This study was designed to quantify the responses of leaf expansion, stomatal conductance, and transpiration of four genotypes of vegetable amaranth [Amaranthus tricolor L. (Hin Choi), A. tricolor L. (Co. 2), A. blitum L. (WS80-192), and A. cruentus L. (RRC 1027)] to soil drying. Two greenhouse experiments were conducted during 1999 and 2000. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Leaf expansion rates, stomatal conductances, and transpiration rates of the stressed plants were determined relative to those of nonstressed plants, and expressed as relative leaf expansion (RLE), relative stomatal conductance (RSC), and relative transpiration (RT), respectively. The rate of soil water extraction differed among genotypes, with RRC 1027 depleting soil water fastest and Hin Choi slowest. Whereas in 1999 all genotypes were equally efficient in soil water use, RRC 1027 extracted a greater volume of transpirable soil water than the other genotypes in 2000. The responses of RLE, RSC, and RT to FTSW were well described by linear-plateau models which allowed calculation of soil-water thresholds for leaf expansion (CL), stomatal conductance (CS), and transpiration (CT). Values for CL were higher than for CS and CT. CL was similar for the four genotypes in each year, whereas, CS and CT differed among genotypes. CS and CT was lowest for Hin Choi and highest for WS80-192. Differences of CL, CS, and CT between the two experiments might have been due to the different soils used in the experiments and the different evaporative demands during the drought cycles. Under drought stress, the reduction of transpiration of vegetable amaranth was due mainly to reduction of stomatal conductance, not to reduction of leaf expansion. The relative reduction of dry weight caused by drought stress was positively correlated with CS or CT across the four genotypes. Variation in CS and CT among amaranth genotypes revealed different responses to drought stress, which could make them suitable for different drought situations.
C. Alt, H. Kage, and H. Stützel
Concepts of above-ground dry matter partitioning in cauliflower [Brassica oleracea L. (Botrytis Group)] as dependent on nitrogen (N) supply and light environment are presented. Leaf and stem partitioning depends on a functional relationship between stem dry weight and leaf area, independent of N status. Dry matter partitioning into the inflorescence is sink-limited (potential capacity) at the beginning, and source limited (daily available assimilates) later. The intrinsic specific growth rate of the inflorescence is dependent on leaf N content. The model is parameterized and evaluated with data from field experiments. Applied to an independent data set, the model predictions of proportions of inflorescence, leaf, and stem on total dry matter corresponded with measurements (r = 0.84, 0.92 and 0.22, respectively) for different N fertilization rates and light treatments.
P.W. Masinde, H. Stützel, S.G. Agong, and A. Fricke
Plant growth and osmotic adjustment of spiderplant were investigated in a glasshouse and under field conditions. Two fast-growing genotypes (P-landrace and P-commercial) and a slow-growing landrace (G-landrace) were grown under soil water deficit and watered conditions. The fraction of transpirable soil water (FTSW) was used as an indicator of water availability in pots. In the greenhouse, transpiration was determined by changes in daily pot weights and the ratio of transpiration of plants in soil water deficit to watered treatments expressed as normalized transpiration ratio (NTR). Water use in the field experiment was determined by gravimetric methods. The fast-growing genotypes had a higher rate of soil drying due to a higher rate of leaf area development. They were also more sensitive to soil water deficit with NTR beginning to decline at FTSW of 0.55-0.77 as compared to 0.29 for the slow-growing landrace. Also, the fast growing genotypes had FTSW thresholds for the stem elongation rate of 0.35-0.55 as compared to 0.20 for the slow growing landrace. The rate of leaf development declined when 40% to 60% of available water in the soil was removed, regardless of genotype. Leaf area of plants under field conditions decreased when the soil moisture was <60% field capacity. Under severe soil water deficit stress in pots, plants partitioned more biomass to roots than above ground; however, biomass partitioning between leaves and stems was not influenced by soil water deficit. Spiderplant showed limited osmotic adjustment (OA) in the range of 0.10-0.33 MPa at the highest soil water deficit (FTSW = 0). Thus, spiderplant is mainly a drought avoiding species. To achieve maximum growth, it is necessary to keep FTSW above 0.6.