Dynamic characteristics of leaf area index and plant height of winter wheat influenced by irrigation and nitrogen coupling and their relationships with yield

Abstract: Leaf area and plant height are 2 important characteristic parameters of canopy structure, which represent the crop vertical extension and horizontal expansion respectively and are closely related to dry matter accumulation and yield formation by influencing the interception and distribution of photosynthetic active radiation. The dynamic process of crop LAI (leaf area index) and height could be influenced by many factors, of which water and nitrogen are the two major ones limiting crop growth and development. Some researchers have analyzed the dynamic process of LAI quantitatively under high yielding conditions, however the dynamic characteristics of LAI under water and nitrogen stress are still not clear. The effect of different cultivars, cultivation practices on the plant height at maturity were studied by most former research, while less information was found about how the management practices influence the dynamic process of plant height. Therefore, this study was aimed to clarify the effects of water and nitrogen on wheat LAIand plant dynamics and to quantify the relationship of LAI and plant height with crop yield. A field experiment during 2012-2014 was conducted with 3 irrigation levels and 4 nitrogen input levels in Guanzhong Plain of Shaanxi Province, and LAIs and plant heights measured were fitted with thermal time using the modified Logistic model and Richards mathematical model, respectively. The results demonstrated that the performances of both models were pretty good with an nRMSE(normalized root mean squared error) of 8% for LAI and an nRMSE of 4% for plant height, respectively. Irrigation enhanced the maximum growth rate of LAI, which increased from 0.004 cm2/(cm2•℃•d) under rainfed condition to 0.006 cm2/(cm2•℃•d) under irrigation condition in over-wintering and jointing period. The corresponding maximum LAI and average LAI increased from 3.50 and 2.64 cm2/cm2 to 4.11 and 3.15 cm2/cm2, respectively. Nitrogen fertilizer application shortened the thermal time from sowing to the maximum growth rate and maximum value of LAI occurring, and improved the maximum growth rate of LAI. Hence, the maximum LAI and average LAI increased from 1.87 and 1.35 cm2/cm2 with no nitrogen input to 4.57 and 3.82 cm2/cm2 with 210 kg/hm2 input, respectively, and no further significant increase was observed with more nitrogen input. Irrigation enhanced the maximum plant height from 62 to 66 cm, which was resulted from the longer thermal time from sowing to the maximum plant height occurring under the irrigation condition. The thermal time from sowing to the rapid growth stage and to the maximum plant height occurring was shortened and extended by nitrogen fertilizer application, respectively,which brought about a plant height increasing from 58 to 65 cm with 105 kg/hm2 input, and then plant height was leveled off with more nitrogen input. Due to more rainfall in the early growth stage, the plant height in 2014 was about 10 cm higher than that in 2013. The stepwise regression analysis of dynamic characteristics of wheat yield with LAI and plant height showed that grain numbers per square meter was only related with average LAI, while 1 000-grain weight depended largely on the maximum plant height. Furthermore, the final yield was determined by both average LAI and the maximum plant height with a larger contribution from average LAI. This research revealed the mechanism that how irrigation and nitrogen fertilizer application affected the plant leaf area and height growth, and then further impacted on yield. The result provides a set of technical support for reasonable regulation of crop community structure.