Bud Microclimate and Fruitfulness inVitis viniferaL.

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The effect of light on bud fruitfulness of Thompson Seedless (TS), Flame Seedless (FS), Chardonnay (CH), and Cabernet Sauvignon (CS) was studied under field conditions in the San Joaquin Valley of California. Light and temperature effects were also assessed in growth chambers on TS and FS. In field vines, discrete shoot light exposure levels were established by pruning and shoot positioning. Bud light interception was measured weekly from April through August and continuously during seven days at the end of April. Potential fruitfulness was assessed during dormancy by bud dissections. Observed fruitfulness was assessed visually three weeks after budbreak. Shoot light exposure had a significant effect on potential fruitfulness of all cultivars and on observed fruitfulness of TS and FS. The most fruitful cultivar was CH, followed by CS, FS, and TS. Maximum potential fruitfulness in TS and CS occurred at approximately one-third of full sunlight, whereas in CH and FS fruitfulness continued to increase with irradiance. Fruitfulness in TS and FS was not affected by bud orientation or location within the canopy. Midday light interception by individual buds did not correlate with their particular fruitfulness. However, when data from individual buds was pooled by shoot, shoot light microclimate significantly correlated with potential fruitfulness. Maximum fruitfulness in TS and FS under controlled conditions occurred at 25°C but was drastically reduced at 32°C in TS and at 18°C in FS. Again, there was no relation between individual bud light exposure and fruitfulness.