No abstract

No abstract

This paper is a section of the book “Drought phenotyping in crops: from theory to practice” (Monneveux Philippe and Ribaut Jean-Marcel eds, published by CGIAR Generation Challenge Programme. Texcoco, Mexico). The section describes recent experience in drought phenotyping in rice which is one of the most drought-susceptible crops. The section contains genetic and genomic resources for drought adaptation and methods for selection of drought-resistant varieties in rice. In appendix, there is experience from Thailand on integration of direct selection for grain yield and physiological traits to confer drought resistance.

Tropical maize (Zea mays L) is characterized by large tassels, broad, long leaves above the ear, and by a low harvest index. This study was conducted to evaluate direct and correlated responses to selection for reduced tassel primary branch number and for reduced leaf area density above the ear (LADAE). Leaf area density above the ear was defined as (number of leaves above the ear ✕ mean area of leaves EL + 1, EL + 3)/(plant height — ear height) ✕ land area occupied per piano, where EL refers to the position of the ear leaf. Six cycles of full‐sib recurrent selection for each of these traits was conducted in each of two populations (‘Tuxpeño‐l’ and ‘Antigua‐República Dominicana’). In a third population, ‘ETO Blanco’, there was simultaneous selection for both traits. Evaluation of Cycles 0 and 6 of single trait selections and Cycle 0,2,4, and 6 of combined trait selection, plus checks, was at several plant densities at three locations in Mexico over 2 yr. Significant (P = 0.05) mean gains per cycle for tassel branch number, LADAE, grain yield, and harvest index were: (i) Tuxpeño‐l: selection for reduced tassel branch number:−7.4, not significant (NS), 2.2, and 2.4%; selection for reduced LADAE: −1.2, −3.8, NS, and 3.8%; (ii) Antigua‐República Dominicana: selection for reduced tassel branch number: −8.6, −1.6, 2.4, and 2.2%; selection for reduced LADAE: −5.7, −2.6, 2.0, and 2.2%; and (iii) ETO Bianco, selection for reduced tassel branch number and LADAE: −6.8, −1.8, 2.5, and 2.9%, respectively. The correlation between LADAE and mean maximum leaf width, after removing the population effect, was 0.95**, significant at the 0.01 probability level (13 df. In all populations, selections resulted in significant increases in the predicted optimum plant density for grain yield, significant decreases in the anthesis‐to‐silking interval, and a shorter interval from planting to 50% silking.

Reduced plant height is considered a desirable attribute in most maize (Zea mays L.) breeding programs. This study was conducted to evaluate direct and correlated responses to selection for reduced plant height in tropical maize. Fifteen cycles of visual, full‐sib recurrent selection for reduced plant height were conducted in one lowland tropical maize population, ‘Tuxpeño Crema I’. Cycles 0, 6, 9,12, and 15 were evaluated for correlated responses in plant morphology, grain yield, dry matter production and distribution, leaf area index, and duration and rate of kernel growth at three plant densities at two or three locations in Mexico over a period of 2 yrs. Selection resulted in a linear reduction in plant height, from 282 to 179 cm (2.4% per cycle). The estimate of broad‐sense heritability for plant height was 0.84. Optimum plant density for yield increased from 48 000 to 64 000 plants ha−1, an increase of 2.1% per cycle. As height was reduced, yield, when determined at the optimum plant density for yield for each generation, increased by 4.4% per cycle. Later cycles showed reduced total lodging, reduced barrenness, earlier flowering, and fewer leaves per plant. When grown at or near their optimum densities for grain yield, leaf area index, and total dry matter per unit area at harvest for selected cycles were similar. Yield improvement was associated with a linear increase in harvest index from 0.30 to 0.45 from the original to the 15th cycle. At 50% silking, total aboveground dry weight and its proportion as stem were reduced; kernel number per unit area at harvest increased from 1592 to 2667 kernels m −2 when grown at or near optimum density, and there was an increase in crop dry weight production from anthesis to black layer formation. Selection for reduced plant height had little effect on the effective filling period or kernel weight.