Phenological and physiological responses of safflower (Carthamus tinctorius) to varying photoperiod regimes

Introduction

About 820,000 hectares of safflower are harvested each year worldwide, spread across latitudes from 15° N to 45° N, where natural day length during the growing season ranges from roughly 10 to 16 hours [1] . That range matters because safflower is classified as a quantitative long-day plant: flowering is promoted by longer photoperiods but can still occur under shorter ones, albeit with a delay [2] . How large that delay is-and what it means for biomass partitioning, chlorophyll dynamics, and final yield-remains incompletely understood. Photoperiod governs the transition from vegetative to reproductive growth mainly through the CONSTANS-FLOWERING LOCUS T (CO-FT) signalling cascade [3] . In model species like Arabidopsis, longer days stabilise CO protein in the afternoon, boosting FT transcription and triggering floral meristem identity. Safflower shares that general framework, but cultivarspecific thresholds and the downstream effects on canopy physiology haven't been mapped for most commercial lines [4,5] . From a production standpoint, growers in arid zones such as central Saudi Arabia face the added complication that extending the vegetative phase under short winter days raises water demand just when evapotranspiration is already high [6] . Conversely, spring sowings with naturally longer days may push the crop to flower before it builds enough leaf area for maximum light interception [7] . Getting the photoperiod-phenology balance right is therefore both a scientific and a practical challenge. This research aimed to: (i) quantify the change in phenological timing of safflower across three discrete photoperiod treatments, (ii) track chlorophyll content, leaf area index, and biomass through the growth cycle, and (iii) link those physiological responses to final seed yield and its components.

Materials and Methods

Research Area and Climate

The experiment was conducted in the controlled-environment growth chambers of the Plant Science Complex, King Saud University, Riyadh (24°43′ N, 46°37′ E), Saudi Arabia, between January and June 2023. Riyadh has a hot desert climate (Köppen: BWh) with mean annual temperature of 26.1 °C and annual rainfall below 100 mm. Chamber conditions were set to 25/18 °C (day/night), 55±5% relative humidity, and photosynthetically active radiation (PAR) of 450 µmol m⁻² s⁻¹ supplied by LED panels. Only photoperiod differed among treatments.

Materials

Seeds of safflower cv. Gila, a spineless semi-dwarf line, were obtained from the Saudi Grains Organization seed bank. Plants were grown in 12 L pots filled with a 2:1 sand-peat mixture amended with slow-release NPK (15-15-15) at 3 g pot⁻¹. Irrigation was maintained at 75% field capacity using weightbased monitoring. Chlorophyll extraction used dimethyl sulphoxide (DMSO); absorbance was read at 663 and 645 nm [8] . Leaf area was measured with a LI-3100C area meter (LI-COR Biosciences).

Methods: Three photoperiod treatments-8 h (short day, SD), 12 h (neutral day, ND), and 16 h (long day, LD)-were applied from emergence onward. Each treatment occupied a separate chamber, and four replicate pots (one plant per pot) constituted a completely randomized design within each chamber. Phenological stages recorded included days to rosette completion, stem elongation, first visible bud, first open floret, 50% flowering, and physiological maturity, following the BBCH scale adapted for safflower [9] . Chlorophyll and leaf area index (LAI) were measured at 15-day intervals from 30 days after sowing (DAS) until maturity. Biomass was partitioned into leaves, stems, and reproductive structures at harvest. Yield components-heads per plant, seeds per head, 1000-seed weight, and seed yield per plant-were recorded after air-drying to 8% moisture. Data were analysed by one-way ANOVA in SAS 9.4, with means compared by Tukey's HSD at p < 0.05 [10] . Total biomass didn't differ between SD and ND (49.4 vs. 50.1 g), but partitioning shifted markedly (Table 2). ND plants allocated 48.1% of dry matter to reproductive organs versus 39.1% in SD plants. The LD treatment reduced total biomass by 20.5% because the compressed vegetative phase limited canopy development. The pie chart (Figure 1) shows that SD plants spent 38.2% of their total cycle in vegetative growth, nearly double the 21.7% recorded for LD plants. Reproductive phase duration was similar across treatments (3.7-4.1%), indicating that photoperiod mainly controls the onset rather than the length of reproduction.

Results

Discussion

The 23.6-day delay in flowering under 8 h confirms that safflower cv. Gila retains a quantitative long-day response, consistent with earlier classification of the species [2,4] . The CO-FT signalling model predicts exactly this pattern: shorter photoperiods destabilise CO protein in the evening, lowering FT transcript levels and postponing floral transition [3] . That Gila still flowered under 8 h-unlike obligate long-day speciessupports the quantitative rather than qualitative nature of its response.

The higher chlorophyll a under SD (2.67 mg g⁻¹ vs. 1.94 under LD) likely reflects the longer leaf lifespan afforded by delayed senescence. Leaves that remain photosynthetically active for more days accumulate and retain pigment pools [11] . But the added carbon capture didn't translate into higher seed yield because the shortened reproductive window under SD limited grain filling. The 12 h treatment struck the best balanceenough vegetative growth for robust LAI yet sufficient reproductive time for seed development, producing 18.3 g plant⁻¹. The 14.7% reduction in 1000-seed weight under LD deserves attention from growers who rely on spring sowing at latitudes above 35° N, where natural day length exceeds 14 h during the critical filling period [12] . Hastened maturity may suit doublecropping rotations but carries a yield penalty. One limitation is that chamber conditions can't fully replicate field light quality and temperature fluctuations; confirmatory field trials are needed [13] .

Conclusion

References

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