Influence of water deficit and canopy senescence pattern on sunflower root functionality during the grain-filling phase.

LISANTI, S.; R.DATSIRA; A.J. HALL; C.CHIMENTI

Mar del Plata/Balcarce

Conferencia; 18th International Sunflower Conference; 2012

International Sunflower Association

·   Root system size and viability are critical for water and nutrient uptake and grain yield realization. In spite of the potential importance of the issue for grain yield determination, root system functionality during the grain-filling phase has received little attention. The study reported here aimed at documenting root system functionality (water uptake capacity and respiration) dynamics during this phase and its responses to water deficit and to contrasting intrinsic patterns of canopy senescence. ·   Plots of two sunflower (Helianthus annuus L.) hybrids of contrasting canopy senescence dynamics (Aguará 6 “stay-green” [SG, lower leaf senescence rate] and  CF 101 “fast dry down” [FDD, higher leaf senescence rate]) were exposed to 2 levels of soil water availability (irrigation [control] and drought) during grain filling.  Drought was imposed by suspending irrigation 8 days before anthesis. Measurements of root system variables (length density and viability [capacity to reduce tri-phenyl tetrazolium] in the 0-40 cm soil stratum), root respiration (in situ), stem xylem flow and leaf area were carried out. ·   At the end of anthesis, control SG live root length density (LRLD) was 24% greater (p<0.05) than that of control FDD.  After this stage, LRLD values declined in both hybrids at a similar rate (0.007 cm root/cm3 soil.day). Thus, root senescence overlaps with the embryo filling phase. Water deficit hastened by 9 days the start of root senescence in both hybrids (p<0.10), and significantly (p<0.05) increased root senescence rate with respect to control values (by 38% in SG and 53 % in FDD). Root respiration showed a pattern similar to LRLD. Greatest rates (0.65 gCO2 m soil-2h-1) were achieved during anthesis and were similar (p>0.10) in both hybrids. Rates of decrease of root respiration in the control treatment were similar in both hybrids (p>0.10). Drought significantly (p<0.10) increased root respiration decrease rate by 42% in SG (0.0091 vs. 0.00128 g CO2 m soil-2h-1, control vs. drought) and by 57% for FDD (0.0092 vs. 0.0144 gCO2 m soil-2h-1, control vs. drought). Leaf area index (LAI) values did not differ (p>0.10) between hybrids at the beginning of anthesis (BA) (SG = 4.6; FDD = 4.8). Decrease in LAI started to be significant at 14 days after (BA) in control treatments and from (BA) in drought treatments. SG-Control senescence rate was 17 %  lower  than FDD (p<0.10).  Under drought treatments, FDD leaf senescence rate was 20% greater than that of SG (p<0.05). Changes in transpiration rates (mm/d) correlated with root senescence dynamics. Greatest values of transpiration were measured at (BA) (10 mm/d SG, 8 mm/d FDD). 14 days after BA transpiration flow in control treatments started to decrease at a rate that was similar between hybrids (p>0.10. Exposure to water deficit produced significant differences (p=0.05) between hybrids in transpiration dynamics. FDD showed a significant decline (p=0.05) in transpiration flow beginning 8 days after cessation of irrigation, while in SG this did not occur until 10 days later. ·   We conclude that both water deficit and intrinsic canopy senescence dynamics can profoundly affect root functionality during grain-filling. The effects of these factors and their interactions, especially under drought, on yield merit focused attention in future research Heretofore there was poor or no evidence about root system behavior during the grain-filling phase. This study documents, for the first time in sunflower and many other crops, the dynamics of root system functionality during this critical development phase. In addition, it has served to highlight the effects of drought, canopy senescence patterns and their interactions on root functioning.