Floret development of durum wheat in response to nitrogen availability

Abstract

In Mediterranean durum wheat production, nitrogen (N) fertilization may be important to stabilize and increase yields. Wheat yield responses to N fertilization are usually related to grains per m(2), which in turn is the consequence of processes related to floret development (floret initiation followed by floret death/survival) during stem elongation. The literature is rather scarce in terms of the relevance of floret developmental dynamics, determining the final number of grains in general terms and in particular regarding responsiveness to N. The aim of this study was to determine whether durum wheat responses to N under different water regimes are related to the dynamics of development of floret primordia to produce fertile florets. During the 2006-2007 and 2007-2008 growing seasons, experiments with a factorial combination of two N levels (0 and 100 or 250 kg N ha(-1)) and two levels of water availability (rainfed and irrigated) were carried out (although the water regime was only effective in the second season). The response of yield was largely a consequence of that in grain number per spike. Floret initiation was similar for both N levels in each experiment and water regime, for which the survival of a higher proportion of initiated florets was critical in the response of the crop. The diminished rate of floret abortion during the late part of stem elongation in response to N was associated with a slightly accelerated rate of floret development which allowed a higher proportion of the primordia initiated to reach the stage of fertile floret by flowering.

Fig. 1.

Images illustrating different aspects of the preparation and setting up of the experiments. Seeds were manually placed at precise regular intervals, to maximize uniformity of the stand, on masking tape (a), then these 1 m linear strips were covered with tissue paper and placed in the rows of each experimental unit (b). The consequence of the delicate handling for sowing was the achievement of experimental units with almost perfectly uniform microcrops (c). (d) The general appearance of the experiment with all the experimental units covered with an anti-bird mesh.

Fig. 2.

Accumulated rainfall, mean temperature, and mean daily global radiation for 10 d periods during the 2006–2007 (experiment 1, top panel) and 2007–2008 (experiment 2, bottom panel) growing seasons at the experimental site (Lleida, Spain). Arrows indicate the average dates of sowing (S), seedling emergence (E), jointing (J), flowering (F), and maturity (M) for wheat.

Fig. 3.

Illustration of wheat floral development as shown schematically (a) and in images taken during the experiments (b) from early spikelet primordia differentiation through terminal spikelet to anthesis, with details of selected floret developmental stages. In both panels (a and b) first the development of the spikelets within the spike is shown up to terminal spikelet initiation, and from then the development of floret primordia (that successfully reach the stage of fertile florets at anthesis) within spikelets is shown. In b, there is also an illustration of the different degree of development of spikelets selected to analyse the developmental progress of its floret primordia. The bottom panel (c) illustrates the floral development dynamics with time (upward-facing arrow) for florets that develop normally towards achieving the stage of fertile floret at anthesis and setting grains afterwards. The downward-facing arrows show the floral degeneration process either early or late during development, as observed microscopically during the experiments. The pictures and drawings are not to scale; as a reference, the width of a floret in W 3.5 is ∼0.10 mm, in W5 0.15 mm, in W7.5 0.30 mm, and in W10 1.60 mm.

Fig. 4.

Dynamics of the number of living floret primordia from jointing to flowering under high and low N availabilities (filled and open symbols, respectively) for basal, central, and apical spikelets of the main shoot spikes of durum wheat in experiments carried out in Lleida (NE Spain) during 2006–2007 (experiment 1) and during 2007–2008 under irrigated and rainfed conditions (experiment 2).

Fig. 5.

Effect of nitrogen availability on floret development of durum wheat during the stem elongation phase after jointing for the 3–4 florets most proximal to the rachis florets in each of the three spikelet categories considered for experiments carried out in Lleida (NE Spain) during 2006–2007 (a) and during 2007–2008 under irrigated (b) and rainfed conditions (c). Floret development was assessed through frequent determination of floret stages following the scores given by the scale of Waddington et al. (1983).

Fig. 5.

Effect of nitrogen availability on floret development of durum wheat during the stem elongation phase after jointing for the 3–4 florets most proximal to the rachis florets in each of the three spikelet categories considered for experiments carried out in Lleida (NE Spain) during 2006–2007 (a) and during 2007–2008 under irrigated (b) and rainfed conditions (c). Floret development was assessed through frequent determination of floret stages following the scores given by the scale of Waddington et al. (1983).