Evidence for lack of photosynthesis net gain during 2°C treatment
Under the cold acclimation condition of a PPFD of 4 μmol m-2s-1and 2°C both FR and FS plants maintained a healthy green color after 6 wk at 2°C, and resumed growth when re-hydrated and returned to 25°C. When the mean foliage dry weight (FDW) of six plants per cultivar (and each treatment interval) were plotted against durations of 0, 2, 4, or 6 wk at 2°C, the slopes representing rate of FDW change of each cultivar were not significantly different from zero (data not shown). It was concluded that plants were near or at light compensation.
Evidence of foliage water content decrease in FR and FS plants at 2°C
To evaluate the laboratory system in relation to reported field results, changes in foliage water content were measured in FR and FS plants after 5 wk at 2°C. Foliage water content decreased to 0.77 g H2O·g FFW-1 in FS plants compared to 0.63 g H2O·g FFW-1 in FR plants. This reflected a greater reduction of foliage water content for FR plants since the starting amount of each was 0.85 g H2O·g FFW-1. Thus even with this approximate measurement of plant water status a differential in foliage water content of FR and FS was demonstrated after 2°C exposure.
A further test was performed to determine to what extent foliage water content decreased in plants maintained under our laboratory conditions. Foliage water content values were determined from the foliage wet weight and dry weight values as described in methods. The 48 hr drying at 80°C ensured the complete drying of the sample and thus accurate determination of foliage dry weight. A similar reduction in foliage water content of FR and FS plants was shown after 4 wk at 2°C (Fig. ). However after 6 wk there was a greater reduction of foliage water content in FR plants, amounting to 14% compared to only 5.8% for FS plants. Thus a differential reduction in foliage water content of FR plants is comparable to that observed in northern and southern-selected HRWW genotypes observed in the field by Martin [11
] and Nass [14
] in late autumn.
Figure 1 A Foliage water content (FWC) of plants from a freeze resistant (FR) and a freeze susceptible (FS) wheat cultivar during 6 wk at 2° C. Five-day seedlings were transplanted one cylinder-1, each cylinder contained 161 g ODS and maintained at about (more ...)
Evidence for differential plant water uptake between FR and FS plants at 2°C
Having ascertained that at 2°C and low light intensity foliage water content differentially decreased in FR and FS plants as described for field grown plants, we proceeded to gravimetrically measure plant water uptake from the soil. The design of the plant soil system for measuring plant water uptake was such that the possibility of water loss via evaporation was minimized. Thus reduction in weight of the system could only be attributed to removal of water from the soil by plants or plant water uptake. Mean plant water uptake was determined after 2, 4, and 6 wk at 2°C (Fig. ). After 2 wk no significant difference was observed for FR and FS plants, 64 g H2O·g FDW-1 and 58 g H2O·g FDW-1, respectively. Thereafter plant water uptake for FS plants remained effectively constant, whereas that by FR plants increased to 102 g H2O by the end of 6 wk (Fig. ). In summary, plant water uptake occurred for both FR and FS plants at 2°C. However over the entire 6 wk water uptake by FR plants was greater than that by FS plants. Similar results were also obtained in a 5 wk study comparing plant water uptake between FR and FS plants.
Detection of maximum plant water uptake by FR plants at 2°C
The above findings that FR plants with the greater plant water uptake have significantly lower foliage water content than FS plants could reflect more rapid loss of foliage water by FR plants, or limiting soil water available to FR due to its rapid rate of plant water uptake. To test the latter, FR plant water uptake was measured with increased initial soil water content. The initial amount of water was increased from 50 mL cylinder-1 (Fig. 1) to 58 mL (Fig. ) or to 81 mL (Fig. ) by increasing cylinder size and the soil mass while maintaining soil matric potential constant.
Figure 2 Comparison of PWU and FWC of FR plants supplied with A: 58 or B: 81 mL of water prior to 2°C exposure. Five-day seedlings were grown 23 d at 25°C in plant cylinders containing either A: 165 or B: 225 g ODS at a Ψm of about -0.003 (more ...)
The rate of plant water uptake was calculated from a slope of plant water uptake plotted against time in wk at 2°C. From the 58 mL supply the rate was 20.3 g H2O·wk-1 (Fig. ), compared to 19.5 g H2O·wk-1 from the 81 mL supply (Fig. ). Similarity of slopes suggested that 58 mL was adequate to sustain maximum water uptake of FR plants. However, the availability of more water initially from the 81 mL supply resulted in a foliage water content decrease that approached that of plants initially provided with 58 mL. Thus we provided evidence that inadequate soil water during the 5 wk period was not the cause of the decrease in FR foliage water content.
Foliage water content and plant water uptake of northern-selected HRWW genotypes at 2°C
In order to determine whether the relationship between foliage water content reduction and plant water uptake was a general phenomenon among other northern-selected HRWW genotypes, four additional HRWW cultivars were studied. These cultivars together with FR have excellent freeze survival records and are used as standards in the Northern Great Plains field trials [18
]. Foliage water content changes are depicted in Fig. &. Foliage water content of Winoka, Norstar, and Rose was shown to be the same after 2 wk as before 2°C exposure (Fig. ). After 6 wk, foliage water content of Winoka, Norstar, and Rose all decreased to similar amounts of 0.63, 0.62, 0.64 g H2
respectively, compared to the 0 wk value of 0.82 g H2
Figure 3 Foliage water content (FWC) of 'Norstar' and 'Rose' (A), and in a separate experiment 'Arapahoe' and 'YTO-117' (B), northern adapted genotypes compared to the reference FR cultivar (Winoka) after soil drying 2 and 6 wk at 2°C. Five-day seedlings (more ...)
Foliage water content among Winoka, Arapahoe, and YTO-117 plants were compared (Fig. ). All values decreased slightly after 2 wk, compared to that of 0.81 g H2O·g FFW-1 before 2°C exposure (Fig. ). However after 6 wk at 2°C foliage water content decreased by Winoka, Arapahoe, and YTO-117 plants to 0.60, 0.55 and 0.64 g H2O·g FFW-1 respectively compared to the initial 0.81 g H2O·g FFW-1 (Fig. ). A slightly larger foliage water content value after 6 wk for Winoka in Fig. compared to that in Fig. suggests lower relative humidity occurred during 2°C exposure in Experiment B.
Thus, it was demonstrated that the magnitude of foliage water content decrease in these four additional genotypes was similar to that in FR (Winoka) at 2°C. As a group, foliage water content decreased appreciably after 6 wk.
Results of plant water uptake are depicted in Fig. &. Water uptake by Winoka, Norstar, and Rose plants is shown in Fig. . After 2 wk at 2°C, water uptake by Rose lagged slightly behind Winoka and Norstar. However after 6 wk water uptake by Norstar and Rose were equal to Winoka, all increasing appreciably compared to plant water uptake values at 2 wk (Fig. ). Plant water uptake by Winoka, Arapahoe, and YTO-117 was compared (Fig. ). After 2 wk the amount was similar for these genotypes, with water uptake being somewhat larger than in Expt. A, based on the response of Winoka. After 6 wk, the pattern of water uptake by plants was also greater, although more variable (Fig. ).
Figure 4 Cumulative plant water uptake (PWU) of 'Norstar' and 'Rose' (A), and in a separate experiment 'Arapahoe' and 'YTO-117' (B) plants, northern adapted genotypes compared to the reference FR cultivar Winoka after soil drying 2 and 6 wk at 2°C. PWU (more ...)
Under the laboratory conditions used in this study, the results of decreasing foliage water content with duration at 2°C was similar to the findings of Fowler and Carles [20
], Nass [14
] and Martin [11
]. Their studies were performed on field grown plants and showed decreasing tissue water content during the Fall period. Fu et al
] also demonstrated that tissue water content decreased during cold acclimation in the field or in an environmental chamber. In our study, decrease in foliage water content correlated with an increase in plant water uptake (Fig. &).
Based on the assumption that stomata opened during the cold acclimation period, we believe that more effective water movement occurs in FR compared to FS plants at 2°C. Osmotic potential difference between FR and FS plants account for this phenomenon. DeNoma et al. [19
] observed that during low non-freezing temperature in the field a greater decrease in crown osmotic potential occurred in freeze-resistant compared to more susceptible HRWW cultivars. We have obtained similar osmotic potential results with leaf tissue (data not shown).