2012 Annual Report
We also made considerable progress in understanding how temperature affects tuber respiration during low temperature-induced sweetening (LTS) in resistant and susceptible genotypes. Premier Russet, GemStar Russet, and Defender differ substantially in their resistance to low temperature sweetening (LTS) and associated metabolism. Gemstar and Premier have moderate and high resistances, respectively, while Defender sweetens and loses processing quality progressively during storage at 9oC and has virtually no resistance to LTS at 4.4 and 6.7oC. At 4oC, Premier maintained low levels of sucrose (suc), glucose (glc) and fructose (fru), GemStar accumulated suc with little inversion to glc and fru, and Defender accumulated glc and fru but comparatively little suc. Respiratory acclimation responses to temperature change (9 to 4oC) reflected the varying degrees of sweetening of these cultivars and were thus diagnostic of the LTS phenotype. In response to an immediate drop from 9oC to 4oC, tuber respiration decreased 42%, reaching a minimum within 45 h for all cultivars, and then increased to a new maximum over the next 4.5 d before decreasing to establish a constant basal rate at 4oC over the next 14 days (acclimation response). The acclimation maxima were 56, 28, and 14% greater than the final resting basal respiration rates at 4oC for Defender, Gemstar, and Premier, respectively. The acclimation responses thus correlated with the extent of LTS from these genotypes and likely reflected the level of metabolic energy required to catabolize starch to suc, glc and fru end products. The ratio of fru to glc during LTS was an effective predictor of LTS resistance in these genotypes. Heat stress during tuber maturation abolished the LTS resistant phenotype of Premier. This information is critical in selecting durable modes of LTS resistance that cannot be compromised by late season heat stress. Studies to determine how in-season nitrogen management affects the ability of tubers to retain processing quality after harvest were also completed. Changes in foliar and tuber fresh weight, sucrose, reducing sugars, and specific gravity over time and in response to four rates of in-season N were profiled over three growing seasons for cvs Alturas and Premier Russet. On average, increasing N rate delayed the attainment of 50% harvest index (HI), increased foliar and tuber biomass at 50% HI, shifted maximum foliar growth later, increased foliar biomass, reduced the HI at maximum foliar growth, delayed vine senescence, and increased tuber yield. Vine growth was more sensitive to in-season N than tuber yield. Source/sink imbalances occurred when tuber growth dominated plant growth at maximum foliar development (HI>50%), resulting in significantly lower final yields. Physiological maturity (PM) was estimated as the average of days after planting to reach maximum yield, maximum specific gravity, minimum sucrose, and minimum reducing sugars in the stem ends of tubers. PM ranged from 143 to 158 DAP and occurred later in the season with increasing level of N. Hence, tubers grown with high N were less mature (physiologically younger) at harvest than tubers grown with less N. Minimizing the interval between PM and harvest resulted in longer retention of fry processing quality (low reducing sugars) in storage. Nitrogen rate therefore affects the retention of processing quality indirectly by influencing the timing of attainment of tuber PM in relation to harvest.