitrogen dependence

The growth coefficients, k′1, of whole plants and their parts were obtained by kinetic analysis of primary data on growth in nutrient solutions that differed only in the concentration 1.7, 8.4, 42, 210, or 1050 ppm nitrogen (N), each at about five different light intensities (I) from 300 to 3600 ft-c. Maximal values kmL and kmN were computed for nonlimiting levels; and, efficiencies (1/K)L and (1/K)N were computed for vanishingly low levels of light and nitrogen. The growth coefficients of the leaves or other organs were not preferentially affected by the N supply, but “stem” growth still dominated at saturating levels of N and I. The influence of N on growth was greatest at high I and vice-versa, by hyperbolic dependencies. Computations of maxima, as kmL.N, from these saturation curves yielded a value of 0.36 per day for whole plants, and this may be close to the genetically determined absolute maximum value. Plant growth in 1.7 and 8.4 ppm N was nearly saturated by 3600 ft-c I, but the growth of plants in nonlimiting N supply was only 70% saturated by that light intensity. Nitrogen at 1050 ppm saturated plant growth at all I including 3600 ft-c, when k′1 = kmn, and also at saturating I, when kmL = kmL.N. At and below 2200 ft-c I plant growth was saturated by only 42 ppm N, and growth at 300 ft-c I was virtually independent of N from the nutrient solutions. At nonlimiting I plant growth was halved by only about 12 ppm N; and this, as 50% nitrogen stress “NS50,” is proposed as a useful criterion. Light and nitrate N were most efficiently used for growth at the sites of their absorption. Roots were 41% less efficient than laminae in the use of photosynthate at nonlimiting I, and laminae used N 50% less efficiently than roots at nonlimiting N. These priorities might be considered as primary morphogenic effects of these environmental factors, effects whose identification is difficult with growth coefficients and impossible by secondary "growth analysis." At 1.7 ppm nutrient N, chlorophyll was subject to net photobleaching after 3 weeks growth. The perturbing effect of the removal of half-used endosperm was reflected in halved plant size but the growth coefficient was not decreased, an observation of consequence to literature on growth effects.