Modern crop cultivars are products of human modification on wild or early crop cultivars through breeding, in order to meet man’s interest [1] . Issues of high yields and quality have been great motivation in substituting old cultivars with modern cultivars in Solanum tuberosum [2] , Brassica napus ssp. Napobrassica [3] , wheat [4] , and soybeans [5] . However, it was argued that crop improvement has failed to increase the basis for assimilation (relative growth rate and relative leaf area) despite increasing harvestable organs [6] . Old wheat and oat cultivars were reported to have higher leaf area and growth rate than their modern cultivars [7] . Consequently, low gaseous exchange is reported in modern cultivars compared to old or wild cultivars for cereal crops [6] . It was not known whether the heritage potatoes collectively known as Taewa in New Zealand outweigh modern potatoes in photosynthetic capacity; unravelling of this controversy under different water and nitrogen situation is vital for Taewa growers to easily manage them in the modern production systems.

Potato gaseous exchange is strongly influenced by water and nitrogen (N) apart from leaf features and other environmental factors [8] . Nitrogen is very much prioritised for leaf development before enhancing photosynthetic capacity in potato [9] . This is why leaf ageing comparably declines assimilation rates [10] . A decrease in conductance and assimilation in potato because of low internal carbon concentration (Ci) and high photosynthesis/ stomatal conductance (A_n/g_s) ratio was also reported under water deficit [11] . This contradicted an argument that an increase in Ci simulates the stomata aperture to reduce g_s [12] . Controversies still remains in gaseous exchange between heritage potatoes of New Zealand and modern potato cultivars at different water and N levels. Progressions of photosynthetic capacity in modern potatoes from heritage potatoes, Taewa was investigated to enable grower manage inputs such as fertilizer and irrigation for optimum production. The response of Taewa to water and N is important to bridge the yield gap between Taewa and modern potato [2] through optimisation of the interaction between cultivars and crop management. This study compared photosynthetic capacity and leaf water potential of Taewa and modern potatoes grown at different water and N regimes under glasshouse and field conditions.

Gaseous exchange is reported to greatly differ with leaf age [10] [19] , genotypes [20] , atmospheric water demand or irrigation [21] [22] , nitrogen [10] [23] and climatic factors in potatoes [24] . Severe water stress is said to accelerate leaf VPD in plants [25] [26] . The implication of a high VPD gradient results in leaf water deficits declining A_n and T rate [25] . However, the extent of these environmental factors effects on gaseous exchange [24] , depends on genotypes and leaf age.

Photosynthetic WUE and A_n initially increased before declining with time and crop age, in all cultivars, regardless of irrigation and N treatment. This result has also been reported by [10] . Figures for photosynthetic WUE and A_n were remarkably high, from 20 to 50 DAE. Generally, modern potatoes had high A_n within the first three weeks from emergence, but Taewa had extended high A_n up to 65DAE. The tendency for photosynthetic WUE and A_n decrease was greatest in the modern potatoes (Agria and Moonlight), possibly due to early maturity, compared to Taewa. A high A_n within 20 DAE as observed in the glasshouse was once reported [10] . It was then concluded that this period had raised A_n due to tuberisation. A rise in A_n with tuberisation in potato has also been reported [27] . In these studies, heritage potatoes, Taewa delayed tuberisation, hence their extended high gaseous exchange.

The highest photosynthetic WUE and A_n in the field study was on 48DAE, in both Taewa and modern potatoes. Contrary to the findings on glasshouse by Ghosh [10] . The photosynthetic capacity trend in both glasshouse and field experiment still reflected tuberisation and it declined with age [10] [22] . This suggests that the period of tuberisation, the likely cause of high A_n [27] , is not static at 21 DAE, as observed in the glasshouse, but it ranges from three weeks to seven weeks from plant emergence and is longer in unimproved cultivars. The extension of high gaseous exchange in Taewa proves that growth stages and maturity period for Taewa are different and longer compared to modern potato cultivars.

Taewa, achieved high photosynthetic WUE in the glasshouse and in 2010/2011 by maintaining high A_n at low Ci, compared to Agria and Moonlight. Taewa and Moonlight had comparable g_s and T in the glasshouse. In 2009/2010, Moe Moe and Agria had comparable photosynthetic WUE, A_n and insignificant g_s, T, Ci and VPD. However, emphasis on glasshouse and the 2009/2010 field study results found that A_n and photosynthetic WUE in Agria, steadily reduced when both g_s and T are very high while the 2010/2011 results suggest that Agria does not steadily increase A_n even if g_s and T are low or comparable to Taewa cultivars, Moe Moe. The main driver of these differences was Ci, which was high in the modern potatoes and low in Taewa.

Photosynthetic WUE is negatively correlated with Ci, particularly in modern cultivars whilst A_n and g_s relationship is curvilinear (Figure 4). Consequently, Ci increase and too high g_s in modern potatoes reduced A_n and photosynthetic WUE. This finding agrees with other studies, which have indicated that increased Ci reduces A_n [12] and A_n and g_s relationship is curvilinear [28] . For this reason, Taewa differs from modern potatoes in photosynthetic WUE, in the way Ci manipulates stomata apertures. It is very possible that modern potatoes have changed gaseous exchange behaviour through breeding [12] , thereby resulting in disparity in photosynthetic WUE with heritage cultivars, Taewa. The heritage cultivars have not undergone several breeding processes to change its gaseous exchange like modern potatoes [11] . This state may assist Taewa to endure water stress.

Water deficit increases leaf VPD in potato [25] , which consequently reduces g_s and photosynthetic capacity [23] [29] [22] . In our studies, the A_n, g_s and T increased with irrigation and N, whereas leaf VPD and Ci declined with irrigation and N: Although N had no much significant influence as irrigation. It was also observed that VPD was significantly influenced by water deficit, thus resulting in low A_n and photosynthetic WUE. This shows that a reduction of irrigation, below optimal levels, affects gaseous exchange in both Taewa and modern potatoes: Although this differs. Nevertheless, Taewa exhibited exceptionally high photosynthetic WUE characteristics under

Note: Insignificance is shown by same letters in columns or rows (p > 0.05).

Note: Insignificance is shown by same letters in columns or rows (p > 0.05).

water stress. This ability suggests they are well adapted to low water supply as once reported in old wheat compared to their modern wheat [30] and among potato genotypes [31] .

There are potato genotypic variation in A_n under well watered and limited water [31] . However, the response to water deficit was primary regulated by stomatal closure followed by mesophyllic activity when water stress was severe, as also observed in this study and in most C₃ plants [32] . The Ci for rain-fed treatment in the 2009/2010 field study increased, thus signifying severe water stress to have fully induced A_n and photosynthetic WUE reduction in modern potatoes. This is factual, because Ci is greatly affected by mesophyllic activity [31] and it is inversely related to A_n [12] . However, this study results differ from Olensinski study [23] , who found that Ci was not affected by water deficit and it also opposes Liu’s findings [15] that photosynthetic WUE was higher under deficit irrigation than full irrigation.

Photosynthetic WUE for rain-fed was lower than irrigated crops, possibly because A_n for rain-fed decreased, while g_s and T remained in the same range with the irrigated potato in the glasshouse and in the 2009/2010 field experiment, which is contrary to other studies [22] and [15] . In 2010/2011, both water and N deficits steadily decreased g_s as once reported by Schapendonk [31] . Consequently, T and A_n reduced under rain-fed and low N, as reported by Olesinski [23] . In contrast to rain-fed, partially irrigated potato achieved the highest Ci, g_s and T, which consequently increased photosynthetic capacity, whilst reducing photosynthetic WUE due to high T. On the other hand, full irrigation moderately increased T and g_s with reduced Ci, hence the high photosynthetic WUE and A_n. This result suggests that high water stress decreased g_s whilst partial water stress reduced the resistance with great T and Ci fluxes. Full irrigation stabilised g_s, Ci and T, resulting in high A_n and photosynthetic WUE. Consequently, photosynthetic WUE was highest with FI and lowest with PI, despite its high A_n. The high fluxes in Ci and T under PI reduced its photosynthetic WUE. This finding confirms that water and N deficiencies limit photosynthetic capacity [23] [32] , whereas FI and PI improve photosynthetic capacity [22] . However, PI failed to use water sparingly on a canopy basis, as reported under a partial root zone drying irrigation strategy [22] , because of very high T soon after irrigation.

Water stress increased leaf water potential in both 2009/2010 and 2010/2011 experiments. However, the leaf water potential results for the 2009/2010 study did not demonstrate genotypic variability on leaf water potential possibly due to low drought intensity compared to 2010/2011, when they differed in leaf water potential, as once reported in the Andean region between water regimes [33] . The 2009/2010 study finding illustrated that potato leaf water potential is not as sensitive as gaseous exchange to water stress and hence it was not a very reliable indicator for cultivar tolerance to water stress, as formerly observed [23] . Regardless to Olesinskiobersation and 2009/2010 study findings, the 2010/2011 experiment showed that leaf water potential can also be used as an indicator for water stress among potato cultivars and for irrigation guidance. Leaf water potential possibly provided good comparison between Taewa and modern potato cultivars when drought intensity was high in 2010/ 2011 compared to the 2009/2010 growing season. The reason is that a high water vapour gradient results in leaf water deficits declining A_n and T rate [25] because of stomatal closure induced by high leaf water potential approached after maximum leaf transpiration and water deficits [19] .

Taewa demonstrates high tolerance to drought and low N compared to modern potatoes. An examination of the gaseous exchange behaviour in Taewa, compared to Agria (as presented above) coupled with the leaf water potential results, supports Taewa’s superior photosynthetic capacity under water stress. Furthermore, the leaf water potential for FI and PI is not statistically different in Taewa while different in modern potato, Agria. This indicate that Taewa (specifically Tutaekuri), can be scheduled at PI without more water stress while modern potato, Agria, need FI for same leaf water potential with Taewa at PI. Taewa does not require a high amount of N and water for A_n maintenance because is not bred for high water and N use efficiency as once reported between wild and modern potatoes [34] [35] . Nevertheless, optimal water is a requirement in Moe Moe and the modern potatoes for maximum gas exchange during the growing season.

There were significant variations on gaseous exchange and leaf water potential between heritage and modern potatoes under different water and N regimes. It has been proven that photosynthetic WUE and A_n increased by irrigation and N in all cultivars. Taewa outweighed modern potatoes on photosynthetic capacity and leaf water potential resilience to water stress in glasshouse and during 2010/2011 experiment. The usual drift of seasonal photosynthetic capacity increasing with tuberisation before declining with time was also observed, except that the decline was high in modern potatoes. The fact was that modern potatoes were more controlled by high Ci compared to Taewa under water stress. That resulted on the build-up propositions of great disparity in photosynthetic capacity among these potato cultivars. Nevertheless, Taewa cultivars are exceptionally able to adapt to low water and N supply. An examination of the g_s and leaf water potential behaviour of Taewa at different soil moisture and N compared to Agria, indicates their competency to decrease T and maintain high photosynthetic capacity under water and N deficit environments. This study indicates that Taewa can be scheduled at PI and N without more water and N stress while modern potatoes need FI and N.