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CROP PHYSIOLOGY & METABOLISM

Diurnal and Seasonal Photosynthesis in Two Asparagus Cultivars with Contrasting Yield

^a Dep. of Plant and Microbial Sciences, Univ. of Canterbury, Private Bag 4800, Christchurch, New Zealand
^b New Zealand Institute of Crop and Food Research, P.O. Box 4704, Christchurch, New Zealand

* Corresponding author (m.turnbull{at}botn.canterbury.ac.nz)

To investigate physiological characters underpinning spear yield in asparagus (Asparagus officinalis L.), diurnal and seasonal changes in photosynthetic parameters were measured under field conditions in two cultivars with contrasting yield. Seasonal patterns in photosynthetic parameters were strongly dependent on cladophyll developmental stage in both cultivars. The greatest photosynthetic rates of 8.94 ± 0.54 µmol m^-2 s^-1 for the high-yielding cultivar (ASP-69) and 6.50 ± 0.38 µmol m^-2 s^-1 for the low-yielding cultivar (ASP-03) were observed in fully expanded cladophyll tissue measured in mid-summer (February) when both photon flux density (PFD) and temperature were at a maximum. A significant decline in net photosynthetic rate (A) was measured in April, when plants experienced colder night temperatures and shorter day lengths. A close correlation between A and stomatal conductance (g_s) (r = 0.84) was observed. Timing of cladophyll initiation and duration did not appear to be significant factors contributing to cultivar difference in photosynthesis. Variation in photosynthetic capacity between the two cultivars was related to significant differences in cladophyll thickness and specific leaf weight (SLW). The results substantiate the conclusion that both metabolic and anatomical factors play significant roles in determining differences in photosynthetic capacity between the two asparagus cultivars studied.

Abbreviations: A, net photosynthetic rate • A_max light saturated net photosynthetic rate • A_sat, light and CO₂ saturated net photosynthetic rate • C_a, atmospheric CO₂ concentration • C_i, intercellular CO₂ concentration • g_s, stomatal conductance • J_max potential rate of RuBP regeneration • L_stom, relative stomatal limitation on photosynthesis • R_d, dark respiration rate • SLW, specific leaf weight • V_cmax, maximum rate of carboxylation

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