Photosynthetic characteristics and yield of potato in potato/maize intercropping systems with different row number ratios

Intercropping of potato with maize has been a traditional cropping system in China widely practiced in mountain regions. However, yields of potato/maize systems have been limited by inappropriate potato-maize row ratios. A potato/maize intercropping field experiment with two different potato-maize row ratios was carried out to determine dynamic changes in a range of potato features. The elements analyzed included leaf area index (LAI), special leaf weight (SLW), chlorophyll a (Chla), chlorophyll b (Chlb), total chlorophyll (Chla+Chlb) and chlorophyll a/b ratio in potato at different developmental stages. Also gas exchanges at potato tuber development and yield stages were studied using monocropped potato as the control. The ratio 2︰2 meant two rows of potato to two rows of maize and that 3︰2 meant three rows of potato to two rows of maize. The results showed that while intercropping significantly reduced LAI, SLW and Chla/b, it increased Chla, Chlb and Chla+Chlb of potato at all growth stages. Compared with the 2︰2 intercropping system, the 3︰2 intercropping system had significantly higher LAI at all developmental stages; lower SLW at tuber initiation stage; higher SLW at vegetative growth and starch accumulation stages; higher Chla, Chlb and Chla+Chlb at tuber development stage; lower Chla, Chlb and Chla+Chlb at starch accumulation stage; and higher Chla/b at both tuber development and starch accumulation stages. Also Chla and Chla+Chlb declined much slowly in the 3︰2 than in the 2︰2 potato/maize system. Furthermore, intercropped potato induced declines in net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and stomatal limitation (Ls). It, however, increased intercellular CO₂ concentration (Ci) and water use efficiency (WUE). Variations in Pn, Gs and Ci suggested that intercropping limited photosynthetic activity in potato due to non-stomatal processes. Significantly higher Pn, Gs, Tr and Ci and lower WUE were noted in the 3︰2 than in the 2︰2 intercropping system. Also in comparison with monocropping, both the number and weight of big tubers (fresh tuber weight >50 g) per plant markedly dropped under intercropping. The number and weight of small tubers (fresh tuber weight ≤50 g) per plant also dropped in the 2︰2 cropping system. However, while the number of small tubers per plant improved, the weight of the small tubers largely remained unchanged in the 3︰2 cropping system. Both the number and weight of small tubers per plant were higher in the 3︰2 than in the 2︰2 intercropping system. No significant difference was noted in the number and weight of big tubers per plant between the 3︰2 and 2︰2 intercropping systems. This suggested that increasing potato rows more or less affected small tubers and not big tubers in potato/maize systems. In conclusion, intercropping substantially decreased tuber yield at harvest due to changes in photosynthetic characteristics of potato in potato/maize systems. The land equivalent ratios were 0.88 and 1.24 in the 2︰2 and 3︰2 intercropping systems, respectively. This suggested that the 3︰2 row ratio performed better than the 2︰2 row ratio in potato/maize intercropping systems.