Response to temperature
Short photoperiods (i.e. long nights) and cool temperatures induce tuber formation in potato (Ewing, 1997). Potato yields are particularly sensitive to high-temperature stress because tuber induction (Reynolds and Ewing, 1989; Gawronska et al., 1992) and development (Krauss and Marschner, 1984) can be directly inhibited by even moderately high temperatures. There is an interaction between photoperiod and temperature: the higher the temperature, the shorter is the photoperiod required for tuberization for any given genotype (Snyder and Ewing, 1989). High temperatures can also adversely affect tuber quality by causing 'heat sprouting' which is the premature growth of stolons from immature tubers (Wolfe et al. 1983; Struik et al., 1989).
Although photosynthesis in potato is repressed by high temperature (Ku et al., 1977), it is often not as sensitive to temperature as are tuberization and partitioning of carbohydrates to the tuber reproductive sink (Reynolds et al., 1990; Midmore and Prange, 1992). Therefore, moderately high temperatures can significantly reduce tuber yields even when photosynthesis and total biomass production are relatively unaffected.
In their comprehensive study, Reynolds and Ewing (1989) documented a distinction between the effects of air and soil temperature on potato physiology and yield. Cooling the soil (17—27°C) at high air temperatures (30-40°C) neither relieved any of the visible symptoms of heat stress on shoot growth nor repressed the tuberization induction signal from the leaves. This was reflected in the lack of tuberization by leaf-bud cuttings. Heating the soil (27-35°C) at cool air temperatures (17-27°C) had no apparent detrimental effect on shoot growth or induction of leaves to tuberize. However, in each case, hot soil essentially eliminated tuber development. They concluded that the induction of leaves to tuberize is affected principally by air rather than soil temperature, but that expression of the tuberization signal from the leaves can be blocked by high soil temperature.
There are various morphological responses of potato to high temperature (see review by Ewing, 1997) in addition to reduction in tuber number and size. Plants grown under high temperature are taller, with longer internodes. Leaves tend to be shorter and narrower, with smaller leaflets, and the angle of the leaf to the stem is more acute. Axillary branching at the base of the mainstem increases, more flowers are initiated, and fewer flower buds abscise. At warm temperatures, compared with cool ones, leaf and stem dry weights often increase (at the expense of tuber growth), and the leaf/stem ratio decreases.
Considerable genetic variability in response to high temperature has been reported for potato. Reynolds et al. (1990) observed significant differences in photosynthetic response to a 9-day heat treatment (40/30°C day/night temperature) among several accessions reported to vary in temperature sensitivity. The differences in photosynthetic rates were attributed to a number of factors, including temperature effects on leaf chlorophyll loss and senescence rate, stomatal conductance, and dark reactions of photosynthesis. Snyder and Ewing (1989) compared six cultivars and noted a tendency for the tuberization of early-maturing types to be less negatively affected by high temperatures (30/25°C day/night) than were late-maturing varieties, in which raising the temperature caused up to a 50% reduction in tuber dry weight. Reynolds and Ewing (1989) also reported genotypic variation in tuberization after exposure to high temperatures among the 319 accessions they tested.
The challenge for breeders in attempting to develop heat-tolerant potato varieties is that a genotype possessing tolerance to one aspect of heat stress may not necessarily be tolerant to other aspects. High temperature can reduce yields by affecting ability of seed tubers to sprout, photosynthetic or dark respiration rates, tuberization, partitioning of assimilates to developing tubers, and other processes, each of which may be under separate genetic control. There are also secondary reactions to stress, such as resistance to drought and increased disease pressure, that often are concomitant with high temperatures.
Response to [CO2]
Potato possesses the C3 photosynthetic pathway, and the tubers are a large 'sink' for carbohydrates. Typically, as much as 70-80% of total dry weight at maturity is in the tubers (Moorby, 1970; Wolfe et al., 1983). Several reviews of the CO2-enrichment literature have concluded that sustained stimulation of photosynthesis by elevated [CO2] is most likely in C3 plant species, such as potato, which have a large, indeterminate sink capacity for photosynthates (Stitt, 1991; Wolfe et al., 1998). The experimental data for potato have not always corroborated this hypothesis, however. Collins (1976) found no significant effect of elevated [CO2] on tuber number but did document a significant increase in tuber size and overall yield from CO2 enrichment. Wheeler and Tibbits (1989) found that raising the [CO2] to 1000 |mmol mol-1 increased tuber yield by only 2% and 12% for the varieties Norland and Russet Burbank, respectively. Goudriaan and de Ruiter (1983) reported a slight reduction in yield of potato grown at elevated - compared with ambient -[CO2], particularly when nutrients limited growth potential. They also observed mild leaf damage associated with starch accumulation (reflecting insufficient sink capacity) in plants grown in high [CO2]. All of these studies were conducted in growth chambers or greenhouses, and the observed variation in [CO2] response may in part be attributable to variation in pot size used, which can affect below-ground sink capacity and the magnitude of downward acclimation of photosynthesis at elevated [CO2] (Arp, 1991; Sage 1994). In one study (Wheeler et al, 1991) in which a relatively large pot size (19 L) was used and plants were grown at [CO2] of 1000 compared with 350 |mmol mol-1, the varieties Norland, Russet Burbank and Denali had yield increases of 23, 35 and 40%, respectively.
A comprehensive evaluation of genotypic variation in potato yield response to [CO2] under field conditions and with unrestricted rooting volume is needed. We have conducted experiments for one growing season with potato (var. Katahdin) in the field (Arkport fine sandy loam soil) in canopy chambers and found a yield benefit of about 70% at 700 compared with 350 |mmol CO2 mol-1 at ambient (non-stress) temperature (Fig. 10.7).
Temperature x atmospheric [CO2] interaction
There have not been many studies with potato in which both [CO2] and temperature were manipulated and the interaction evaluated. Cao et al. (1994) reported a greater tuber yield benefit from CO2 enrichment (1000 compared with 500 |mmol mol-1) at a constant 20°C temperature compared with 16°C. This may reflect one of the most important and most frequently observed [CO2] x temperature interactions: the stimulation of C3 photosynthesis by elevated [CO2] increases as temperatures increase within the non-stress temperature range (e.g. 15—30°C). A primary reason for this is that as temperatures increase, oxygenation by the key photosynthetic enzyme, Rubisco, increases relative to carboxylation, thereby increasing the benefit from CO2 enrichment (Jordan and Ogren, 1984).
The effect of elevated [CO2] on yield response to heat stress will be particularly important to potato farmers in regions where an increase in the frequency of high-temperature stress events occurs concomitantly with an increase in [CO2]. Wolfe and Boese (unpublished) compared the yield response to a [CO2] doubling in field canopy chambers maintained at non-stress temperatures (daytime maximum temperature near 25°C) with the yield response in chambers allowed to develop moderate heat stress (daytime maximum temperatures near 35°C). Averaged across [CO2] treatments, tuber yields were reduced by 85% in the high-temperature stress treatment, while total biomass was relatively unaffected by temperature (Fig. 10.7). This corroborates earlier findings reviewed above: potato tuberization and tuber development are more sensitive to heat stress than is either photosynthesis or total biomass accumulation. Tuber yield was increased by 71.5% (statistically significant at P < 0.05) by a [CO2] doubling in the non-stress temperature treatment. Tuber yields in plants exposed to high temperatures were extremely low, regardless of [CO2] treatment, and there was no statistically significant [CO2] effect. These results should be viewed as preliminary since they are based on a single growing season and one variety (Katahdin), but they suggest that elevated [CO2] will not mitigate the negative effects of high-temperature stress on tuberization and yield.
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