Changes in source-sink relationships during grain-filling and its effects on sunflower root system viability and canopy functionality

LISANTI, S.; J.M.CABARCOS; A.J. HALL; C.CHIMENTI

Mar del Plata/Balcarce

Conferencia; 18th International Sunflower Conference; 2012

International Sunflower Association

ABSTRACT ·   After flowering, grains become the main sink for carbohydrates (C) derived from current photosynthesis or from temporary storage, reducing the amount of (C) partitioned to the root system. During grain filling in intact plants, size and functionality of the root system are reduced.  This suggests a trade-off between grain formation and root viability, but other effects of grain formation also need to be considered (e.g., altered leaf senescence, changes in leaf photosynthetic capacity).  Here we report results from an experiment that aimed to document root and leaf responses to a severe reduction in demand for (C) for grain formation. ·   Source-sink relationships during grain-filling were manipulated by removal of  all floral primordia at the beginning of anthesis in crops of two sunflower (Helianthus annuus L.) commercial hybrids of contrasting leaf senescence dynamics (Aguará 6 “stay-green” (SG, lower leaf senescence rate) and  CF 101 “fast dry down” (FDD, higher leaf senescence rate));  resulting in a factorial experiment (repeated in two successive years) of 4 treatments: 1) SG Control: 2) SG F-; 3) FDD Control; 4) FDD F-, distributed in randomized complete blocks (n=3). During the grain-filling phase of the control plants, the dynamics of leaf photosynthetic capacity (Pmax, measured at 2.000 μmol/m2.s PAR on the uppermost expanded leaf)), LAI, organ biomass and root viability (capacity to reduce tri-phenyl tetrazolium) in the 0-40 cm soil stratum were followed in all treatments. ·   In both years Pmax remained stable (average value: 39.6 μmolCO2/m2.s) for 14 days after the beginning of anthesis (DAA), and showed no significant differences (p>0.10) between the four treatments. Pmax declined linearly as from 14 DAA at rates (average slope = -0.84 μmolCO2/m2.s.day), which were statistically indistinguishable between treatments. LAI values at anthesis were statistically indistinguishable between treatments (4.0 SG control, 4.0 FDD control, 4.2 SG F- y 3.9 FDD F-). The SG (F-) treatment maintained the initial LAI value until 35 DAA, while the FDD (F-) treatment did so until 43 DAA, but this difference between hybrids was not significant (p>0.10). In the control treatment both hybrids maintained their initial LAI until 15 DAA, after which LAI declined linearly at rates of  -0.11 IAF/day in SG and -0.14 IAF/day in FDD, which differed significantly (p < 0.05) between hybrids. The hastening of the initiation of LAI loss in the controls with respect to the (–F) treatments was significant (p = 0.05). Both hybrids attained, in the control treatment, their maximum values of live root length density (LRLD) at 10 DAA, with SG reaching a significantly (p=0.05) greater value than FDD (0.40 vs. 0.32 cm root/cm3 soil). In the (F-) treatments LRLD increased continuously until 25 DAA in both hybrids, achieving LRLD values significantly (p=0.05) greater than those of the (F) treatments ( + 28% greater in SG hybrid,  and + 21% in FDD), but this difference between hybrids was not significant (p>0.10). At the end of the experiment, root dry weight (the only organ to show major and significant responses to treatment) in the (F-) treatment showed a 175% increase (in relation to control) in FDD and a 350% one in SG. ·   We conclude that flower primordium removal at anthesis produces complex responses during grain filling. These clearly included increased LRLD and root dry weight, as well as delayed loss of LAI in the (F-) treatments. In contrast, Pmax of the uppermost expanded leaves was not altered in these treatments. Intrinsic canopy senescence patterns (i.e., SG vs. FDD) modulated these responses to some degree, but this was not generalized across variables. ·   For the first time, in sunflower and many other crops, this study shows how the root system is strongly affected by (C) reductions supply during grain-filling phase. It is a starting point to achieve a better understanding and quantification of the potential trade-offs (in carbohydrates and other plant resources) between grains and roots. Future studies will require the development of quantitative response functions to floral primordium ablation for the variables studied in these experiments as well as for additional ones.