Influence of water deficit and canopy senescence pattern on Helianthus annuus (L.) root functionality during the grain-filling phase

- We compared root functionality, during grain-filling, under control and drought conditions of stay green (SG) and fast-dry-down (FDD) sunflower hybrids.
- The SG hybrid root system senesced slower than that of the FDD hybrid, under both irrigation and drought.
- Root functionality determined with minirhizotron imaging showed good agreement with capacity for triphenyl tetrazolium chloride reduction.
- Crop transpiration was greater in the SG than FDD hybrid during most of grain-filling phase, under both irrigation and drought.
- Root senescence preceded canopy senescence in both hybrids and levels of water supply.

Root system size and functionality during the grain-filling phase are critical for water uptake and grain yield realization. In spite of its potential importance, root system functionality during this phase has received little attention. The study reported here aimed at documenting root system functionality dynamics between anthesis and physiological maturity, and its responses to water deficit, in two sunflower (Helianthus annuus L.) hybrids of contrasting intrinsic patterns of canopy senescence. In experiments repeated in two separate years, crops of the hybrids Aguará 6 (stay green [SG]) and CF 101 (fast dry down [FDD]) were exposed to two levels of soil water availability (irrigation and drought) during grain filling, and the temporal dynamics of root and crop system variables (live root length density [LRLD], root respiration, sap flow rate, green leaf area index [GLAI] and the percentage of live roots [LR%]) were followed. The highest values for all variables were observed close to anthesis, and these differed little or not at all between hybrids within water regimes; but decreased thereafter at rates that differed between hybrids, water supply treatment and response variable. Drought hastened the beginning of root and leaf senescence in of both hybrids. The start of the decline in GLAI in the SG hybrid was significantly (p < 0.05) delayed with respect to that of the FDD hybrid under both water regimes, so that the SG hybrid maintained a significantly (p < 0.05) greater canopy area for the duration of the experiments under both water regimes. The SG hybrid also exhibited significantly (p < 0.05) higher values of LRLD, and root respiration, as well as significantly (p < 0.05) lower rates of LR% decrease under both water treatments. On days of high evaporative demand the SG hybrid showed significantly (p < 0.05) greater rates of sap flow under both water regimes. These response patterns were consistent across the 2 years of experimentation. Importantly, when the dynamics of all measured variables were contrasted, it was found that root senescence preceded canopy senescence. We conclude that the SG syndrome in sunflower is associated with improved root functionality during grain filling under both irrigation and drought conditions.