Pollen production in sunflower (Helianthus annuus L.) is affected by air temperature and relative humidity during early reproductive growth

During microsporogenesis, sunflower florets might be exposed to episodes of day- and night-time temperatures exceeding 30 and 20 degrees C, respectively, that can affect pollen availability and consequently plant's yield. The relationship between air temperature and relative humidity during the meiotic phase of microsporogenesis, and the quantity and quality of pollen produced by the flowers, were studied in two sunflower hybrids during two years. The hybrids were grown on irrigated plots in Bahia Blanca (38 degrees 45' S; 62 degrees 11'W) in three planting dates (PD) at a density of 5.6 plants/m(2). Flowers were consecutively taken as anthesis progressed, sectorizing the capitulum in three regions (external, middle and internal), to determine pollen grain number per flower (PGF) and pollen viability (PV). Both average air temperature (AT, degrees C) and relative humidity (RH, %) from reproductive stage R1 to R2 [36 to 48 days from emergence (DFE), respectively] were calculated for each PD, hybrid and capitulum sectors. Pollen viability was high (> 90%) with no differences between PD, capitulum sectors and hybrids. The number of PGF, with respect to AT (19 to 31 degrees C) from R1 to R2, was adjusted to a second-order polynomial (R-2=0.53, n=24; p<0.0004), with a maximum of 45,000 PGF between 21 and 25 degrees C. Above and below these values, the number of PGF showed a reduction of approximately 20%. There also was a significant positive relationship between pollen production and air RH. A maximum of 45,679 PGF was observed at 57% RH adjusting a second-order polynomial (R-2=0.58, n=24; p<0.0001).Temperatures over 26 degrees C were supraoptimal for pollen production. It is concluded that pollen production in sunflower is determined by the cumulative effects of both air temperature and relative humidity above a critical value and not only by temperature effects at a specific developmental stage before flower opening. It remains unknown whether this observation is associated with a direct effect on the mitosis or indirectly with changes in the contribution of current photoassimilates to anthers during the meiotic phase of the microsporogenesis.